{'fraction_non_alphanumeric': 0.04281767955801105, 'fraction_numerical': 0.05074149462052922, 'mean_word_length': 4.359174133229451, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 3, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
The first symbiotic stars from the LAMOST survey

Symbiotic stars are interacting binary systems with the longest orbital periods. They are typically formed by a white dwarf, a red giant and a nebula. These objects are natural astrophysical laboratories for studying the evolution of binaries. Current estimates of the population of Milky Way symbiotic stars vary from 3000 up to 400000. However, the current census is less than 300. The Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) survey can obtain hundreds of thousands of stellar spectra per year, providing a good opportunity to search for new symbiotic stars. In this work we detect 4 of such binaries among 4,147,802 spectra released by the LAMOST, of which two are new identifications. The first is LAMOST J12280490-014825.7, considered to be an S-type halo symbiotic star. The second is LAMOST J202629.80+423652.0, a D-type symbiotic star.

T eff ∼ 3000 − 4000 K. They are the most numerous (∼ 80%) and their orbital periods are of the order of a few years. D-types contain Mira-type variables and warm (T ∼ 1000 K) dust shells. Their orbital periods are larger than 15 years (Mikołajewska 2003). D ′ -types contain F-or G-type cool giant stars surrounded by a dust shell with a temperature not higher than 500 K (Allen 1984b). There are only 8 D ′ -types out of 188 symbiotic stars in the Belczyński et al. (2000) catalogue. The presence of both an evolved giant that suffers from heavy mass loss and a hot component copious in ionizing photons results in rich and luminous circumstellar environment. In particular, very different surroundings are expected, including both neutral and ionized regions, dust forming regions, accretion disks, interacting winds and jets, which makes symbiotic stars a natural laboratory for studying the binary star interaction and evolution , and references therein).
Symbiotic stars are proposed as probable progenitors of type Ia supernovae (SNe Ia) (Hachisu et al. 1999;Chen et al. 2011), as rapid mass accretion from the cool giant component may lead the WD to reach the Chandrasekhar mass limit (Han & Podsiadlowski 2004;Wang & Han 2012). SNe Ia have been successfully used as a cosmological distance indicator (Riess et al. 1998;Perlmutter et al. 1999), however, the exact nature of the progenitors of SNe Ia is still not clear. SNe Ia PIF 11kx, with a broad Hα line in its late spectrum, may result from a symbiotic star (Dilday et al. 2012). Large numbers of symbiotic stars can help us to study the progenitors of SNe Ia. Hence, searching for new symbiotic stars is of extreme importance.
Current estimates for the total Galactic symbiotic population vary considerably from 3000 (Allen 1984b), 30000 (Kenyon et al. 1993) to 3-4×10 5 (Magrini et al. 2003). However, there are less than 300 Galactic symbiotic stars known (Belczyński et al. 2000;Miszalski et al. 2013;Miszalski & Mikołajewska 2014;Rodríguez-Flores et al. 2014). This incompleteness is due to the lack of deep all-sky surveys at wavelengths where symbiotic stars can be efficiently selected against other stellar sources (Belczyński et al. 2000;Miszalski & Mikołajewska 2014;Mikołajewska et al. 2014). Most of the known symbiotic stars have been discovered by large Hα surveys, e.g. the INT Photometric Hα Survey and the AAO/UKST SuperCOSMOS Hα Survey, and then confirmed by the deep spectroscopy and long-term I-band light curves of the candidates. The Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) survey can obtain hundreds of thousands of stellar spectra per year, providing a good opportunity to find new symbiotic stars. Since no effort has been made from the large spectroscopic observation survey, we devise a selection method to find out symbiotic stars from the data released by LAMOST (including DR1, DR2 and DR3). As a result, we detect 4 symbiotic star spectra, of which two are new identifications, the other two have been previously identified by Downes & Keyes (1988).
The structure of the paper is as follows. Section 2 briefly introduces the LAMOST survey and our method to select symbiotic stars. Section 3 describes the 4 symbiotic stars we identify. Section 4 is the discussion. We conclude in Section 5.

