arXiv subjects

Ray Jayawardhana

Publications and source records attributed to Ray Jayawardhana.

At least 127 records · Page 7Linked to original sources

arXiv2007

Multiplicity Among Young Brown Dwarfs and Very Low Mass Stars

Characterizing multiplicity in the very low mass (VLM) domain is a topic of much current interest and fundamental importance. Here we report on a near-IR AO imaging survey of 31 young brown dwarfs and VLM stars, 28 of which are in Chamaeleon I, using the ESO VLT. Our survey is sensitive enough to detect equal mass binaries down to separations of 0.04-0.07" (6-10 AU at 160 pc) and, typically, companions with mass ratios as low as 0.2 outside of 0.2" (~30 AU). We resolve the suspected 0.16" (~26 AU) binary Cha_Halpha 2 and present two new binaries, Hn 13 and CHXR 15, with separations of 0.13" (~20 AU) and 0.30" (~50 AU); the latter is one of the widest VLM systems discovered to date. We do not find companions around the majority of our targets, giving a binary frequency of 11% [+9,-6], thus confirming the trend for a lower binary frequency with decreasing mass. By combining our work with previous surveys, we arrive at the largest sample of young VLMOs (72) with high angular resolution imaging to date. Its multiplicity fraction is in statistical agreement with that for VLMOs in the field. In addition we note that many field stellar binaries with lower binding energies and/or wider cross sections have survived dynamical evolution and that statistical models suggest tidal disruption by passing stars is unlikely to affect the binary properties of our systems. Thus, we argue that there is no significant evolution of multiplicity with age among brown dwarfs and VLM stars in OB and T associations between a few Myr to several Gyr. Instead, the observations to date suggest that VLM objects are either less likely to be born in fragile multiple systems than solar mass stars or such systems are disrupted very early.

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arXiv2007

The Gemini Deep Planet Survey -- GDPS

We present the results of the Gemini Deep Planet Survey, a near-infrared adaptive optics search for giant planets and brown dwarfs around nearby young stars. The observations were obtained with the Altair adaptive optics system at the Gemini North telescope and angular differential imaging was used to suppress the speckle noise of the central star. Detection limits for the 85 stars observed are presented, along with a list of all faint point sources detected around them. Typically, the observations are sensitive to angular separations beyond 0.5" with 5-sigma contrast sensitivities in magnitude difference at 1.6 micron of 9.5 at 0.5", 12.9 at 1", 15.0 at 2", and 16.5 at 5". For the typical target of the survey, a 100 Myr old K0 star located 22 pc from the Sun, the observations are sensitive enough to detect planets more massive than 2 Mjup with a projected separation in the range 40-200 AU. Second epoch observations of 48 stars with candidates (out of 54) have confirmed that all candidates are unrelated background stars. A detailed statistical analysis of the survey results, yielding upper limits on the fractions of stars with giant planet or low mass brown dwarf companions, is presented. Assuming a planet mass distribution dN/dm m^{-1.2} and a semi-major axis distribution dN/da a^{-1}, the 95% credible upper limits on the fraction of stars with at least one planet of mass 0.5-13 Mjup are 0.28 for the range 10-25 AU, 0.13 for 25-50 AU, and 0.093 for 50-250 AU. The 95% credible interval for the fraction of stars with at least one brown dwarf companion having a semi-major axis in the range 25-250 AU is 0.019 (-0.015/+0.083), irrespective of any assumption on the mass and semi-major axis distributions. The stars HD 14802, HD 166181, and HD 213845 have been resolved into binaries for the first time.

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arXiv2007

A Comprehensive View of Circumstellar Disks in Chamaeleon I: Infrared Excess, Accretion Signatures and Binarity

We present a comprehensive study of disks around 81 young low-mass stars and brown dwarfs in the nearby ~2-Myr-old Chamaeleon I star-forming region. We use mid-infrared photometry from the Spitzer Space Telescope, supplemented by findings from ground-based high-resolution optical spectroscopy and adaptive optics imaging. We derive disk fractions of 52 (+/-6) % and 58 (+6/-7) % based on 8-micron and 24-micron colour excesses, respectively, consistent with those reported for other clusters of similar age. Within the uncertainties, the disk frequency in our sample of K3-M8 objects in Cha I does not depend on stellar mass. Diskless and disk-bearing objects have similar spatial distributions. There are no obvious transition disks in our sample, implying a rapid timescale for the inner disk clearing process; however, we find two objects with weak excess at 3-8 microns and substantial excess at 24 microns, which may indicate grain growth and dust settling in the inner disk. For a sub-sample of 35 objects with high-resolution spectra, we investigate the connection between accretion signatures and dusty disks: in the vast majority of cases (29/35) the two are well correlated, suggesting that, on average, the timescale for gas dissipation is similar to that for clearing the inner dust disk. The exceptions are six objects for which dust disks appear to persist even though accretion has ceased or dropped below measurable levels. Adaptive optics images of 65 of our targets reveal that 17 have companions at (projected) separations of 10-80 AU. Of the five <20 AU binaries, four lack infrared excess, possibly indicating that a close companion leads to faster disk dispersal. The closest binary with excess is separated by ~20 AU, which sets an upper limit of ~8 AU for the outer disk radius. (abridged)

