{'fraction_non_alphanumeric': 0.03860480866915002, 'fraction_numerical': 0.030358956992888587, 'mean_word_length': 4.1451345936057145, 'pattern_counts': {'":': 0, '<': 11, '<?xml version=': 0, '>': 7, 'https://': 0, 'lorem ipsum': 0, 'www.': 1, 'xml': 0}, 'pii_count': 2, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 6, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Multiwavelength Study of NGC 281 Region

We present a multiwavelength study of the NGC 281 complex which contains the young cluster IC 1590 at the center, using deep wide-field optical UBVI_c photometry, slitless spectroscopy along with archival data sets in the near-infrared (NIR) and X-ray. The extent of IC 1590 is estimated to be ~6.5 pc. The cluster region shows a relatively small amount of differential reddening. The majority of the identified young stellar objects (YSOs) are low mass PMS stars having age<1-2 Myr and mass 0.5-3.5 M_\odot. The slope (\Gamma) of the mass function for IC 1590, in the mass range 2<M/M_\odot \le 54, is found to be -1.11+-0.15. The slope of the K-band luminosity function (0.37+-0.07) is similar to the average value (~0.4) reported for young clusters. The distribution of gas and dust obtained from the IRAS, CO and radio maps indicates clumpy structures around the central cluster. The radial distribution of the young stellar objects, their ages, \Delta(H-K) NIR-excess, and the fraction of classical T Tauri stars suggest triggered star formation at the periphery of the cluster region. However, deeper optical, NIR and MIR observations are needed to have a conclusive view of star formation scenario in the region. The properties of the Class 0/I and Class II sources detected by using the Spitzer mid-infrared observations indicate that a majority of the Class II sources are X-ray emitting stars, whereas X-ray emission is absent from the Class 0/I sources. The spatial distribution of Class 0/I and Class II sources reveals the presence of three sub-clusters in the NGC 281 West region.


Introduction
H II regions have been studied quite extensively in recent years on account of their close association with star formation. There seems to be two modes of star formation associated with H II regions depending on the initial density distribution of the natal molecular cloud. One is the cluster mode which gives birth to a rich open clusters and the other is the dispersed mode which forms only loose clusters or aggregates of stars. Presumably, the former takes place in centrally condensed, massive clouds, whereas the latter occurs in clumpy, dispersed clouds (see e.g., Ogura 2006). These clusters/aggregates of stars emerging from their natal clouds can be the laboratories to address some of the fundamental questions of star formation. Trends in their evolutionary states and spatial distribution can help distinguish between various star formation scenarios such as spontaneous or triggered star formation. Triggered star formation is a complex pro- * saurabh@aries.res.in cess and makes an interesting and important topic of star formation. The formation of massive stars feeds energy back into the nearby environments, irradiating, heating and compressing the remain of the natal molecular cloud. This feedback can have either destructive or constructive effects, but it is not clear which dominates in a given cloud or overall in a galaxy. Many examples exist in our Galaxy as well as in other galaxies where spatial distributions of young stellar objects (YSOs) and their ages suggest for triggered star formation (see e.g. Walborn  The HII region NGC 281/Sh2-184 (α 2000 = 00 h 52 m , δ 2000 = +56 • 34 ′ or l=123 • .07, b= -6 • .31) is located at a relatively high Galactic latitude and has the centrally located cluster IC 1590 (Guetter & Turner 1997, Henning et al. 1994). The brightest member of IC 1590 is an O-type Trapezium-like system HD 5005, whose component stars HD 5005ab (unresolved), HD 5005c, and HD 5005d have spectral types of O6.5 V, O8 V, and O9 V, respectively (Walborn 1973; Abt 1986; Guetter & Turner Saurabh Sharma et al. [Vol. , 1997). Despite many measurements, the distance estimates of NGC 281 varies from 2.0 kpc to 3.7 kpc (cf. Sato et al. 2008). Recently, using the VLBI observations of an associated H 2 O maser source Sato et al. (2008) derived a trigonometric parallax of 0.355±0.030 milli arcsec, corresponding to a distance of 2.81 ± 0.24 kpc. The NGC 281 region provides an excellent laboratory for studying in detail star formation through the interaction of high mass stars with their surrounding cloud. Of special interest in this region is the possibility of triggered star formation occurring on two different scales: the large-scale (∼300 pc) supernovae-triggered formation of the first generation OB stars and their associated cluster (Megeath et al. 2002(Megeath et al. , 2003, and the subsequent, triggered sequential and ongoing star formation on a smaller scale (∼1-10 pc); the latter is taking place in an adjoining molecular cloud (NGC 281 West) probably through an interaction with an HII region (the NGC 281 nebula) excited by the first generation OB stars (Elmegreen & Lada 1978;Megeath & Wilson 1997;Sato et al. 2008).
The southwestern quadrant of the NGC 281 nebula is obscured by the adjoining molecular cloud NGC 281 West. Ongoing star formation in NGC 281 West is indicated by the presence of H 2 O maser emission and IRAS sources within this cloud near its clumpy interface between the HII region. This star formation may have been triggered by the interaction of the molecular cloud with the HII region (Elmegreen & Lada 1978;Megeath & Wilson 1997). The NGC 281 molecular cloud complex was mapped both in 12 CO (J=1-0) and 13 CO (J=1-0) emission lines by Lee & Jung (2003). The central radial velocity of the NGC 281 West molecular cloud, V LSR = 31 kms −1 (Lee & Jung 2003) agrees well with that of the H 2 O maser emission in the cloud (Sato et al. 2007). Megeath et al. (2002Megeath et al. ( , 2003 suggested that this cloud complex was formed in a fragmenting super-bubble, which gave birth to the first generation OB stars, and these OB stars have then ionised the surrounding gas which subsequently triggered next generation star formation in the neighboring clouds (Sato et al. 2008).
Though both low-mass and high-mass star-forming regions can be studied at variety of wavelengths ranging from radio waves to X-rays, however most of the present knowledge about the HII region/open cluster NGC 281 has been inferred from studies outside the optical region. Henning et al. (1994) made a multiwavelength studies of the NGC 281/IC 1590 region including Stromgren photometry of the bright cluster stars. The first detailed U BV CCD photometry of 279 stars for the cluster was published by Guetter & Turner (1997). Their photometry terminates at a magnitude limit that is marginally brighter than the expected brightness of pre-main sequence (PMS) and T Tauri stars in the embedded cluster region. Keeping above discussion in mind we feel that NGC 281 is an appropriate target for a deep and wide field optical/infrared photometry. In this paper, we present deep wide-field optical U BV I c data and slitless spectroscopy. We supplement them with archival data collected from the surveys such as Chandra, Spitzer, 2MASS, IRAS and NVSS (NRAO VLA Sky Survey). Our aim is to understand the global scenario of star formation under the effects of massive stars in the whole NGC 281/IC 1590 region. In Section 2, we describe our optical CCD photometric and slitless spectroscopic observations and briefly the data reduction. In Section 3, we discuss the archival data set used in the present study. In the ensuing sections, we present the results and discuss star formation scenarios in the NGC 281 region.

