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Spectroscopy of Very Low Mass Stars and Brown Dwarfs in IC2391: Lithium depletion and Halpha emission

We have obtained intermediate-resolution optical spectroscopy of 44 candidate very low mass members of the nearby young open cluster IC2391. Of these, 26 spectra are totally new, 14 were already analyzed in a previous paper and another four are in common. These spectra, taken at the Cerro Tololo 4-meter and Magellan I and II telescopes, allow us to confirm 33 of them as likely cluster members, based on their spectral types, presence of Li, and Halpha emission. Among these new cluster members is CTIO-160 (M7), the first IC2391 candidate to satisfy all criteria for being a substellar member of the cluster, including detection of the Li 6708A doublet. With the enlarged membership, we are able to locate the lithium depletion boundary of the cluster more reliably than in the past. Based on comparison to several theoretical models, we derive an age of 50+-5 Myr for IC2391. We also estimate new ages for the Alpha Per and Pleiades clusters; our ages are 85+-10 Myr and 130+-20 Myr, respectively. We derive an estimate of the initial mass function of IC2391 that extends to below the substellar limit, and compare it to those of other well-studied young open clusters. The index of the power law mass function for IC2391 is alpha=0.96+-0.12, valid in the range 0.5 to 0.072 M(sun).

on comparison to several theoretical models, we derive an age of 50±5 Myr for IC 2391. We also estimate new ages for the Alpha Per and Pleiades clusters; our ages are 85±10 Myr and 130±20 Myr, respectively. We derive an estimate of the initial mass function of IC 2391 that extends to below the substellar limit, and compare it to those of other well-studied young open clusters. The index of the power law mass function for IC 2391 is α=0.96±0.12, valid in the range 0.5 to 0.072 M ⊙ .

Introduction
Hundreds of open clusters 1 are known in the Galaxy. However, few among them are well characterized (size, distance and reddening) and fewer still are close enough to allow their stellar population to be investigated in detail. Only the Pleiades, the Hyades, Alpha Per and a few other clusters have been systematically studied. Our goal is to add other clusters to this list, and IC 2391 has been the focus of some of our efforts.
IC 2391 is a young cluster with an estimated age, based on Main Sequence isochrone fitting, of 35 Myr (Mermilliod 1981). It is one of the nearest clusters, with a Hipparcos distance modulus of (m−M) o =5.82±0.07 (Robichon et al. 1999). The interstellar reddening in its direction is very low, E(B − V )=0.04 or 0.06, as estimated by Becker & Fenkart (1974) and Patten & Simon (1996), respectively. This last work also estimated a distance of (m − M) o =5.95±0.10, which is the value we will use here.
In Barrado y Navascués, Stauffer and Patten (1999, hereafter Paper I), we presented some spectra of very low mass members of the cluster and a preliminary age estimate, 53 Myr, based on the lithium depletion technique (see Stauffer et al. 1998;Basri, Marcy & Graham 1996). Subsequently, in Barrado y Navascués et al. (2001a, hereafter Paper II), we conducted an extensive photometric survey in the optical which yielded a substantial population of candidate members, both very low mass stars and brown dwarfs. The combination of this database with infrared photometry from 2MASS (Skrutskie et al. 1997, see also Cutri et al. 2003) allowed us to extract from the initial sample those objects which might be interlopers, based on the analysis of different color-magnitude and color-color diagrams. In the introduction to Paper II, we described the results achieved by previous studies, both those papers that looked for new cluster members, more massive than those presented in Paper II, and those papers that studied the properties of true cluster members (X-ray emission, rotation, lithium abundance and so on). Since then, the only other works that have been published dealing with properties of members of this cluster are Randich et al. (2001) -spectra-and Allen et al. (2003 -the luminosity function and the age. Here, we present medium resolution optical spectra of a large number of candidate members discovered in Paper II and consider the membership of those candidates by studying their spectral types, Hα emission, sodium and lithium content; derive again, in a more accurate way thanks to the larger number of objects, the location of the lithium depletion boundary of the cluster and hence its age; and study the Initial Mass Function (IMF).