The LAMOST Survey
LAMOST is a quasi-meridian reflecting Schmidt 4.0 m telescope located in Xinglong Station of National LAMOST telescope dedicates for spectral survey of celestial objects in the whole northern sky (Cui et al. 2012;Zhao et al. 2012).
The current data of LAMOST contain data release 1 (DR1), data release 2 (DR2) and data release 3 (DR3). The number of DR1, DR2 and DR3 spectra are 2204860, 1443843 and 499099, respectively (see Table 1). The spectra are classified as stars, galaxies, quasars and unknowns (mainly due to the poor quality of their spectra). DR1 includes both 1,487,200 spectra obtained during the first year of the general survey, and 717,660 spectra which were observed during the pilot survey ). Due to the lack of network of photometric standard stars, the flux calibration is relative. The LAMOST spectra are unambiguously identified by a plate identifier, a spectrograph identifier and a fiber identifier (Song et al. 2012;Ren et al. 2014).
Based on the LAMOST data, lots of works have been done.

Optical spectra of symbiotic stars
Optical spectra of known symbiotic stars have been catalogued by Allen (1984b) and Munari & Zwitter (2002 and a factor of ∼ 4 weaker but similar emission feature at λ7088Å, which only exists in symbiotic stars. The λ6830Å band profile has a typical width of about 20Å and it often ranks among the 10 most intense lines in the optical region of symbiotic stars (which may reach 5% of the intensity of Hα  (Allen 1984a). The black and red lines are the spectra (continuum plus lines) and continua of the objects, respectively. We have normalized the flux into the range of [0, 1] following Eq. (2).
In S-type stars (e.g. V2416 Sgr in Figure 1 D ′ -type stars (see Figure 6; note that this is one of our 4 identified LAMOST symbiotic binaries) have similar emission lines as for D-types. The most conspicuous property of D ′ -type stars is the presence of a G-or F-type giant instead of a cooler star (Mürset & Schmid 1999).
According to above described features, we use the following criteria to classify an object as a symbiotic star (Mikolajewska et al. 1997;Mikołajewska et al. 2014;Belczyński et al. 2000):

Selection Method
Based on the spectral properties of symbiotic stars and the feature of LAMOST data, we devise a selection method to find symbiotic stars. This follows a three-step procedure.
(i) In order to directly compare the spectra, we first normalize the flux into the range of [0,1] following where F max , F min are the maximum and minimum fluxes of each spectrum, respectively.
(ii) Unlike the vast majority of spectra observed by LAMOST, symbiotic stars display Balmer emission lines, especially the Hα and the Hβ lines. We therefore select symbiotic stars by analyzing the intensity of the Hα and Hβ lines, i.e. I(Hα) and I(Hβ). These intensities are obtained from the following equation, where F c is the continuum fitted by the Global Continuum Fit method of Lee et al. (2008)  It is important to emphasize that fitting the continuum of M-type or later type spectra is subject to systematic uncertainties at the λ5500 − 7500Å range, as shown in Figure 2. In order to efficiently select symbiotic stars, it becomes important to set proper values for the intensity of the Hα and Hβ lines. In our work, they were set to I(Hα) > 0.5 and I(Hβ) > 0.2, which efficiently exclude spectra displaying the Notes: (1) Total stands for the number of the spectra we have used in each data release of the LAMOST. (2) Star, M-type columns stand for stars and M type spectra, respectively. (3) Step(ii) and Step(iii) columns stand for the selected out spectra by step(ii), step(iii) in Section 2.3. Notes: Basic properties of the symbiotic stars, which are ordered by right ascension (R. A.) of the epoch J2000.0.
The first and third symbiotic stars have been indentified by Downes & Keyes (1988), the others are new symbiotic stars.

(iii)
The Balmer series and other emission lines in symbiotic stars are very prominent, hence the continuum is generally flat in the optical spectrum (see Figure 1). We therefore average the normalized continuum F n in the 4000 − 7300Å wavelength range and exclude all spectra with the average value being above 0.4.
After performing the above steps over the entire LAMOST spectroscopic database, we select 4758, 7631 and 2377 spectra in DR1, DR2, and DR3, respectively, as symbiotic star candidates (see Table 2). Visual inspection of the spectra revealed that most of these were planetary nebula and B[e] stars. Finally, we find four symbiotic stars, two of which have been catalogued in the past, the other two are new identifications (see Table 3). Figures 3-6 show the LAMOST spectra of the 4 symbiotic stars we have identified.