Optical photometry
The CCD U BV I c observations of the NGC 281 region were obtained by using the 105-cm Schmidt telescope of the Kiso Observatory, Japan on November 21, 2004 and November 27, 2005. The CCD camera used a SITe 2048 × 2048 pixel 2 TK2048E chip having a pixel size of 24 µm. At the Schmidt focus (f/3.1), each pixel corresponds to 1.5 arcsec and the entire chip covers a field of ∼ 50 × 50 arcmin 2 on the sky. The read-out noise and gain of the CCD are 23.2 e − and 3.4 e − /ADU , respectively. A number of short and deep exposure frames were taken. The average FWHM of star images was found to be ∼3 arcsec. The observed region is shown in Fig. 1. The Kiso data were standardized by observing the cluster together with the standard stars in the SA 98 field (Landolt 1992) on 07 January 2005 using the 2048 × 2048 pixel 2 CCD camera mounted on the f/13 Cassegrain focus of the 104-cm Sampurnanand telescope of Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital. In this set up, each pixel of the CCD corresponds to ∼ 0.37 arcsec and the entire chip covers a field of ∼ 13 × 13 arcmin 2 on the sky. To improve the signal to noise ratio, this observation was carried out in the binning mode of 2 × 2 pixel. The read-out noise and gain of the CCD are 5.3 e − and 10 e − /ADU respectively. The FWHMs of the star images were ∼ 2 arcsec. The log of these observations is given in Table 1.
The CCD data frames were reduced by using computing facilities available at ARIES, Nainital. Initial processing of the data frames was done using the standard tasks available from IRAF 1 and ESO-MIDAS 2 data reduction packages. Photometry of cleaned frames was carried out by using the DAOPHOT-II software (Stetson 1987). The PSF was obtained for each frame by using several uncontaminated stars. Magnitudes obtained from different frames were averaged. When brighter stars were saturated on deep exposure frames, their magnitudes have been taken from short exposure frames. We used the DAOGROW program in constructing of an aperture growth curve required for determining the difference between aperture and profile fitting magnitudes. Calibration of the instrumental magnitudes to those in the standard system was done by using the procedures outlined by Stetson (1992).
No. ] NGC 281: Star formation 3 To translate the instrumental magnitudes to the standard magnitudes the following calibration equations, derived using a least-square linear regression, were used: where U, B, V and I c are the standard magnitudes and u,b,v and i are the instrumental aperture magnitudes normalized for 1 second of exposure time and X is the airmass. We have ignored the second-order colour correction terms as they are generally small in comparison to other errors present in the photometric data reduction. The standard deviations of the standardization residuals, ∆, between standard and transformed V magnitude and (U − B),(B − V ) and (V − I) colours of the standard stars are 0.006, 0.025, 0.015 and 0.015 mag, respectively. Short exposure data of the cluster region taken on the standardization nights, were standardized by using the above equations and coefficients. The standard magnitudes and colours of more than 50 stars obtained from these short exposures were further used to standardize the deep observations taken with Kiso Schmidt. The standard deviations of the residual of secondary standards are of the order ∼0.02 mag. The typical DAOPHOT errors in magnitude as a function of corresponding magnitude in different pass-bands for the Kiso Schmidt observations are found to increase with the magnitude and become large (≥ 0.1 mag) for stars fainter than V ≃ 20 mag. The measurements beyond this magnitude were not considered in the analysis.

Completeness of the data
To study luminosity functions (LFs)/ mass functions (MFs), it is very important to make necessary corrections in data sample to take into account the incompleteness that may occur for various reasons (e.g. crowding of the stars). We used the ADDSTAR routine of DAOPHOT II to determine the completeness factor (CF). The procedures have been outlined in detail in our earlier works (Pandey et al. 2001(Pandey et al. , 2005. Briefly, the method consists of randomly adding artificial stars of known magnitude and position into the original frame. The frames are rereduced using the same procedure used for the original frame. The ratio of the number of stars recovered to those added in each magnitude interval gives the CF as a function of magnitude. In the case of optical CCD photometry, the incompleteness of the data increases with magnitude as expected. The CF as a function of V magnitude is given in Table 2. Table 2 indicates that our optical data have a 95% completeness at V ∼ 16.5 mag, which corresponds to a stellar mass of ∼ 2 M ⊙ for a PMS star having an age of ∼2 Myr (cf. Fig. 11).

Comparison with previous studies
We have carried out a comparison of the present photometric data with those available in the literature. The difference ∆ (literature -present data) as a function of V magnitude is given in Table 3. The comparison indicates that the present V mag and B − V colour are in good agreement with the CCD and photoelectric photometry by Guetter & Turner (1997), whereas the ∆(U − B) shows a systematic variation with the V magnitude in the sense that the present (U − B) colours become blue with increasing V magnitude.

Slitless grism spectroscopy
Spectra of some PMS stars, specifically classical T-Tauri stars (CTTSs) show emission lines, among which usually Hα is the strongest. Therefore, Hα surveys have often been used to identify PMS stars. We observed the NGC 281 region in the slitless mode with a grism as the dispersing element using the Himalayan Faint Object Spectrograph Camera (HFOSC) instrument during two observing runs on 10 October 2005 and 16 August 2006. This yields panoramic images where the star images are replaced by their spectra. A combination of a 'wide Hα' interference filter (6300 -6740Å) and Grism 5 (resolution = 870) of HFOSC was used without any slit. The central 2K × 2K pixels of the 2K × 4K CCD were used in the observations. The pixel size is 15 µm with an image scale of 0.297 arcsec pixel −1 . The observed sky area is shown in Fig. 1 as a large white box which was covered by four field-of-views of ∼ 10 × 10 arcmin 2 each. For each field-of-view we secured three spectroscopic frames of longer exposure with the grism in, and one direct frame of shorter exposure with the grism out for the identification purpose. The log of the observations is given in Table 1. Emission line stars with enhancement over the continuum at the Hα wavelength are visually identified.

2MASS
Near-infrared (NIR) JHK s data for point sources in the NGC 281 region have been obtained from the Two Micron All Sky Survey (2MASS) Point Source Catalogue. The 2MASS data reported to be 99% complete up to ∼ 16,15,14.7 mag in J,H,K S bands respectively 3 . To secure the photometric accuracy, we used only the photometric data with the quality flag ph-qual=AAA, which endorses a S/N≥ 10 and photometric uncertainty < 0.10 mag. The NIR data are used to identify the Classical T-Tauri stars (CTTSs) and Weak line T-Tauri stars (WTTSs) (cf. §4.3).