Sample.
The spectra presented in this paper correspond to IC 2391 candidate members discovered by two different groups. The first (generally brighter) set was selected from Patten & Simon (1996) and Patten & Pavlovsky (1999). The second group was extracted from Paper II, where we presented a large sample of low mass stars and brown dwarf candidates in the range 12 < I C < 21, discovered in a deep optical and infrared photometric search. An initial subset of this survey was previously published in Paper I, and it includes medium resolution spectroscopy, allowing the confirmation of the membership of most of them. These spectra were obtained in January 1999 with the Ritchey-Chrétien spectrograph at the Cerro Tololo Interamerican Observatory (CTIO) 4 m telescope and have a spectral resolution equivalent to 2.7Å (see Paper I for details).
In the present study, we have carried out spectroscopic observations at medium resolution for a total of 44 candidate members. Of these, 26 have been observed for the first time, while 18 were already analyzed in Paper I. Four out of these 18 have been reobserved. We especially selected those objects located around the lithium depletion boundary in a colormagnitude diagram (generally very cool stars with masses slightly larger than the substellar limit at 0.072 M ⊙ ). Figure 1 displays the I magnitude versus the (R−I) color in the Cousins system. Proposed IC 2391 members are included as crosses. Open circles represent those candidates with spectra (in some cases at quite low signal-to-noise) from Paper I, whereas candidates with new spectroscopic data are illustrated with solid circles in the diagram. An empirical ZAMS from Barrado y Navascués et al. (2001b) is displayed as a solid line, as well as several 50 Myr isochrones (Baraffe et al. 1998;Siess et al. 2000;D'Antona & Mazzitelli 1997;long-dashed, dotted and short dashed, respectively). The location of the lithium depletion boundary (LDB) estimated in Paper I is also included. Due to the lack of space and since the main goal of Paper I was to establish the location of the LDB of the cluster, we did not publish all the spetra of that sample. We now present them in Figure 2. Table 1 lists all the targets observed both in Paper I -except the three non-members analyzed there-and in this paper, and includes accurate position (from the 2MASS survey, Cutri et al. 2003), optical as well as near infrared photometry, and information regarding how the spectra were collected (telescope, instrument and date).

Multifiber spectroscopy from CTIO.
On 1999 March 10th and 13th, we collected multifiber medium resolution spectroscopy with the Hydra II bench spectrograph at the CTIO 4m telescope under a shared risk program. The grating KPGL-D was used with the OG-570 filter. The achieved resolution was 2.7Å as measured in ThAr comparison spectra observed with the same set-up, and covering the spectral range 6300-8500Å.
The total exposure time was 11.5 hours, but we divided this time into 12 individual observations of 1 hour each (except the last one, which lasted only 30 minutes) during these two nights. Each exposure was processed individually with the IRAF 2 package "hydra". We used dome flats. After the extraction of each 1-D spectrum and its calibration in wavelength, we combined all the data corresponding to the same target into a final spectrum using a median algorithm in order to remove the hits by cosmic rays. No correction for telluric lines or instrumental profile was carried out. The final spectra can be seen in Figure 3.

Spectroscopy from Magellan I & II.
During two different observing runs mainly devoted to other goals, we were able to complement the previous data with additional spectra collected at the Magellan 6.5m twin telescopes, located at Las Campanas Observatory. The first campaign took place on 2002 December 11-14th. We used Magellan I and the MIKE echelle spectrograph. MIKE provides spectral resolution that is actually too high for our purposes. For this reason and in order to improve the final signal-to-noise ratio, we degraded the resolution by rebinning the original data during the read-out to 2 and 8 pixels in the spatial and spectral directions, respectively, thus obtaining spectra with a resolution of 0.55Å and complete spectral coverage of 4500-7250Å. Four IC 2391 candidate member were observed with MIKE. We show the Hα order from these spectra in Figure 4a. The second run took place on 2003 March 11th, and five additional candidates were observed, in this case with Magellan II and the B&C spectrograph, using the 1200 l/mm grating, yielding a 2.3Å resolution. These last spectra can be seen in Figure 4b (probable non-members are not included in the diagram). In most cases, we took three individual exposures of 1200 seconds each, fully reduced each exposure separately, and then added together the three resultant 1D spectra at the end. Additional details can be found in Barrado y Navascués et al. (2004). Table 2 lists our results, including the derived spectral type, the measured Hα, sodium and lithium equivalent widths at 6563, 8200 and 6708Å, respectively, and the estimated effective temperature and lithium abundance. We used calibrations by Bessell (1979), Leggett (1992) and Basri et al. (2000), and the (R − I) C color and the spectral types for the T ef f determination, and curves of growth by Zapatero  for the lithium abundance. In the subsequent section we analyze and discuss these results.

