arXiv subjects

K. L. Luhman

Publications and source records attributed to K. L. Luhman.

105 records · Page 6Linked to original sources

arXiv2004

Spectroscopy of Candidate Members of the Eta Cha and MBM12 Young Associations

We present an analysis of candidate members of the Eta Cha and MBM 12A young associations. For an area of 0.7 deg^2 toward Eta Cha, we have performed a search for members of the association by combining JHK_s photometry from 2MASS and i photometry from DENIS with followup optical spectroscopy at Magellan Observatory. We report the discovery of three new members with spectral types of M5.25-M5.75, corresponding to masses of 0.13-0.08 M_sun by theoretical evolutionary models. Two and three of these members were found independently by Lyo and coworkers and Song and coworkers, respectively. Meanwhile, no brown dwarfs were detected in Eta Cha down to the completeness limit of 0.015 M_sun. For MBM 12A, we have obtained spectra of three of the remaining candidate members that lacked spectroscopy at the end of the survey by Luhman, all of which are found to be field M dwarfs. Ogura and coworkers have recently presented four "probable" members of MBM 12A. However, two of these objects were previously classified as field dwarfs by the spectroscopy of Luhman. In this work, we find that the other two objects are field dwarfs as well.

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arXiv2004

A Census of the Chamaeleon I Star-Forming Region

Optical spectroscopy has been obtained for 179 objects that have been previously identified as possible members of the cluster, that lack either accurate spectral types or clear evidence of membership, and that are optically visible (I<18). I have used these spectroscopic data and all other available constraints to evaluate the spectral classifications and membership status of a total sample of 288 candidate members of Chamaeleon I that have appeared in published studies of the cluster. The latest census of Chamaeleon I now contains 158 members, 8 of which are later than M6 and thus are likely to be brown dwarfs. I find that many of the objects identified as members of Chamaeleon I in recent surveys are actually field stars. Meanwhile, 7 of 9 candidates discovered by Carpenter and coworkers are confirmed as members, one of which is the coolest known member of Chamaeleon I at a spectral type of M8 (~0.03 M_sun). I have estimated extinctions, luminosities, and effective temperatures for the members and used these data to construct an H-R diagram for the cluster. Chamaeleon I has a median age of ~2 Myr according to evolutionary models, and hence is similar in age to IC 348 and is slightly older than Taurus (~1 Myr). The measurement of an IMF for Chamaeleon I from this census is not possible because of the disparate methods with which the known members were originally selected, and must await an unbiased, magnitude-limited survey of the cluster.

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arXiv2004

Spectroscopy of Ultra-Cool Dwarfs

Ultra-cool dwarfs have been defined as dwarfs with spectral types of M7 or later (Kirkpatrick, Henry, & Irwin 1997) and thus include the new L and T spectral classes. The previous IAU report on ultra-cool dwarfs in 1999 described the optical and IR spectral features that characterize these spectral types. In this report, I review the latest progress on the spectral classification of ultra-cool dwarfs, which are divided into two categories: old dwarfs in the field (>1 Gyr) and their young progenitors in the nearest star-forming regions and open clusters (1-100 Myr). Because the former are nearby and the latter are young, these two groups are relatively bright and lend themselves to discovery and detailed study. Technically, the luminosity class of young ultra-cool objects is closer to subgiant than dwarf; nevertheless, I include them in this report on ultra-cool dwarfs.

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arXiv2003

A Census of the Young Cluster IC 348

We present a new census of the stellar and substellar members of the young cluster IC 348. We have obtained images at I and Z for a 42'x28' field encompassing the cluster and have combined these measurements with previous optical and near-infrared photometry. From spectroscopy of candidate cluster members appearing in these data, we have identified 122 new members, 15 of which have spectral types of M6.5-M9, corresponding to masses of 0.08-0.015 M_sun by recent evolutionary models. The latest census for IC 348 now contains a total of 288 members, 23 of which are later than M6 and thus are likely to be brown dwarfs. From an extinction-limited sample of members (A_V<=4) for a 16'x14' field centered on the cluster, we construct an IMF that is unbiased in mass and nearly complete for M/M_sun>=0.03 (<=M8). In logarithmic units where the Salpeter slope is 1.35, the mass function for IC 348 rises from high masses down to a solar mass, rises more slowly down to a maximum at 0.1-0.2 M_sun, and then declines into the substellar regime. In comparison, the similarly-derived IMF for Taurus from Briceno et al. and Luhman et al. rises quickly to a peak near 0.8 M_sun and steadily declines to lower masses. The distinctive shapes of the IMFs in IC 348 and Taurus are reflected in the distributions of spectral types, which peak at M5 and K7, respectively. These data provide compelling, model-independent evidence for a significant variation of the IMF with star-forming conditions.