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arXiv2007

Emission Line Variability of the Accreting Young Brown Dwarf 2MASSW J1207334-393254: From Hours to Years

We have obtained a series of high-resolution optical spectra for the brown dwarf 2MASSW J1207334-393254 (2M1207) using the ESO Very Large Telescope with the UVES spectrograph during two consecutive observing nights (time resolution of ~12 min) and the Magellan Clay telescope with the MIKE spectrograph. Combined with previously published results, these data allow us to investigate changes in the emission line spectrum of 2M1207 on timescales of hours to years. Most of the emission line profiles of 2M1207 are broad, in particular that of Halpha, indicating that the dominant fraction of the emission must be attributed to disk accretion rather than to magnetic activity. From the Halpha 10% width we deduce a relatively stable accretion rate between 10^(-10.1...-9.8) Msun/yr for two nights of consecutive observations. Therefore, either the accretion stream is nearly homogeneous over (sub-)stellar longitude or the system is seen face-on. Small but significant variations are evident throughout our near-continuous observation, and they reach a maximum after ~8 h, roughly the timescale on which maximum variability is expected across the rotation cycle. Together with past measurements, we confirm that the accretion rate of 2M1207 varies by more than one order of magnitude on timescales of months to years. Such variable mass accretion yields a plausible explanation for the observed spread in the accretion rate vs. mass diagram. The magnetic field required to drive the funnel flow is on the order of a few hundred G. Despite the obvious presence of a magnetic field, no radio nor X-ray emission has been reported for 2M1207. Possibly strong accretion suppresses magnetic activity in brown dwarfs, similar to the findings for higher mass T Tauri stars.

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arXiv2007

Rotation and activity of pre-main-sequence stars

We present a study of rotation (vsini) and chromospheric activity (Halpha EW) based on an extensive set of high-resolution optical spectra obtained with MIKE on the 6.5m Magellan Clay telescope. Our targets are 74 F-M dwarfs in the young stellar associations Eta Cha, TW Hydrae, Beta Pic, and Tuc-Hor, spanning ages from 6 to 30 Myr. While the Halpha EW for most F and G stars are consistent with pure photospheric absorption, most K and M stars show chromospheric emission. By comparing Halpha EW in our sample to results in the literature, we see a clear evolutionary sequence: Chromospheric activity declines steadily from the T Tauri phase to the main sequence. Using activity as an age indicator, we find a plausible age range for the Tuc-Hor association of 10-40 Myr. Between 5 and 30 Myr, we do not see evidence for rotational braking in the total sample, thus angular momentum is conserved, in contrast to younger stars. This difference indicates a change in the rotational regulation at 5-10 Myr, possibly because disk braking cannot operate longer than typical disk lifetimes, allowing the objects to spin up. The rotation-activity relation is flat in our sample; in contrast to main-sequence stars, there is no linear correlation for slow rotators. We argue that this is because young stars generate their magnetic fields in a fundamentally different way from main-sequence stars, and not just the result of a saturated solar-type dynamo. By comparing our rotational velocities with published rotation periods for a subset of stars, we determine ages of 13 (7-20) Myr and 9 (7-17} Myr for the Eta Cha and TWA associations, respectively, consistent with previous estimates. Thus we conclude that stellar radii from evolutionary models by Baraffe et al. (1998) are in agreement with the observed radii within +-15%. (abridged)