Spectral types.
For most of our targets, spectral types were derived by comparison with spectral standards and/or other cluster members whose spectral types were known, in a similar manner to Kirkpatrick et al. (1999) and Martín et al. (1999), using several spectral indices defined in the red side of the optical spectrum. These standards, whose spectral types go from K7 to M8, were observed with the same spectral set-up. In the case of the echelle spectra collected with the MIKE spectrograph, we estimated the spectral types using the TiO band-head which starts at 7053Å. The depth of this feature is very sensitive to the effective temperature and it is an excellent spectral type indicator. In this case, in addition to our IC 2391 candidates, we observed standards of almost every spectral sub-class in the vicinity of M5, with half a sub-class steps. A visual inspection, comparing IC 2391 members with the standard stars and verifying the spectral classification by direct comparison among the IC 2391 objects themselves, was also carried out. We believe that the uncertainty of the IC 2391 spectral types we have derived is of order half a subclass.
Five of our fainter targets (I C ∼20 mag), observed with HYDRAII, have spectra with a very low signal-to-noise and we have not attempted spectral classification.
The spectral type of several of our brighter candidates is in strong disagreement with membership in the cluster, since they do not correspond to the value expected from the optical and infrared colors. This is the case of two stars -CTIO-002 and CTIO-067-of K spectral type, probable background giants. These two stars, as well as the fainter objects, have no detectable Hα in emission or the line is seen in absorption (see next subsection).

Hα emission.
The strength of the Hα emission can be used as a criterion to establish the membership of a cluster candidate. As stated before, two objects of K spectral type, warmer than the expected values from their colors, lack Hα emission and, therefore, they can be classified as likely non-members. The same argument is valid for three out of the five faint objects located at the end of the cluster sequence (I C ∼20), since we would have expected, at least, some emission. Note, however, that this criterion is a statistical one, and membership cannot be completely ruled out for these three objects or confirmed for the other two. In any case, the data suggest that there is a strong pollution rate for this range, about 60%.
The comparison of the Hα distribution between clusters of different ages is displayed in Figure 5. For clarity, we have only included the cluster associated to the multiple star σ Orionis (Wolk 1996) and the Alpha Per cluster. The data come from Béjar et al. (1999); Barrado y Navascués et al. (2001c; Zapatero , Prosser (1992Prosser ( , 1994; Prosser & Randich (1998);Stauffer et al. (1999). We note that the σ Orionis cluster, with an age close to 5 Myr, has a significant number of stellar and substellar members with Hα far beyond the limits of the figure and might contain about 20 % of classical TTauri stars and substellar analogs Jayawardhana et al. 2003), which are characterized, among other things, by strong, asymmetric and broad Hα emission lines. We find that the distribution of Hα emission in IC 2391 is very similar to that for the somewhat older Alpha Persei cluster.
Only one star belonging to IC 2391 stands out in the figure, CTIO-059. This object is shown with the average value corresponding to the two observation we carried out in January 1999 at CTIO, but in fact it was observed in two consecutive nights, and showed very different values (49.5 and 18.8Å, see Paper I). Therefore, it seems that we detected a flare in that cluster member at that time. Figure 6 displays the Hα equivalent width versus time. There might be some variability on short time scales, within the same night. This variability could be related with rotation (see, for instance, possible modulation due to rotation in CTIO-038 or CTIO-074). Variability on a longer time scale might be present too, but the dataset is too sparse and we cannot confirm this at the present time. In any case, it would be very interesting to compare this information with light curves derived for the same objects.

Sodium doublet at 8200Å.
We have also measured the equivalent widths of the sodium doublet at 8200Å. The strength of this alkali feature is sensitive to gravity (see, for instance, Martín et al. 1996). Since IC 2391 members are much younger than field objects of similar spectral types, and should have larger radii, it is possible to use this characteristic as a youth indicator. All our targets but one (CTIO-046) have W(Nai) in agreement with a young age and, therefore, with membership. The large equivalent width measured in CTIO-046 indicates that it is a more evolved object, which confirms our other indications that this star is probably not a member of the cluster.

Lithium at 6707Å.
Figure 7 and 8 display the area around the Lii 6708Å for all our spectra. The vertical dashed line indicate the location of this feature. In some cases, the signal-to-noise is indeed good, and lithium is unambiguously detected; see, for example, the cases of CTIO-145 and 038 (January 1999), CTIO-081 (March 1999), CTIO-195, 192 and 026 (December 2002) and CTIO-160 (March 2003). Other cases, such as the two spectra of CTIO-077 (January and March 1999), are less certain. In any case, the new data allow us a significant improvement in the determination of the location of the LDB (Section 3.2).