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arXiv2003

New Low-Mass Members of the Taurus Star-Forming Region

Briceno et al. recently used optical imaging, data from the Two-Micron All-Sky Survey (2MASS), and follow-up spectroscopy to search for young low-mass stars and brown dwarfs in 8 square degrees of the Taurus star-forming region. By the end of that study, there remained candidate members of Taurus that lacked the spectroscopic observations needed to measure spectral types and determine membership. In this work, we have obtained spectroscopy of the 22 candidates that have A_V<=8, from which we find six new Taurus members with spectral types of M2.75 through M9. The new M9 source has the second latest spectral type of the known members of Taurus (~0.02 M_sun). Its spectrum contains extremely strong emission in H_alpha (W~950 A) as well as emission in He I 6678 A and the Ca II IR triplet. This is the least massive object known to exhibit emission in He I and Ca II, which together with the strong H_alpha are suggestive of intense accretion.

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arXiv2002

Discovery of an Edge-On Disk in the MBM 12 Young Association

We report the discovery of a spatially-resolved edge-on protoplanetary disk in the ~2-Myr-old MBM 12 young association. Our near-infrared images of LkHa 263C (MBM 12A 3C), obtained with the Hokupa'a adaptive optics system on the Gemini North telescope, clearly show two elongated reflection nebulosities separated by a dark lane, a morphology well-matched by scattered light models of an optically thick (at near-infrared wavelengths) edge-on disk. An optical spectrum of the scattered light nebulosity obtained with the Keck II telescope exhibits a spectral type of M0 +/- 0.5 (T_eff = 3850 +/- 100 K) for the central star and contains H_alpha and forbidden emission lines, which may indicate the presence of a jet. The absence of a near-infrared point source implies A_K > 9.5 toward the unseen central star. The disk is flared and has a radius of ~150 AU (at a distance of 275 pc) and an inclination of 87 degrees. The aspect ratio of the model disk in the J-band is 0.72. There is possible evidence for dust settling to the disk midplane. LkHa 263C is 4.115" from the 0.415" binary LkHa 263 A and B (MBM 12A 3A and 3B), which is itself 15.5" from LkHa 262 (MBM 12A 2). Thus, LkHa 263C may be the first disk to be clearly resolved around an individual star in a young quadruple system. The detection of a faint edge-on disk near a bright star demonstrates both the high angular resolution and the high sensitivity that can be achieved with adaptive optics imaging on large telescopes.

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arXiv2001

An Adaptive Optics Search for Companions to Stars with Planets

We have performed a Keck adaptive optics (AO) imaging survey of 25 extrasolar planetary systems discovered by the radial velocity programs. Typically, the high-resolution (FWHM~0.04") near-infrared images are able to detect point sources at dH=10 at 1", L and T dwarfs at 0.5" to 2.0", and stars and brown dwarfs at 3-10 AU and 10-100 AU from the targets. The AO images reveal 15 faint sources (H=14-20) near the stars HD 37124, HD 168443, HD 187123, and HD 190228 out to a search radius of 3.3. We have obtained AO images at a second epoch for five of the candidate companions at Keck and Gemini Observatories. The resulting measurements of proper motions relative to the primary stars indicate that these five sources are background stars. Given that the four primaries are near the Galactic plane (|b|<6 deg), the remaining faint sources are also likely to be background stars rather than substellar companions. We present the companion detection limits for each target as a function of separation and compare them to the sensitivities of radial velocity surveys, demonstrating the complementary nature of the two techniques.