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arXiv2007

Evolution of brown dwarf disks: A Spitzer survey in Upper Scorpius

We have carried out a Spitzer survey for brown dwarf (BD) disks in the ~5 Myr old Upper Scorpius (UpSco) star forming region, using IRS spectroscopy from 8 to 12μm and MIPS photometry at 24μm. Our sample consists of 35 confirmed very low mass members of UpSco. Thirteen objects in this sample show clear excess flux at 24μm, explained by dust emission from a circum-sub-stellar disk. Objects without excess emission either have no disks at all or disks with inner opacity holes of at least ~5 AU radii. Our disk frequency of 37\pm 9% is higher than what has been derived previously for K0-M5 stars in the same region (on a 1.8 sigma confidence level), suggesting a mass-dependent disk lifetime in UpSco. The clear distinction between objects with and without disks as well as the lack of transition objects shows that disk dissipation inside 5 AU occurs rapidly, probably on timescales of <~10^5 years. For the objects with disks, most SEDs are uniformly flat with flux levels of a few mJy, well modeled as emission from dusty disks affected by dust settling to the midplane, which also provides indirect evidence for grain growth. The silicate feature around 10μm is either absent or weak in our SEDs, arguing for a lack of hot, small dust grains. Compared with younger objects in Taurus, BD disks in UpSco show less flaring. Taken together, these results clearly demonstrate that we see disks in an advanced evolutionary state: Dust settling and grain growth are ubiquituous in circum-sub-stellar disks at ages of 5 Myr, arguing for planet forming processes in BD disks. For almost all our targets, results from high-resolution spectroscopy and high-spatial resolution imaging have been published before, thus providing a large sample of BDs for which information about disks, accretion, and binarity is available. (abridged)

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arXiv2006

Infrared Spectroscopy of the Ultra Low Mass Binary Oph 162225-240515

Binary properties are an important diagnostic of the star and brown dwarf formation processes. While wide binaries appear to be rare in the sub-stellar regime, recent observations have revealed Ophiuchus 162225-240515 (2MASS J16222521-2405139) as a likely young ultra-low-mass binary with an apparent separation of ~240 AU. Here, we present low-resolution near-infrared spectra of the pair from NTT/SOFI (R~600) and VLT/ISAAC (R~1400), covering the 1.0-2.5um spectral region. By comparing to model atmospheres from Chabrier & Baraffe and Burrows et al., we confirm the surface temperatures to be T_A = (2350+/-150) K and T_B = (2100+/-100) K for the two components of the binary, consistent with earlier estimates from optical spectra. Using gravity sensitive K I features, we find the surface gravity to be significantly lower than field dwarfs of the same spectral type, providing the best evidence so far that these objects are indeed young. However, we find that models are not sufficiently reliable to infer accurate ages/masses from surface gravity. Instead, we derive masses of M_A = 13 (+8/-4) M_J and M_B = 10 (+5/-4) M_J for the two objects using the well-constrained temperatures and assuming an age of 1-10 Myr, consistent with the full range of ages reported for the Oph region.

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arXiv2006

The Planetary Mass Companion 2MASS1207-3932 B: Temperature, Mass and Evidence for an Edge-On Disk

We present J-band imaging and H+K-band low-resolution spectroscopy of 2MASS1207-3932 AB, obtained with VLT NACO. For the putative planetary mass secondary, we find J = 20.0+/-0.2 mag. The HK spectra of both components imply low gravity, and a dusty atmosphere for the secondary. Comparisons to synthetic spectra yield Teff_A ~ 2550+/-150K, and Teff_B ~ 1600+/-100K, consistent with their late-M and mid-to-late L types. For these Teff, and an age of 5-10 Myrs, evolutionary models imply M_A ~ 24+/-6 M_Jup and M_B ~ 8+/-2 M_Jup. Independent comparisons of these models to the observed colors, spanning ~I to L', also yield the same masses and temperatures. Our primary mass agrees with other recent analyses; however, our secondary mass, while still in the planetary regime, is 2-3 times larger than claimed previously. This discrepancy can be traced to the luminosities: while the absolute photometry and Mbol of the primary agree with theoretical predictions, the secondary is ~ 2.5+/-0.5 mag fainter than expected in all bands from I to L' and in Mbol. This accounts for the much lower secondary mass (and temperature) derived earlier. We argue that this effect is highly unlikely to result from a variety of model-related problems, and is instead real. This conclusion is bolstered by the absence of any luminosity problems in either the primary, or in AB Pic B which we also analyse. We therefore suggest grey extinction in 2M1207B, due to occlusion by an edge-on circum-secondary disk. This is consistent with the observed properties of edge-on disks around T Tauri stars, and with the known presence of a high-inclination evolved disk around the primary. Finally, the system's implied mass ratio of ~0.3 suggests a binary-like formation scenario. (abridged)