Confirmed members at the substellar limit.
So far, including the data published in Paper I, we have collected medium resolution spectroscopy for 47 candidate members (out of the 206 identified in Paper II and a handful of brighter objects). The membership of another three (CTIO-040,  was rejected in Paper I. Our targets were selected from those classified as probable and possible members in Paper II based on their optical and infrared photometry (132 objects in total). In that study, we estimated the pollution rate for that subsample -candidates identified for the first time in Paper II-as 25%. Out of the 47 spectroscopically observed candidates (including some VXRP and PP objects), nine have been classified as non-members (the membership of three out of this nine, namely CTIO-040, CTIO-094 and VXR 27, was rejected in Paper I) and another five as possible members (four from the CTIO sample). Of them, 38 objects (the CTIO-xxx) were discovered and presented in Paper II and another six objects -brighter-come from previous studies (from Simon 1996 andPavlovsky 1999). Therefore, taking into account these 38 CTIO objects -including 4 possible members in the CTIO sample-, and CTIO-040 and CTIO-094 -the non-members discussed in Paper I, the pollution rate is in the range 20-30 % for the CTIO sample, depending on how the possible members are counted.
3.2. Lithium depletion boundary and cluster age.

Lithium equivalent width versus magnitudes and colors.
The initial lithium depletion boundary for IC 2391 was located at I C =16.2±0.2 and (R − I) C =1.91 in Paper I. Figures 7 and 8 contain the area around the Lii6708Å doublet for the bona fide members. The initial estimate of the LDB location is confirmed, but there are two stars -PP07 and CTIO-206-that show lithium in their spectra despite the fact they are brighter than the LDB. Figures 9a and 9b display color-magnitude diagrams using optical and infrared data. We have used I C from Paper II in the y-axis for the first case and Ks from 2MASS in the second. In both cases these two stars are clearly above the LDB. Actually, CTIO-206, the fainter of these two, might be a binary composed of two very low mass stars of almost the same mass, which would solve the puzzle. However, the other star, PP07, is well above the LDB, by about 2 magnitudes. In both cases, the Nai(8200) equivalent width is in agreement with a young object.
A similar situation has been discovered in the Pleiades cluster by Oppenheimer et al. (1997), and their interpretation was that the two supposed Pleiades members (HHJ339=HCG332 and HHJ409=HCG509) are, in fact, young interlopers in the line of sight. Recently, Deacon & Hambly (2003) have derived membership probabilities for them. Although they are very low (P=0.155 and P=0.284, respectively), these values are not conclusive, and membership cannot be completely excluded.
The alternative would be that there is a mechanism which can prevent lithium depletion during the pre-main sequence phase in M dwarfs. Note that some warmer Alpha Per and Pleiades members (K spectral type) may inhibit lithium depletion using a mechanism related to rotation or magnetic field strength (see, for instance, Soderblom et al. 1993;García López, Rebolo & Martín 1994;Randich et al. 1998;D'Antona et al. 1998). However, our medium resolution spectrum of PP07 does not seem to be broader than the rest (although it would have been very difficult to detect anything with a projected rotational velocity of less than ∼100 km/s), and its activity in Hα is average compared with other cluster members of the same color or spectral type.
Another, more speculative, possibility is that these two objects are, in fact, bona-fide very low mass members of the cluster, which have recently swallowed a companion (a brown dwarf). The sudden additional mass accretion could explain their location in these colormagnitudes diagrams and the strong lithium feature in the spectrum. This mechanism has been invoked to explain the tendency among planet-harboring stars to be metal rich Gonzalez et al. 2001). Possible evidence of planetary engulfment has been presented by Israelian et al. (2001;, although it has been called into question by others (Reddy et al. 2002).
In any event, Figure 9 clearly shows the lithium chasm (i.e., the lack of lithium for late K and early M in the cluster). Figure 10 helps to determine with greater degree of accuracy the location of the chasm and, therefore, its cool border, the LDB. Objects with lithium are displayed with solid symbols (circles for our data, squares for data from Randich et al. 2001), whereas open symbols represent objects without it (triangles for our upper limits). The LDB keeps its location at (R − I) LDB C ∼1.9. Our first detection of lithium is located at I C =16.286, whereas the last star without it has a magnitude of I C =16.144 (Ks=13.394 and 13.587, respectively, if 2MASS data are used instead). Since the adopted distance and reddening are (m − M) 0 =5.95±0.10 and E(B − V )=0.06, equivalent to A I =0.112 and A K =0.021 (Rieke & Lebofsky 1985), these values yield M(I C ) LDB = 10.15 and M(Ks) LDB = 7.52. The distance yielded by Hipparcos would locate the LDB 0.13 mag fainter than these values.