CHAN DRA X-ray data
Since YSOs are very strong X-ray emitters (as strong as log L X /L bol ∼ 10 −3 ) and they can be detected behind column densities as large as N HI ∼ 10 23 cm 2 (Linsky et al. 2007), X-ray imaging of star forming regions and young clusters is valuable for identifying these sources. , and the satellite roll angle (i.e. the orientation of the CCD array relative to the north-south direction) was 225 • .5 for all observations. The exposures were obtained in the very faint data mode with a 3.2 s frame time using the ACIS-I imaging array as the primary detector. ACIS-I consists of four front illuminated 1024 × 1024 CCDs with the pixel size of ∼ 0.492 arcsec and the combined field of view of ≈ 16.9 × 16.9 arcmin 2 . The S2 and S3 CCDs in ACIS-S were also enabled, however in the present study we used the ACIS-I data only. The detailed information on Chandra and its instrumentation can be found in the Chandra Proposer's Guide (POG) 4 . To detect sources, we have merged the event-list files of all observations. The X-ray observed region is again shown in Fig. 1 by a black box.

Data reduction and Source detection
We analyzed the data reprocessed by the Chandra X-Ray Center on 2006 April 5 (ASCDSVER 7.6.7.1). The data were reduced by using the Chandra Interactive Analysis of Observations (CIAO; Fruscione et al. 2006) software (ver. 4.1; CALDB ver. 4.2). Light curves from the on-chip background regions were inspected for large background fluctuations that might have resulted from solar flares, and none were found. We have filtered the data for the energy band 0.5 to 7.5 keV. After filtering in energy, the time integrated background is 0.11 counts arcsec −2 . Source detection was performed on the mergeevent list by using the 1.7 keV exposure map with the PWDetect 5 code (Damiani et al. 1997), a wavelet-based source detection algorithm. The significance threshold was set to 5 σ so as to ensure a maximum one spurious source per field. We have detected 379 sources, out of which 9 sources either fell on the unexposed areas of the CCD or were doubly detected. This implies that a total of 370 X-ray sources were detected in the NGC 281 field. An IDL-based program ACIS Extract (AE; Broos et al 2010) was used to extract the photons from each candidate source in a polygonal region which closely matches with the local PSFs. The source free regions around the source were considered as background. AE provides the Poisson probability of not being a source. We have not considered those sources which have the probability of being non-existence > 0.01. Sixteen such sources were found in the catalogue. After removing these sources the catalogue consists of 354 X-ray sources. Further, the mediandetected photon energy for the point sources were determined by using the ACIS Extract software package. We estimated the background AGN rate within the Chandra field of view using the Chandra Deep Field (Brandt et al. 2001). At the 0.5-2 keV limiting flux of 5.6 × 10 −16 erg 4 See http://asc.harvard.edu/proposer/POG 5 See http://www.astropa.unipa.it/progetti ricerca/PWDetect/ s −1 cm −2 , we expect to find 79 to 127 background objects within the Chandra field of view. The optical, 2MASS and IRAC counterparts of the X-ray sources were searched within a match radius of 1 arcsec and the data are given in Table 4. A sample of the table is given here, whereas the complete table is available in the electronic form only. Out of 354 X-ray sources, 193 and 90 sources have NIR and optical counterparts respectively. All the optical counterparts of X-ray sources have NIR counterparts also. The location of X-ray sources in NIR colour-colour diagram has been used to identify the probable WTTSs/ Class III sources. The completeness of the X-ray data has not been estimated. Since we are using the WTTSs/ Class III sources to study the spatial distribution of these sources and to support the results obtained on the basis of rather complete optical and NIR data, the incompleteness of the X-ray data will not have any significant effect on the results presented in this study.

Spitzer IRAC data
The Spitzer mid-infrared (MIR) surveys have enabled detailed censuses of YSOs in star forming regions. The classification of young stars as protostellar Class I or more evolved Class II sources with optically thick discs is best accomplished by using their broadband spectral energy distributions (SEDs) (Muench et al. 2007).
We have used archived MIR data observed with Infrared Array Camera (IRAC). We obtained basic calibrated data (BCD) using the software Leopard. The exposure time of each BCD was 10.4 sec and for each mosaic, 72 BCDs have been used. Mosaicking was performed by using the MOPEX software provided by Spitzer Science Center (SSC). All of our mosaics were built at the native instrument resolution of 1.2 arcsec pixel −1 with the standard BCDs. In order to avoid source confusion due to crowding, PSF photometry for all the sources was carried out. We used the DAOPHOT package available with the IRAF photometry routine to detect sources and to perform photometry in each IRAC band. The FWHM of every detection is measured and all detections with a FWHM >3.6 arcsec are considered resolved and removed. The detections are also examined visually in each band to remove non-stellar objects and false detections. The sources with photometric uncertainties < 0.2 mag in each band were considered as good detections. A total of 347 sources were detected in the 3.6 and 4.5 µm bands, whereas only 35 sources could be detected in all the four bands.
Aperture photometry for well isolated sources was done by using an aperture radius of 3.6 arcsec with a concentric sky annulus of the inner and outer radii of 3.6 and 8.4 arcsec, respectively. We adopted the zero-point magnitudes for the standard aperture radius (12 arcsec) and background annulus of (12-22.4 arcsec) of 19.670, 18.921, 16.855 and 17.394 in the 3.6, 4.5, 5.8 and 8.0 µm bands, respectively. Aperture corrections were also made by using the values described in IRAC Data Handbook (Reach et al. 2006). The necessary aperture correction for the PSF photometry was then calculated from the selected isolated sources and were applied to the PSF magnitudes of all the sources. The 2MASS, optical and X-ray counterparts of the IRAC sources were searched for within a match radius of 1 arcsec. These counterparts are given in Table 5. A sample of the table is given here, whereas the entire table is available in the electronic form. The completeness of the data in the 3.6, 4.5, 5.8 and 8.0 µm bands having S/N > 5 (error ≤ 0.2 mag) is found to be ∼ 16.0, 15.5, 13.0 and 12.0 mag, respectively.

IRAS
The data from the IRAS survey in the four bands (12, 25, 60 and 100 µm) for the NGC 281 region have been used to study the spatial distribution of warm and cold interstellar dust. One cold IRAS point sources is identified in the cluster region and its details are given in Table 6.