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arXiv2006

Deficit of wide binaries in the eta Chamaeleontis young cluster

We have carried out a sensitive high-resolution imaging survey of stars in the young (6-8 Myr), nearby (97 pc) compact cluster around eta Chamaeleontis to search for stellar and sub-stellar companions. Given its youth and proximity, any sub-stellar companions are expected to be luminous, especially in the near infrared, and thus easier to detect next to their parent stars. Here, we present VLT/NACO adaptive optics imaging with companion detection limits for 17 eta Cha cluster members, and follow-up VLT/ISAAC near-infrared spectroscopy for companion candidates. The widest binary detected is ~0.2", corresponding to the projected separation 20 AU, despite our survey being sensitive down to sub-stellar companions outside 0.3", and planetary mass objects outside 0.5". This implies that the stellar companion probability outside 0.3" and the brown dwarf companion probability outside 0.5" are less than 0.16 with 95% confidence. We compare the wide binary frequency of eta Cha to that of the similarly aged TW Hydrae association, and estimate the statistical likelihood that the wide binary probability is equal in both groups to be < 2e-4. Even though the eta Cha cluster is relatively dense, stellar encounters in its present configuration cannot account for the relative deficit of wide binaries. We thus conclude that the difference in wide binary probability in these two groups provides strong evidence for multiplicity properties being dependent on environment. In two appendices we derive the projected separation probability distribution for binaries, used to constrain physical separations from observed projected separations, and summarize statistical tools useful for multiplicity studies.

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arXiv2006

Spectroscopy of Young Planetary Mass Candidates with Disks

It is now well established that many young brown dwarfs exhibit characteristics similar to classical T Tauri stars, including infrared excess from disks and emission lines related to accretion. Whether the same holds true for even lower mass objects, namely those near and below the Deuterium-burning limit, is an important question. Here we present optical spectra of six isolated planetary mass candidates in Chamaeleon II, Lupus I and Ophiuchus star-forming regions, recently identified by Allers and collaborators to harbor substantial mid-infrared excesses. Our spectra, from ESO's Very Large Telescope and New Technology Telescope, show that four of the targets have spectral types in the ~M9-L1 range, and three of those also exhibit H_alpha. Their luminosities are consistent with masses of ~5-15 M_{Jupiter} according to models of Chabrier, Baraffe and co-workers, thus placing these four objects among the lowest mass brown dwarfs known to be surrounded by circum-sub-stellar disks. Our findings bolster the idea that free-floating planetary mass objects could have infancies remarkably similar to those of Sun-like stars and suggest the intriguing possibility of planet formation around primaries whose masses are comparable to those of extra-solar giant planets. Another target appears to be a brown dwarf (~M8) with prominent H_alpha emission, possibly arising from accretion. The sixth candidate is likely a background source, underlining the need for spectroscopic confirmation.

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arXiv2006

Accretion Disks around Young Stars: Lifetimes, Disk Locking and Variability

We report the findings of a comprehensive study of disk accretion and related phenomena in four of the nearest young stellar associations spanning 6-30 million years in age, an epoch that may coincide with the late stages of planet formation. We have obtained ~650 multi-epoch high-resolution optical spectra of 100 low-mass stars that are likely members of the eta Chamaeleontis (~6 Myr), TW Hydrae (~8 Myr), beta Pictoris (~12 Myr) and Tucanae-Horologium (~30 Myr) groups. Our data were collected over 12 nights between 2004 December - 2005 July on the Magellan Clay 6.5m telescope. Based on H$α$ line profiles, along with a variety of other emission lines, we find clear evidence of on-going accretion in three out of 11 eta Cha stars and two out of 32 TW Hydrae members. None of the 57 beta Pic or Tuc-Hor members shows measurable signs of accretion. Together, these results imply significant evolution of the disk accretion process within the first several Myr of a low-mass star's life. While a few disks can continue to accrete for up to ~10 Myr, our findings suggest that disks accreting for beyond that timescale are rather rare. This result provides an indirect constraint on the timescale for gas dissipation in inner disks and, in turn, on gas giant planet formation. All accretors in our sample are slow rotators, whereas non-accretors cover a large range in rotational velocities. This may hint at rotational braking by disks at ages up to ~8 Myr. Our multi-epoch spectra confirm that emission-line variability is common even in somewhat older T Tauri stars, among which accretors tend to show particularly strong variations. Thus, our results indicate that accretion and wind activity undergo significant and sustained variations throughout the lifetime of accretion disks.