Lithium depletion and a new lithium depletion boundary age estimate.
The evolution of the lithium depletion boundary and the lithium chasm with age, from the empirical point of view, is illustrated in Figure 11, where we show a comparison with the three clusters where this type of data are available (IC 2391, Alpha Per and the Pleiades). Additional information can be found in Rebolo et al. (1996); Stauffer et al. (1998Stauffer et al. ( , 1999; Martín et al. (1998); . Note that a detection of the LDB has been attempted in a fourth cluster, NGC2547, by Oliveira et al. (2003, see also Jeffries et al. 2003), but they were not able to detect it unambiguously.
We have already determined the absolute magnitudes in the I C and Ks filters where the LDB appears in IC 2391. Using bolometric corrections of BC I =0.070 (Bessell 1991;Comerón et al. 2000) and BC K =2.756 (Tinney et al. 1993;Leggett et al. 1996) In all cases, an error of 0.15 mag has been estimated for the location of the LDB boundary, taking into account distances, reddening, and the gap between Li detection and non-detection.
With these values and the predictions of theoretical models, it is possible to estimate the age of each cluster. Figure 12 displays the absolute magnitudes of an object whose lithium has been depleted almost completely (one percent of the original lithium abundance) versus the time. The models correspond to Baraffe (priv.comm That is, we estimate the plausible ages for these three clusters as 50±5, 85±10 and 130±25 Myr, respectively. Note, however, that Jeffries & Naylor (2001) have reevaluated the error budget for them, both experimental and systematic, and estimated that the errors can be significantly larger. As an example, a different I-band bolometric correction (Monet et al. 1992, using  Burke, Pinsonneault & Sills (2004) have also revaluated the the ages for these three clusters plus NGC2547, and examined the errors in the analysis. Their values are 55±6, 101±12 and 148±19 Myr for IC 2391, Alpha Per and the Pleiades (48, 87 and 126 Myr when introducing an ad hoc offset to the I-band bolometric correction). Within the error bars, all these results agree with each other.
As stated in Stauffer et al. (1998;, Barrado y Navascués, , and Paper I -see a summary in Barrado y Navascués, Stauffer & Bouvier (1999)these ages are about a 50% older than the values obtained by fitting isochrones to the upper part of the Main Sequence. Recently, Allen et al. (2003) have derived an age for IC 2391 based on the luminosity function (LF) of the cluster. They obtained a value, 35 Myr, lower than the LDB age and identical to the main-sequence turn-off age (Mermilliod 1981). By fitting the data published in Paper II with models of the LF, they argue that LF has a peak at M(I)=14-15 mag, which should be produced by the deuterium burning, and that the age cannot be ∼50 Myr. Their result supports a recent claim by Song, Bessell and Zuckerman (2002), who state that the LDB age might overestimate the real age for young clusters. However, the analysis by Allen et al. (2003) was carried out prior to our new spectroscopic data, which indicate that the pollution rate in that range (I C ∼20) is very large (∼60 %). The alleged LF peak is not obvious at all now. Additionally, Dobbie et al. (2002) have pointed out that a drop in the LF exists around M7-M8 (Teff∼2500 K), which might be due to dust formation in the atmospheres of these objects. For IC 2391, this happens at about I C ∼19.5 mag.
For these three clusters, the lithium depletion boundaries are located at the spectral types of M5, M6.5 and M6.5 or, in effective temperatures, 3050, 2800 and 2650 K, for IC 2391, Alpha Per and the Pleiades, respectively. When expressed in mass, using models from Baraffe et al. (1998), they take place at 0.12, 0.085 and 0.075 M ⊙ , repectively. All the LDB data are summarized in Table 3.

The first confirmed brown dwarf in the cluster.
So far, although a large number of candidate brown dwarfs were presented in Paper II, none of these candidates were established via spectroscopy. This confirmation implies that: (a) it is a member of the cluster based on all available criteria; (b) it has a bolometric magnitude that would place it below the substellar mass limit if it were a member of the cluster; and (c) there is a detection of the lithium 6708A doublet.
Using our new age estimate and models by Baraffe et al. (2002), the interstellar red-dening and the cluster distance, the substellar limit is located at I C =17.06. Several of the targets in this sample are fainter than this value, and their spectroscopic properties agree with membership. However, their spectra are not good enough to have a lithium detection beyond a doubt, which would confirm the substellar nature. Only in one case, CTIO-160 (whose spectral type is M7), is lithium clearly identified and its nature firmly established, making this object the first brown dwarf unambiguously identified in the IC 2391 cluster. Note, however, that errorbars in the object photometry and the uncertainties in the models are large enough to change the classification of this object.