. Isodensity contours
Internal interaction due to two-body encounters among member stars and external tidal forces due to the Galactic disc or giant molecular clouds can significantly influence the morphology of clusters. However, in the case of young clusters where dynamical relaxation is not important because of their young age, the stellar distribution can be considered as the initial state of the cluster that should be governed by the star formation process in the parent molecular cloud (Chen et al. 2004). To study the morphology of the NGC 281 cluster, we plotted isodensity contours using the 2MASS data as well as table of the identified YSOs (cf. §4.3) in Fig. 2. The isodensity contours indicate an elongated morphology for the cluster. It is interesting to point out that sub-structures can be clearly seen in the 2MASS data (left panel of Fig. 2) towards the south-west of the cluster as well as in the south-east.

Radial stellar surface density profile
To find out the extent and radial stellar density profile of IC 1590 we used the 2MASS data of K 14.3. First the cluster center was determined by using the stellar density distribution in a 100 pixel wide strip along both the X and Y directions around an initially eye estimated center. The point of the maximum density obtained by fitting the Gaussian distribution is considered as the center of cluster. It is found to be α (2000) = 00 h 52 m 39 s .5 ± 1.0 s , To determine the radial surface density profile we assumed a spherical symmetry of stellar distribution and divided the cluster into a number of concentric circles. The projected radial stellar density in each concentric annulus was obtained by dividing the number of stars by its area. The densities thus obtained are plotted in Fig.  3. The error bars are derived by assuming that the number of stars in a concentric annulus follows the Poisson statistics. The horizontal dashed line in the plot indicates the density of contaminating field stars, which is obtained from the reference region ∼ 15 arcmin away toward the northwest from the cluster center (α (2000) = 00 h 51 m 16 s .0, . The extent of the cluster R cl is defined as the projected radius from the density peak to the point at which the radial density becomes constant and merges with the field star density. R cl from the optical data as well as from the NIR 2MASS data is estimated as ∼ 8 arcmin (∼6.5 pc for a distance of 2.81 kpc).
The observed radial density profile of the cluster was parametrized by following the approach of Kaluzny & Udalski (1992). The projected radial density profile ρ(r) is described as: where the core radius r c is the radial distance at which the value of ρ(r) becomes half of the central density ρ 0 . The best fit obtained by the χ 2 minimization technique is shown in Fig. 3. Within the uncertainties the model reproduces well the observed radial density profile of IC 1590. The core radius r c comes out to be 1.7 ± 0.4 arcmin (1.4 ± 0.3 pc) and 2.0 ± 0.3 arcmin (1.6 ± 0.2 pc) for the optical and 2MASS data, respectively.

Reddening
The interstellar extinction in the cluster region is studied by using the Fig. 4 where zero-age-mainsequence (ZAMS) from Schmidt-Kaler (1982) is shifted along the normal reddening vector having a slope of E(U − B)/E(B − V ) = 0.72. The distribution of stars shows a small amount of differential reddening (E(B − V ) ∼ 0.2 mag) in the region with the minimum of ∼ 0.32 mag which corresponds to the foreground extinction.
The reddening for the individual star having a photometric error in the V band σ V ≤ 0.1 mag and of a spectral type earlier than A0 has also been estimated by using the reddening free index Q (Johnson & Morgan 1953). Assuming the normal reddening slope we can construct the reddening- For stars earlier than A0, the value of Q will be less than 0.0. For main-sequence (MS) stars, the intrinsic (B − V ) 0 colour and colour-excess can be obtained from the rela- The individual reddening of stars down to the A0 spectral type is found to vary in the range of E(B − V ) ∼ 0.32 -0.52 mag, implying the presence of a small amount of differential reddening.

Reddening law
The extinction in star clusters arises due to two distinct sources; (i) the general interstellar medium (ISM) in the foreground of the cluster, and (ii) the localised dust associated with the cluster. While for the former component a value of R = 3.1 is well accepted (Wegner 1993;Lida et al. 1995;Winkler 1997), for the intra-cluster extinction the R value varies from 2.42 (Tapia et al. 1991)  To study the nature of the extinction law in the IC 1590 region, we used TCDs as described in Pandey et al. (2000Pandey et al. ( , 2003. The TCDs of the form of (V − λ) vs. (B − V ), where λ is one of the colour bands R, I, J, H, K and L, provide an effective method for separating the influence of the possible abnormal extinction arising within intracluster regions having a peculiar distribution of dust sizes from that of the normal extinction produced by the diffuse interstellar medium (cf. Chini Fig. 5. The contamination due to field stars is apparent in the cluster region. We selected probable field stars having (B − V ) > 0.7 visually, assuming that stars following the slope of the distribution of the reference region are contaminating foreground stars in the cluster region, and they are shown by filled circles. The slopes of the distributions for the probable cluster members (open circles), m cluster are found to be 1.24 ± 0.04, 2.16 ± 0.06, 2.76 ± 0.08, 2.90 ± 0.09 for the E(B−V ) and the ratio of the totalto-selective extinction in the cluster region, R cluster , is then derived using the procedure given by Pandey et al. (2003). R cluster has turned out to be 3.5 ± 0.3. From the photometry of bright cluster members in the I and K bands and a variable-extinction analysis of ZAMS members, Guetter & Turner (1997) also found a value of R as 3.44 ± 0.07, which is comparable to ours. Several studies have already pointed out anomalous reddening laws with high R values in the vicinity of star forming regions (see e.g. Pandey et al. 2003 and references therein). The higher than normal values of R have been attributed to the presence of larger dust grains. There is evidence that within dark clouds accretion of ice mantles on grains and their coagulation due to collision changes the size distribution towards larger dusts. The NIR TCD using the 2MASS data for all the sources lying in the NGC 281 region and having photometric errors less than 0.1 magnitude is shown in the left panel of Fig. 6. All the 2MASS magnitudes and colours have been converted into the California Institute of Technology (CIT) system. The solid and thick dashed curves rep-resent the unreddened MS and giant branch (Bessell & Brett 1988) respectively. The dotted line indicates the locus of unreddened CTTSs (Meyer et al. 1997). All the curves and lines are also in the CIT system. The parallel dashed lines are the reddening vectors drawn from the tip (spectral type M4) of the giant branch ("upper reddening line"), from the base (spectral type A0) of the MS branch ("middle reddening line") and from the tip of the intrinsic CTTS line ("lower reddening line"). The extinction ratios A J /A V = 0.265,A H /A V = 0.155 and A K /A V = 0.090 have been taken from Cohen et al. (1981). We classified sources according to three regions in this diagram (cf. Ojha et al. 2004a). 'F' sources are located between the upper and middle reddening lines and are considered to be either field stars (MS stars, giants) or Class III and Class II sources with small NIR-excesses. 'T' sources are located between the middle and lower reddening lines. These sources are considered to be mostly CTTSs (or Class II objects) with large NIR-excesses. There may be an overlap of Herbig Ae/Be stars in the 'T' region (Hillenbrand et al. 1992).  Fig. 6) indicates that the sources in the NGC 281 region having X-ray emission and lying in the 'F' region above the extension of the intrinsic CTTS locus as well as sources having (J − H) ≥ 0.6 mag and lying to the left of the first (left-most) reddening vector (shown by filled circles) could be WTTSs/Class III sources. Here it is worthwhile to mention that some of the X-ray sources classified as WTTSs/ Class III sources, lying near the middle reddening vector could be CTTSs/ Class II sources. The CTTSs and WTTSs identified in this section are listed in Table 7.