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arXiv2006

Exploring brown dwarf disks: A 1.3 mm survey in Taurus

We have carried out sensitive 1.3 mm observations of 20 young brown dwarfs in the Taurus star-forming region, representing the largest sample of young substellar objects targeted in a deep millimeter continuum survey to date. Under standard assumptions, the masses of brown dwarf disks range from ~<0.4 to several Jupiter masses. Their relative disk masses are comparable to those derived for coeval low-mass stars: most of them are in the ~<1-5% range, and there is no clear change of relative disk mass with object mass from 0.015 to 3 solar masses. Specifically, we do not find evidence for disk truncation, as would be expected in the ejection scenario for brown dwarf origin, although the signature of ejection may be hidden in our non-detections. We use the derived mm fluxes, complemented by mid-infrared data from the Spitzer Space Telescope and ground-based near-infrared images, to construct spectral energy distributions (SEDs) for six of our sources, and model those SEDs with a Monte Carlo radiative transfer code. While the model fits are by no means unique, they allow us to investigate disk properties such as the degree of flaring and minimum radii. In several cases, we find that the SEDs in the mid-infrared exhibit lower flux levels than predicted by hydrostatic models, implying dust settling to the disk midplane. What's more, at least 25% of our targets are likely to have disks with radii >10 AU; models with smaller disks cannot reproduce the mm fluxes even if they are very massive. This finding is in contrast to the results of some simulations of the ejection scenario for brown dwarf formation that suggest only ~5% of ejected objects would harbor disks larger than 10 AU. Our findings imply that ejection is probably not the dominant formation process, but may still be relevant for some brown dwarfs.

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arXiv2005

Variable accretion and outflow in young brown dwarfs

We report on the first dedicated monitoring campaign of spectroscopic variability in young brown dwarfs. High-resolution optical spectra of six targets in nearby star-forming regions were obtained over 11 nights between 2005 January-March on the Magellan 6.5m telescope. We find significant variability in Halpha and a number of other emission lines related to accretion and outflow processes on a variety of timescales ranging from hours to weeks to years. The most dramatic changes are seen for 2M1207, 2M1101 and ChaI-ISO217. We observe possible accretion rate changes by about an order of magnitude in two of these objects, over timescales of weeks (2M1207) or hours (2M1101). The accretion 'burst' seen in 2M1101 could be due to a 'clumpy' flow. We also see indications for changes in the outflow rate in at least three objects. In one case (ISO217), there appears to be a ~1-hour time lag between outflow and accretion variations, consistent with a scenario in which the wind originates from the inner disk edge. Our variability study supports a close to edge-on inclination for the brown dwarf LS-RCrA 1. The fact that all targets in our sample show variations in accretion and/or outflow indicators suggests that studies of young brown dwarf properties should be based either on large samples or time series. As an example, we demonstrate that the large scatter in the recently found accretion rate vs. mass relationship can be explained primarily with variability. The observed profile variations imply asymmetric accretion flows in brown dwarfs, which, in turn, is evidence for magnetic funneling by large-scale fields. We show that accreting sub-stellar objects may harbor magnetic fields with ~kG strength (abridged).

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arXiv2005

Rotation and variability of young very low mass objects

Variability studies are an important tool to investigate key properties of stars and brown dwarfs. From photometric monitoring we are able to obtain information about rotation and magnetic activity, which are expected to change in the mass range below 0.3 solar masses, since these fully convective objects cannot host a solar-type dynamo. On the other hand, spectroscopic variability information can be used to obtain a detailed view on the accretion process in very young objects. In this paper, we report about our observational efforts to analyse the variability and rotational evolution of young brown dwarfs and very low mass stars.

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arXiv2005

Whims of an Accreting Young Brown Dwarf: Exploring Emission Line Variability of 2MASSW J1207334-393254

We report the first comprehensive study of emission line variability in an accreting young brown dwarf. We have collected 14 high-resolution optical spectra of 2MASSW J1207334-393254 (M8), a likely member of the nearby 8-million-year-old TW Hydrae association. These spectra show a variety of emission lines that are commonly seen in classical T Tauri stars. Halpha line in particular shows dramatic changes in shape and intensity in our dataset, both on timescales of several weeks and several hours. In spectra from late-January, the line is relatively weak and only slightly asymmetric. Spectra from mid- and late-March show intense, broad and asymmetric Halpha emission, indicative of on-going disk accretion. We estimate that the accretion rate could have changed by a factor of 5-10 over ~6 weeks in this brown dwarf. March spectra also reveal significant `quasi-periodic' changes in the Halpha line profile over the course of a night, from clearly double-peaked to nearly symmetric. These nightly profile changes, roughly consistent with the brown dwarf's rotation period, could be the result of a redshifted absorption feature coming into and out of our line of sight; when the profile is double-peaked we may be looking into an accretion column, flowing from the inner disk edge on to the central object, indicating that the accretion is probably channelled along the magnetic field lines. Our findings provide strong support for the magnetospheric accretion scenario, and thus for the existence of large-scale magnetic fields, in the sub-stellar regime (abridged).