Mass Function.
We have derived a mass function for IC 2391 using non-dusty models from Baraffe et al. (1998) and the Ic magnitudes. Figure 13 depicts our results. We have assumed different ages, ranging from 25 to 50 Myr (these values are close to the turn-off and the LDB ages, respectively). In any case, when expressed as a power law, the index (α=0.96±0.12) does not depend on the age in this interval (i.e., the derived value is very similar when using these three ages). The MF is valid between a mass of 0.5 M ⊙ and the substellar limit. Below 0.072 M ⊙ , there is a sudden drop, which might be partially explained by the lack of survey completeness beyond I C ∼18.5 for cluster members (0.050 M ⊙ for 50 Myr isochrone from Baraffe et al. 1998). However, we have detected a significant number of candidate members with magnitudes around I C ∼20. Our medium resolution spectroscopy indicates that, despite the strong pollution in this range, about 60 %, some seem to be members of the cluster. Therefore, the gap at I C ∼19, mass∼0.05 M ⊙ might be real. Dobbie et al. (2002) and Jameson et al. (2003) have pointed out that several young clusters show a lack of substellar members of M7-M8 spectral type, more or less in the same location as in the case of IC 2391 (see the case of Alpha Per in the same diagram). They explain this fact as an effect of dust formation at this spectral type and its effect in the luminosity function as a new source of opacity, which would decrease the overall luminosity for the cooler objects. In any event, the number of objects discovered so far in the IC 2391 cluster at the low end of the cluster sequence is too low to confirm this possibility. Figure 13 also contains a comparison with several Mass Functions corresponding to young clusters of different ages, such as Alpha Per, the Pleiades, and M35 all of them derived in the same manner (see Bouvier et al. 1998;Barrado y Navascués et al. 2001b;Barrado y Navascués 2003). The index of the Mass Function power law is very similar in all cases, except in the case of the low mass stellar members of M35, a very rich cluster, where some mass segregation might have taken place due to its older age (175 Myr in the turn-off age scale, Barrado y Navascués et al. 2001d).

Conclusions and Summary
By collecting medium resolution spectroscopy for a significant fraction of IC 2391 candidate members discovered in Paper II, we have established the membership for most of them via their spectral types and Hα emission properties, including dependence with spectral type and variability in a short time scale.
In addition, we have studied the presence of the lithium doublet at 6708Å, located the lithium depletion boundary in the color-magnitude diagram and, with the help of theoretical models, derived an age estimate, 50±5 Myr. The same study was carried out in other two clusters, namely Alpha Per and the Pleiades. Our new age estimate is 85±10 and 130±20 Myr. We have also derived an Initial Mass Function for the low mass end of the IC 2391 cluster, fitting a power law with an index of α=0.96±0.12.
We thank the staff of the CTIO 4m and Magellan telescopes for outstanding support. The anonymous referee has indeed contibuted to the improvement of the paper. DByN is indebted to the Spanish "Programa Ramón y Cajal", AYA2001-1124-CO2 & AYA2003-05355 programs. R.J. acknowledges support from NSF grant AST-0205130. This publication makes use of data products from the Two Micron All Sky Survey. Fig. 1.-Color-magnitude diagram for IC 2391 candidate members. Crosses represent all the available photometric data from Simon & Patten (1996), Patten & Pavlovsky (1999) and Barrado y Navascués et al. (2001a). Open circles correspond to spectroscopic data from Barrado y Navascués et al. (1999), whereas solid circles were observed with HydraII or the Magellan I and II telescopes. We plot different 50 Myr isochrones (short dashed for D'Antona & Mazzitelli 1997, long dashed for Baraffe et al. 1998 and dotted for Siess et al. 2000). The solid line represents an empirical ZAMS.          (1997), whereas the other two display results from I. Baraffe (priv. comm.).     =45.5 and 18.8Å. 4 In emission, with no continuum (very low S/N). 5 Nai(8200) out of range. 6 Classified as "possible" member in Paper I. We have collected a higher S/N spectrum, which indicates the presence of lithium. 7 Teff from (R − I) C color. 8 Teff from the spectral type. 9 The detection of lithium is quite uncertain in this case.