On the basis of MIR data
The NGC 281 region also has MIR observations through the Spitzer Space Telescope towards the south-west direction of the cluster. Since young stars inside cloud clumps are often deeply embedded, these MIR observations can provide a deeper insight into the embedded YSOs. YSOs occupy distinct regions in the IRAC colour plane according to their nature; this makes MIR TCDs a very useful tool for the classification of YSOs.  ). These are Class 0/I sources. Encircled objects represent sources with X-ray emission. A majority of the Class II objects have X-ray emission, whereas none of the Class 0/I sources show xray emission. It is found that four of the probable Class 0/I sources identified on the basis of MIR data lie in the unexposed area of the detector (ACIS-I) of the Chandra telescope, whereas one lies near the edge of detector ACIS-I of the Chandra telescope.
The detection of Class 0/I and Class II sources in all the four IRAC bands is limited mainly by the lower sensitivity of the 5.0 and 8.0 µm channels. Fig. 7 (right panel) shows the IRAC colour-magnitude diagram (CMD) for stars detected in the 3.6 and 4.5 µm bands. Encircled objects represent sources with X-ray emission. Stars having 0.35 ≤ [3.6] − [4.5] ≤ 0.80 mag could be probable Class II stars, whereas stars having [3.6]-[4.5] > 0.80 could be Class 0/I sources. As can be seen, a majority of the Class II sources are X-ray emitting stars, whereas X-ray emission is mostly absent in probable Class 0/I sources. One of the critical astrophysical questions is whether X-ray emission is present in Class 0/I sources at the very onset of star formation when collimated outflows begin (Getman et. al 2007). A few studies report detection of X-ray from Class 0/I protostars, whereas some studies reported that many bona-fide protostars are not detected in X-ray images (cf.  Table 7. To further elucidate the nature of the PMS sources, we derived SEDs for 35 sources (cf. Table 8) using the optical, NIR and MIR photometry. In Fig. 8, we show a sample of three SEDs for three different classes. To classify the evolutionary stage of YSOs using the SEDs, we adopted the classification scheme of Lada et al. (2006), which defines the spectral class index α = dlog λ(F λ )/dlog(λ). We computed the spectral class index α K−8µm , which is the slope of the linear fit to the fluxes at the K s and IRAC 8 µm bands. Objects with are considered as Class I, Flat, Class II and Class III sources respectively. The α indices obtained from the SEDs, in general, confirm the classification obtained from the MIR TCD (cf. Fig. 7 (left)). However, the α K−8µm indices of two sources (Ih and Ij; Class I on the basis of MIR TCD) reveal that these must be Class II sources, whereas two seemingly Class II sources on the basis of the MIR TCD (IIa and IIc) appear to be Class I sources on the basis of the SEDs. The MIR TCD classifies the two sources, namely IIIb and IIIh as Class III sources, whereas α K−8µm indices classify them as Class II objects.  (Fig. 9) reveals that the identified YSOs are probably PMS stars of age 1 Myr. Majority of these stars have masses between 0.5-3.5 M ⊙ .

Optical colour-magnitude diagram
The V /(V − I) CMD for stars lying within the cluster region is shown in the left-hand panel of Fig. 10. A more or less well defined broad MS presumably due to the variable reddening in the cluster region can be noticed down to ∼15 mag. The distribution of stars fainter than V ∼ 15-16 mag deviates towards the red side of the MS indicating the presence of PMS stars in the cluster region. Contamination due to a field star population is also evident in the CMD. To study the LF/MF of the cluster, it is necessary to remove field star contamination from the sample of stars in the cluster region because PMS member stars and dwarf foreground stars both occupy similar positions above the ZAMS in the CMD. In the absence of proper motion data, we used a statistical method to estimate the number of probable member stars in the cluster region. We again utilize the reference region towards the north-west (cf. §4.1.2); it has the same area as that of the cluster region. The middle panel of Fig. 10 shows its V /(V − I) CMD.
To remove contamination due to field stars, we statistically subtracted their contribution from the CMD of the cluster region using the following procedure. The CMDs of the cluster as well as of the reference region were divided into grids of ∆V = 1 mag by ∆(V − I) = 0.4 mag. The number of stars in each grid of the CMDs were counted. After applying the completeness corrections using the CF (cf. Table 2) to both the data samples, the probable number of cluster members in each grid were estimated by subtracting the corrected reference star counts from the corrected counts in the cluster region. The estimated numbers of contaminating field stars were removed from the cluster CMD in the following manner. For a randomly selected star in the CMD of the reference region, the nearest star in the cluster CMD within V ± 0.25 and (V − I) ± 0.125 of the field was removed. Although the statistically cleaned V /(V − I) CMD of the cluster region shown in Fig. 10c clearly shows the presence of PMS stars in the cluster, however the contamination due to field stars at V 17 mag and (V − I) ∼ 1.2 mag can still be seen. This field population could be due to the background population as discussed by Pandey et al. (2006).
The available distance estimates of NGC 281 in the literature varies from 2.0 kpc to 3.7 kpc (cf. Sato et al. 2008). For further analysis we adopt the VLBI trigonometric distance of the maser source of 2.81 ± 0.24 kpc (Sato et al. 2008). Fig. 11 (left panel) shows statistically cleaned dereddened V 0 /(V − I) 0 CMD where stars having spectral type earlier than A0 were individually dereddened (cf. §4.2.1), whereas the mean reddening of the nearby [Vol. , region, estimated from the available individual reddening values in that region, was used for other stars. We have also plotted the ZAMS by Marigo et al. (2008) and the PMS isochrones by Siess et al. (2000) using the distance of 2.81 ± 0.24 kpc. The evolutionary tracks by Siess et al. (2000) for various masses have also been plotted which reveal that majority of the YSOs have masses between 0.5-3.5 M ⊙ . Fig. 11 (left panel) indicates an age spread for the PMS population. To check its reality, we plotted V 0 /(V − I) 0 CMD (assuming a mean E(B − V )=0.4 mag) for the Hα emission stars, NIR-excess stars (probable CTTSs) and X-ray stars (probable WTTSs) (cf. §4.3) in Fig. 11 (right panel). This also indicates an age spread of about 1-5 Myr for these probable PMS stars, supporting the reality of that in Fig. 11 (left panel).
The age and mass of each YSO were estimated by comparing its location with the isochrones. Here we would like to point out that the estimation of the ages and masses  Barentsen at al. (2011) found that uncertainty in the extinction estimation could play a significant role. In the case of NGC 281 region the variable extinction is small (∼ 0.2 mag), hence it should not contribute significantly in the errors. The effect of random error due to photometric error and reddening estimation in determination of ages and masses was estimated by propagating the random errors to their observed estimation by assuming normal error distribution and using the Monte-Carlo simulations. The estimated ages and their error are given in Table 9. The systematic errors could be due to the use of different PMS evolutionary models and the error in distance estimation etc. Barentson et al. (2011) mentioned that the ages may be wrong by a factor of two due to the systematic errors in the model. The presence of binaries may be the another source of error in the age determination. Binarity will brighten the star, consequently the CMD will yield a lower age estimate. In the case of an equal mass binary we expect an error of ∼ 50 -60% in the PMS age estimation. However, it is difficult to estimate the influence of binaries/variables on mean age estimation as the fraction of binaries/variables is not known. In the study of TTSs in the HII region IC 1396, Barentson et al. (2011) presumed that the number of binaries in their sample of TTSs could be very low as close binary lose their disc significantly faster than single stars (cf. Bouwman et al. 2006).
The age distribution of YSOs shown in the Fig. 12 indicates a significant scatter. The inset of Fig. 12 shows the distribution of random errors. A comparison manifests that the age distribution of YSOs shows a significantly larger scatter than that could occur due to random errors. Burningham et al. (2005) have investigated the effect of photometric variability in the apparent age spreads observed in the CMDs of OB associations. They found that the combination of binarity, photometric uncertainty and variability could not explain the observed age spread in the CMDs of OB associations. If the effect of unresolved binaries is not significant, we presume that the main reason of spread in the distribution of YSOs in Fig. 11 could be due to the spread in the ages of YSOs.
The ages of young clusters are usually derived on the dereddened CMDs by comparing the earliest members to post-main-sequence evolutionary tracks if significant evolution has occurred and/or the low mass contracting population to the PMS isochrones. Since the most massive member of the cluster IC 1590 is an O6.5 MS star (Walborn 1973 We consider the points lying above the 5 Myr isochrone in Fig. 11 (left panel) as representing the statistics of the PMS stars in the cluster region. Here, we would like to remind the readers that the filled circles in Fig. 11 may not represent the actual members of the cluster. However, they should represent the statistics of PMS stars in the cluster region and this statistics is used to study its MF only.