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arXiv2005

Constraints on Inner Disk Evolution Timescales: A Disk Census of the eta Chamaeleontis Young Cluster

We present new L' (3.8-micron) observations of stars in the nearby (~97 pc) young (~6 Myr) compact cluster around eta Chamaeleontis, obtained with the European Southern Observatory's Very Large Telescope in Paranal, Chile. Our data, combined with J,H, Ks photometry from the 2-Micron All Sky Survey, reveal that only two of the 12 members surveyed harbor L'-band excesses consistent with optically thick inner disks; both are also likely accretors. Intriguingly, two other stars with possible evidence for on-going accretion, albeit at very low rates, do not show significant infrared excess: this may imply substantial grain growth and/or partial clearing of the inner disk region, as expected in planet formation scenarios. Our findings suggest that eta Cha stars are in an epoch when disks are rapidly evolving, perhaps due to processes related to planet building, and provide further constraints on inner disk lifetimes.

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arXiv2005

The T Tauri Phase Down to Nearly Planetary Masses: Echelle Spectra of 82 Very Low Mass Stars and Brown Dwarfs

Using the largest high-resolution spectroscopic sample to date of young, very low mass stars (VLMS) and brown dwarfs (BDs), we investigate disk accretion in objects ranging from just above the hydrogen-burning limit all the way to nearly planetary masses. Our 82 targets span spectral types from M5 to M9.5, or masses from 0.15 Msun down to ~15 Jupiters. They are confirmed members of the rho Oph, Taurus, Cha I, IC 348, R CrA, Upper Sco and TW Hydrae regions, with ages <1 to ~10 Myr. The sample contains 41 BDs (spectral types >= M6.5). We find that: (1) classical T Tauri-like disk-accretion persists in the BD domain down to nearly the deuterium-burning limit; (2) in addition to H-alpha, permitted emission lines of CaII, OI and HeI are also good accretion indicators, as in CTTs; (3) the CaII 8662A flux is an excellent quantitative measure of the accretion rate (Mdot) in VLMS and BDs(as in CTTs); (4) Mdot diminishes as M^2 -- our measurements support previous findings of this correlation, and extend it to the entire range of sub-stellar masses; (5) the accretor fraction among VLMS and BDs decreases substantially with age, as in higher-mass stars; (6) at any given age, the VLMS and BD accretor fraction is comparable to that in higher-mass stars; and (7) a number of sources with IR disk excesses do not evince measurable accretion, with the incidence of such a mismatch increasing with age: this implies that disks in the low mass regime can persist beyond the main accretion phase, and parallels the transition from the classical to post-T Tauri stage in more massive stars. These strong similarities at young ages, between higher-mass stars and low-mass bodies close to and below the hydrogen-burning limit, are consistent with a common formation mechanism in the two mass regimes. (abridged)

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arXiv2004

A new Classical T Tauri object at the sub-stellar boundary in Chamaeleon II

We have obtained low- and medium-resolution optical spectra of 20 candidate young low-mass stars and brown dwarfs in the nearby Chamaeleon II dark cloud, using the Magellan Baade telescope. We analyze these data in conjunction with near-infrared photometry from the 2-Micron All Sky Survey. We find that one target, [VCE2001] C41, exhibits broad H(alpha) emission as well as a variety of forbidden emission lines. These signatures are usually associated with accretion and outflow in young stars and brown dwarfs. Our spectra of C41 also reveal LiI in absorption and allow us to derive a spectral type of M5.5 for it. Therefore, we propose that C41 is a classical T Tauri object near the sub-stellar boundary. Thirteen other targets in our sample have continuum spectra without intrinsic absorption or emission features, and are difficult to characterize. They may be background giants or foreground field stars not associated with the cloud or embedded protostars, and need further investigation. The six remaining candidates, with moderate reddening, are likely to be older field dwarfs, given their spectral types, lack of lithium and H(alpha).

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