LAMOST J122804.90-014825.7
The presence of strong HeII λ4686 emission implies the presence of a hot component in the object (Figure 3). The spectrum is with a radial velocity of v r ∼ (374 ± 2) km/s. And we can see the faint [FeVII] emission lines with the same Doppler shift in the inset of Figure 3. The spectrum displays also Mgλ5176 and CaII triplet absorption lines. It is classified as K3 type by the LAMSOT pipeline, which stands for the cool giant component of a symbiotic star. So we propose it is an S-type system.
Based on the g, r and i magnitudes, the V magnitude is m V ∼ 13.5 mag (http://classic.sdss.org/dr4/algorithms/sdssUBVRITransform.html), and we assume a typical absolute be about 5 kpc. The Galactic coordinates are l = 291.0062, b = 60.5226, so the vertical distance between the object and the Galactic disk is about 4.3 kpc. It seems to be a halo symbiotic star.
The other Balmer lines, as well as HeI and HeIIλ4686 lines are also clearly present. At the same time there is a faint feature at λλ6830 as shown in Figure 4. The spectrum displays obvious TiO absorption bands and CaII triplet absorption lines, and the star is classified as M2 by the LAMOST pipeline. It is an S-type symbiotic star.

LAMOST J20262980+4236520
The spectrum of this object is shown in Figure 5.  Fig. 4 LAMOST J1194957.58+461520.5 is a S-type symbiotic star, which has been observed by Downes & Keyes (1988), and listed as StHA 169 in Belczyński et al. (2000) catalogue of symbiotic stars. F λ is relative flux. LAMOST J214144.88+024354.4 is a D ′ -type symbiotic star. It is included in the catalogue by Belczyński et al. (2000) as StHA190. The spectrum, shown in Figure 6, is nearly identical to the one presented by Downes & Keyes (1988) in the 4000 − 7300Å wavelength range. The spectrum is classified as G5 type by the LAMOST pipeline, which is consistent with the D ′ -type nature of this symbiotic binary.
High-resolution optical spectra of StHA 190 were analyzed by Smith et al. (2001), who derived the giant temperature, T eff = 5300 ± 150 K and [Fe/H] ∼ 0 based on fitting the spectrum with model atmospheres.  Fig. 6 LAMOST J214144.88+024354.4 is a D ′ -type symbiotic star, included in the Belczyński et al. (2000) list as StHA 190. F λ is relative flux.

DISCUSSION
We used 4,147,802 LAMOST spectra to find symbiotic stars, of which 3,622,479 are classified as stars by the LAMOST pipeline (see Table 2). Since about 1% of the stars are red giants in our solar neighborhood (http://pages.uoregon.edu/jimbrau/astr122/Notes/Chapter17.html), a rough estimate of the number of the red giant stars in these spectra is around 30,000. Magrini et al. (2003) proposed that the number of symbiotic stars can be taken to be 0.5% of the total number of red giants. Therefore there should be more than 100 symbiotic stars to be selected out. But we should note that the estimated number of Galactic symbiotic stars from Magrini et al. (2003) is 1-2 orders of magnitude higher than that given by others as shown in Section 1.
So the real symbiotic stars among the LAMOST stellar spectra may be less than 100. As the first step, we only identified 4 symbiotic binaries, far below what we expected. By comparing the coordinates between the known symbiotic stars listed in the catalogue of Belczyński et al. (2000) and the footprint of LAMOST DR1, there should be 9 known symbiotic stars in the area of the footprint (see Table 4 ). According to the plate design of the LAMOST, for brighter stars (r ≤ 14 mag), only those with 9 ≤ J ≤ 12.5 mag from the 2MASS catalogs are chosen as potential targets (Luo et al. 2015). It immediately rules out 8 of the 9 symbiotic stars. For the fainter stars (r > 14 mag), there are 5 conditions to select potential targets (Yuan et al. 2015), which might exclude the symbiotic star Draco C-1. The StHA 190 has not been excluded, probably because its J-band magnitude is very close to the target selection condition of brighter stars. In the future work, we will apply the selection method to the SDSS (the Sloan Digital Sky Survey) released data to find more symbiotic stars and check its efficiency.

CONCLUSION
We have found 4 symbiotic stars among 4,147,802 spectra released by the LAMOST DR1, DR2 and DR3.
The objects LAMOST J194957.58+461520.5 and LAMOST J214144.88+024354.4 are included in the can be selected out by our method. The LAMOST spectra of these two objects are nearly identical to those provided by Downes & Keyes (1988). LAMOST J12280490-014825.7 is a new symbiotic star and is considered to be a Halo S-type with a radial velocity of v r ∼ (374 ± 2) km/s. LAMOST J20262980+4236520 is also a new symbiotic star classified as D-type. With the operating of the LAMOST, more and more symbiotic star spectra will be obtained.