Initial Mass Function and K-band luminosity function
The distribution of stellar masses that form in a starformation event in a given volume of space is called initial mass function (IMF). Young clusters are important tools to study the IMF since they are too young to lose a significant number of members either by dynamical or by stellar evolution. The MF is often expressed by a power law, N (log m) ∝ m Γ and the slope of the MF is given as: where N (log m) is the number of stars per unit logarithmic mass interval. The classical value derived by Salpeter (1955) is Γ = −1.35.
With the help of the statistically cleaned CMD, shown in Fig. 11 (left panel), we can derive the MF using the theoretical evolutionary models. Since the age of the massive cluster members is thought to be ∼ 2 -4 Myr, the stars having V 13 mag (V 0 12 mag; M 4M ⊙ ) are considered to be still on the MS. For these stars, the LF was converted to a MF using the theoretical models by Marigo al. (2008) (cf. Pandey et al. 2001Pandey et al. , 2005. The data for the three brightest stars, which were saturated in the present photometry, have been taken from Guetter & Turner (1997). The MF for the PMS stars have been obtained by counting the number of stars in various mass bins (shown as evolutionary tracks) having age ≤ 5 Myr in Fig. 11 (left panel). The resulting MF of the cluster is plotted in Fig. 13. Since data incompleteness plays an important role in the estimation of the IMF, we restrict our analysis only to sources having V < 16.5 mag. The present data have a completeness of ∼95% at V =16.5 mag (cf. Table. 2). The slope (Γ) of the MF in the mass range 2 < M/M ⊙ < 54 comes out to be −1.11 ± 0.15, which seems to be slightly shallower than the Salpeter (1955) value (-1.35). Using various combinations of the maximum expected errors in E(B − V ) and the distance, we found that the slope of the MF can vary in the range of −1.08 ± 0.15 to −1.16 ± 0.15. Keeping the errors in the estimation of Γ in mind, it is difficult to decide whether the present slope 'Γ' is different from that of the Salpeter value. Guetter & Turner (1997) have reported the slope of the MF (Γ = −1.00 ± 0.21) for IC 1590 which, within error, is comparable to the value obtained in the present work.
The . In order to obtain the KLF of IC 1590, we again have to examine the effects of incompleteness and field star contamination in our data. The completeness of the data is estimated using the ADDSTAR routine of DAOPHOT as described in §2.1.1. To take into account the foreground/background field star contamination, we used the Besançon Galactic model of stellar population synthesis (Robin et al. 2003) and predicted the star counts in both the cluster region and in the direction of the reference field. We checked the validity of the simulated model by comparing the model KLF with that of the reference field and found that the two KLFs match rather well (Fig.  14a). An advantage of using the model is that we can separate the foregrounds (d < 2.8 kpc) and the background (d > 2.8 kpc) field stars. The foreground extinction towards the cluster region is found to be A V ∼ 1.0 mag.The model simulations with d < 2.8 kpc and A V = 1.0 give the foreground contamination, and that with d > 2.8 kpc and A V = 1.7 mag the background population. We thus determined the fraction of the contaminating stars (fore-ground+background) over the total model counts. This fraction was used to scale the nearby reference region and subsequently the modified star counts of the reference region were subtracted from the KLF of the cluster to obtain the final corrected KLF. This KLF is expressed by the following power-law: dK is the number of stars per 0.5 magnitude bin and α is the slope of the power law. Fig. 14b shows the KLF for the cluster region. This indicates a slope of α = 0.37 ± 0.07 which is similar to the average slopes

Star formation scenario
The star forming region NGC 281 has been attracting attention of the star forming community. It contains a cluster IC 1590 with a Trapezium like system of Otype stars at the center. The ionized hydrogen seems to be associated with two CO molecular clumps (east and west) which have been mapped in 12 CO and 13 CO by Elmegreen & Lada (1978 Elmegreen & Lada (1978) suggested that this region is a site of triggered star formation through the "collect and collapse" mode. However, Megeath & Wilson (1997) claimed that numerical models of imploding spherical clumps can approximately reproduce the kinematic features observed in NGC 281 West, and suggested that "radiation-driven implosion" (RDI) is a more plausible and attractive model. Between NGC 281 East and the central cluster a few bright-rimmed clouds (BRCs) or cometary globules and an IRAS source are located. Some of the Hα stars can be seen around the tip of the BRCs just as observed in several well-known BRCs (see Ogura et al. 2002). Both the CO clumps show the presence of YSOs (IR-excess and Hα stars) around their respective centers.
Whatever the star formation scenario, the ionization/shock fronts caused by high-mass stars of the first generation appears to have initiated the formation of a new generation of stars at the edge of the molecular clumps. The distribution of YSOs and morphological details of the environment around the cluster can be used to infer the star formation history of the NGC 281 region in detail. To know the distribution of YSOs in the region, we have included all the detected YSOs without considering their photometric errors to improve the sample.
IRAS maps can be used to study the distribution of dust and unidentified infrared band (UIB) carriers. Fig.  15 shows the IRAS intensity maps for the NGC 281 region at 12 µm (top left), 25 µm (top right), 60 µm (bottom [Vol. , left) and 100 µm (bottom right). The global features of these maps are quite similar to that of the molecular gas. All the contours show peaks around both the eastern and western clumps indicating the coexistence of warm dust (IRAS 12 and 25 µm), cold dust (IRAS 60 and 100 µm) and molecular gas. NGC 281 East lacks radio continuum emission and its extended distribution of the 12 µm emission towards the south resembles that of the molecular material. As pointed out by Leisawitz, Bash & Thaddeus (1989), in the case of eastern clump, the peak of ionized gas (as seen from the ionized source) is followed by the peaks of IR emission and CO emission from the molecular cloud, respectively. The coincidence of the peak of the IRAS and CO emission, the location of the IRAS point source and the distribution of YSOs indicate ongoing star formation activity in these clumps.  Chauhan et al. 2009). These alignments in NGC 1893 and BRC 14 were attributed to triggered star formation due to a series of RDI process. In both the cases YSOs located away from the ionization sources are found to be younger. Sicilia-Aguilar et al. (2004) have also shown that in the case of the Tr 37/IC 1396 globule region, CTTSs are aligned from the iosizing source towards the direction of the globule, and that most of the younger (∼1 Myr) members appear to lie near or within the globule. They mentioned that it can be indicative of triggered star formation. Fig. 17 (left panel) shows the age distribution of the YSOs as a function of the radial distance from HD 5005, the ionization source of NGC 281, which shows that the YSOs in the cluster region (r 8 ′ ) have ages 5 Myr whereas those lying outside the cluster region are relatively younger and have ages 2 Myr. Fig. 17 (right panel) shows the radial variation of the NIR-excess ∆(H − K), which is defined as the horizontal displacement from the middle reddening vector at the boundary of the 'F' and 'T' regions (see Fig. 6). To quantify the radial variation of the age and NIR-excess, we used only those stars which have the error ≤ 0.1 mag. The distribution of ∆(H − K) also suggests that the sources lying outside the boundary of the cluster (r ∼ 8 ′ ) have relatively larger NIR excess in comparison to those located within the cluster region. However we admit that the above statements are not conclusive in view of the fact that the scatters are large and the differences are subtle. A deeper optical, NIR and MIR observations are needed to have conclusive star formation scenario in the region. A similar trend has been reported in the case of a few BRCs by Chauhan et al. (2009) as well as in a recent study on the IC 1396 region by Barentsen et al. (2011).
The near-IR excess in the case of CTTSs suggest the presence of dusty optically thick discs (Haisch et al. 2001a, Sicilia-Aguilar et al. 2006). Sicilia-Aguilar et al. (2006) found similarities between the decrease in IR excesses and the decrease in accretion rates, and concluded that gas evolution seems to occur somehow parallel to the evolution of the dust grains and the structure of the disc. In a recent study, Sicilia-Aguilar et al. (2009) further confirmed that IR-excess decreases with age. Hence, relatively large NIR excess outside the cluster region (cf. Fig.  17, right panel) could be indicative of relatively younger population outside the cluster region.
To study the evolutionary stages of the NGC 281 region, we have divided it into three sub-regions as shown in Fig. 16, namely Region 1 (IC 1590), Region 2 (NGC 281 West) and Region 3 (NGC 281 East). Figs 18 and 19 show the V /(V −I) CMDs and NIR TCD, respectively, for the three regions. Here also we have used only sources of good quality, i.e., sources having error less than 0.1 mag. X-ray data are not available for Region 3. The CMDs indicate that the age of the YSOs in Region 1 ranges between 1 -5 Myr, whereas a majority of the YSOs in Region 2 have ages < 1 Myr. The YSOs associated with Region 3 indicate an age of ∼ 1-2 Myr for the region. The NIR TCDs (Fig. 19) indicate that the extinction in Regions 2 and 3 is relatively higher in comparison to Region 1.
A comparison of the statistics of the YSOs in these three regions can also give a clue about the evolutionary stages of the regions. Table 10 shows the statistics of the probable CTTSs associated with the three sub-regions. We have given statistics for the sources having error ≤ 0.1 mag as well as for all the detected YSOs without considering their errors. The total number of stars detected in the 2MASS Catalogue and the number of field stars expected in each region, estimated from the nearby reference region (cf. §4.1.2), are also given in Table 10. The percentage of Hα stars and NIR excess stars in each region is estimated after substracting the contribution of expected field stars (89 stars. column 2 of Table 10) from the total stars (also given in column 2 of Table 10) in the subregions. The fraction of detected CTTSs (i.e. NIR-excess stars and Hα stars), for both the samples, is significantly higher in Regions 2 and 3 as compared to the cluster region (Region 1). This is further supported by a comparison of the CTTSs fractions, f CT T S = N CT T S /(N CT T S + N W T T S ) in Region 1 (5-12 %) and Region 2 (25-45 %) (cf. Table 11). The sources flagged as 1 and 2 in column 14 of Table 7 are classified as CTTSs and WTTSs respectively. The WTTSs are identified using the NIR TCD and X-ray data (cf. §4.3.1). The X-ray data is not available for the Region 3. Here it is important to mention that as mentioned in §4.3.1 some of the CTTSs having less or negligible NIR excess might have been classified as WTTSs. If this is true, the f CT T S will have further higher value. Haisch et al. (2001b) have found the disc evolu-tion fraction in the sense that the fraction of stars having a disc decreases with age. Armitage et al. (2003) have also found that, in Taurus-Auriga T-association, f CT T S , decreases with stellar age. A comparison of the disc fractions of Regions 1 and 2 with those given by Haisch et al. (2001b) and Armitage et al. (2003) suggests ages of ∼ 4-5 Myr and ∼ 1-3 Myr for Regions 1 and 2 respectively, which is in fair agreement with the results obtained from the CMDs. Thus the above discussions suggest the propagation of (triggered) star formation in Regions 2 and 3.
Megeath & Wilson (1997) pointed out the presence of two sub-clusters (northern and southern) in NGC 281 West. They concluded that the northern and southern sub-clusters are indeed physically separate sub-clusters resulting from distinct star formation events. Based on the location of the sub-clusters they concluded that the northern sub-cluster appears to be associated with the NW and NE clumps, whereas the southern sub-cluster seems to be associated with S clump. The spatial distribution of the Class 0/I and Class II sources detected by Spitzer observations overlaid on the 2MASS image is shown in Fig. 20. Certainly the YSOs are found to make two sub-clusters. However, a careful look of this figure manifests a third subclustering also. The isodensity contours shown in Fig. 2 also suggest the presence of three sub-clusters. We have visually marked the boundaries of these sub-clusters, designated as 'a', 'b' and 'c'. Some of the detected YSOs are located outside the boundaries of these sub-clusters. Megeath & Wilson (1997) also found that northern subcluster has higher extinction as compared to the southern sub-cluster. A comparison of the morphology of the molecular cloud as observed in C 18 O (2-1) (Megeath & Wilson 1997) and the distribution of YSOs in the 'a' subcluster region indicates that 'a' sub-cluster seems to be associated with the NW and NE clumps as referred by Megeath & Wilson (1997), whereas 'c' sub-cluster seems to be associated with the clump 'S'. Megeath & Wilson (1997) found that the clump 'S' (-34 to -32 kms −1 ) is kinematically and spatially distinct from the NW and NE clumps (-32 to -29 kms −1 ). The velocity distribution suggests that clump 'S' (i.e. sub-cluster 'c') is relatively near to the observer in comparison to the NW clump 'a'. If we assume that the sub-clusters are not associated with each other, the spatial distribution of the YSOs in the sub-cluster 'a' reveals that the Class II sources are relativley near to the ionizing source in comparison to the Class 0/I sources. We do not find this trend in the subcluster 'b'. The distribution of YSOs in sub-cluster 'c' also show a similar trend however the statistics is poor. The distribution of the YSOs detected by using the IRAC data in the case of a few BRCs also shows that the Class 0/I sources are found to be located more away from the ionizing sources as compared to Class II sources. This seems to indicate the propagation of star formation in the NGC 281 West region.

PMS sources: MIR and X-ray observations
As discussed in §4.3.2 that none of the Class 0/I sources show X-ray emission. The high value of A V in the region could be a possible reason for non-detection of X-ray in the Class 0/I sources. X-ray median energy (MedE) is a reliable indicator of absorbing column density through the empirical relation, log N  2007) is valid for the NGC 281 region also, we estimated the MedE for the Class II/III sources detected in the present study as 4 keV. Using the above relation, the absorbing column densities towards these sources have been estimated as N H 10 23 cm −2 , which yields A V 50 mag according to the standard gas-to-dust ratio by Ryter (1996). Here it is interesting to mention that all the six Class 0/I sources detected in the IC 1396 N region (distance ∼0.75 kpc) by Getman et al. (2007) having A V 50 mag show X-ray activity. Non-detection of X-ray emission in probable Class 0/I sources in the NGC 281 region may be due to the detection limit on account of the larger distance. Deeper exposures are required to reach the conclusions.

Summary and conclusions
In this paper, we present a multiwavelength study of the NGC 281 region using deep wide-field optical U BV I c data, slitless spectroscopy along with the archival data from the surveys such as Chandra, Spitzer, 2MASS, IRAS and NVSS. We made an attempt to construct a scenario of the global star formation in the NGC 281 complex by taking into account the effects of massive stars on lowmass star formation. The main results from this study are as follow: • The morphology of the central cluster IC 1590 is found to be elongated. The extent of the cluster is 6.5 pc and the core radius 1.6 pc. The maximum age of the ionizing source of the region could be ∼ 4 Myr. The minimum reddening E(B − V ) min towards the cluster is estimated to be 0.32 mag and the cluster shows a small amount of differential reddening (E(B − V ) ∼ 0.2 mag). The distribution of the YSOs selected on the basis of NIR-excess, Hα emission and X-ray emission show a spread in the CMD. The age distribution of YSOs indicates a non-coeval star formation in and around the cluster. However we would like to caution the readers that the above statement is not conclusive in view of the scatter in the data. Deeper optical, NIR and MIR observations are needed to have a conclusive view for the star formation scenario in the region. The Class 0/I and Class II sources detected by using the Spitzer MIR observations indicate that a majority of the Class II sources are X-ray emitting stars, whereas X-ray emission is absent in Class 0/I sources. The spatial distribution of Class 0/I and Class II sources reveals the presence of three subclusters in the NGC 281 West region. The distribution of the Class 0/I and Class II sources in the 'a' sub-cluster indicates that the Class II sources tend to be located relatively near to the ionizing source. V range IC 1590 Field region (mag) r ≤ 2 ′ 2 ′ < r ≤ 5 ′ r ≤ 5 ′ 9.5-10.5 1.00 1.00 1.00 10.5-11.5 1.00 1.00 1.00 11.5-12.5 1.00 1.00 1.00 12.5-13.5 1.00  Mean and σ are based on N stars in a V magnitude bin.     Table 7. The YSOs identified on the basis of Hα emission, NIR CCD, MIR CCD, MIR CMD and X-ray emis-sion.
The radial distance is from the cluster center.
The complete table is available in the electronic form only.    Table 11. CTTS fraction f CT T S = N CT T S /(N CT T S + N W T T S ).   sources. The contours are plotted above 3 sigma levels with the step size of 5 stars/pc 2 .

Region
The lowest contour represents 17 stars/pc 2 .
(right panel) Isodensity contours of the distribution of the identified YSOs.    The horizontal continuous line shows the adopted division between Class I and Class I/II sources (see Megeath et al. 2004).
(right) IRAC CMD for sources detected in the 3.6 and 4.5 µm bands only.
Encircled sources represent objects with X-ray emission. 2 ) as given in Table 7.    The error bar on Y-axis represent amount of scatter in each bin estimated on the basis of errors associated with age estimates as given in Table 9 and using Monte Carlo simulations.      Chandra X-ray source median energy. Filled circles and squares represent Class II and Class III sources, respectively.. The dashed lines indicate the boundaries of the distribution of similar objects in the IC 1396N region (Getman et al. 2007).