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arXiv2001

On the MBM12 Young Association

I present a comprehensive study of the MBM12 young association (MBM12A). By combining infrared (IR) photometry from the Two-Micron All-Sky Survey (2MASS) survey with new optical imaging and spectroscopy, I have performed a census of the MBM12A membership that is complete to 0.03 Msun (H~15) for a 1.75deg X 1.4deg field encompassing the MBM12 cloud. I find five new members with masses of 0.1-0.4 Msun and a few additional candidates that have not been observed spectroscopically. From an analysis of optical and IR photometry for stars in the direction of MBM12, I identify M dwarfs in the foreground and background of the cloud. By comparing the magnitudes of these stars to those of local field dwarfs, I arrive at a distance modulus 7.2+/-0.5 (275 pc) to the MBM12 cloud; it is not the nearest molecular cloud and is not inside the local bubble of hot ionized gas as had been implied by previous distance estimates of 50-100 pc. I have also used Li strengths and H-R diagrams to constrain the absolute and relative ages of MBM12A and other young populations; these data indicate ages of 2 +3/-1 Myr for MBM12A and 10 Myr for the TW Hya and Eta Cha associations. MBM12A may be a slightly evolved version of the aggregates of young stars within the Taurus dark clouds (~1 Myr) near the age of the IC 348 cluster (~2 Myr).

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arXiv2000

The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Taurus

By combining deep optical imaging and infrared spectroscopy with data from the Two-Micron All-Sky Survey (2MASS) and from previous studies (e.g., Briceno et al.), I have measured the Initial Mass Function (IMF) for a reddening-limited sample in four fields in the Taurus star forming region. This IMF is representative of the young populations within these fields for masses above 0.02 Msun. Relative to the similarly derived IMF for the Trapezium Cluster (Luhman et al.), the IMF for Taurus exhibits a modest deficit of stars above one solar mass (i.e., steeper slope), the same turnover mass (~0.8 Msun), and a significant deficit of brown dwarfs. If the IMF in Taurus were the same as that in the Trapezium, 12.8+/-1.8 brown dwarfs (>0.02 Msun) are expected in these Taurus fields where only one brown dwarf candidate is found. These results are used to test theories of the IMF.

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arXiv2000

The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Young Clusters

We have obtained images of the Trapezium Cluster (140" x 140"; 0.3 pc x 0.3 pc) with the Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). Combining these data with new ground-based K-band spectra (R=800) and existing spectral types and photometry and the models of D'Antona & Mazzitelli, we find that the distributions of ages of comparable samples of stars in the Trapezium, rho Oph, and IC 348 indicate median ages of \~0.4 Myr for the first two regions and ~1-2 Myr for the latter. The low-mass IMFs in these sites of clustered star formation are similar over a wide range of stellar densities and other environmental conditions. With current data, we cannot rule out modest variations in the substellar mass functions among these clusters. We then make the best estimate of the true form of the IMF in the Trapezium by using the evolutionary models of Baraffe et al. and an empirically adjusted temperature scale and compare this mass function to recent results for the Pleiades and the field. All of these data are consistent with an IMF that is flat or rises slowly from the substellar regime to about 0.6 Msun, and then rolls over into a power law that continues from about 1 Msun to higher masses with a slope similar to or somewhat larger than the Salpeter value of 1.35. For the Trapezium, this behavior holds from our completeness limit of ~0.02 Msun and probably, after a modest completeness correction, even from 0.01-0.02 Msun. These data include ~50 likely brown dwarfs. We test the predictions of theories of the IMF against various properties of the observed IMF.

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arXiv2000

Membership and Multiplicity among Very Low-Mass Stars and Brown Dwarfs in the Pleiades Cluster

We present near-infrared photometry and optical spectroscopy of very low-mass stars and brown dwarf candidates in the Pleiades open cluster. The membership status of these objects is assessed. Eight objects out of 45 appear to be non-members. A search for companions among 34 very low-mass Pleiades members (M$\le$0.09 M$_\odot$) in high-spatial resolution images obtained with the Hubble Space Telescope and the adaptive optics system of the Canada-France-Hawaii telescope produced no resolved binaries with separations larger than 0.2 arcsec (a ~ 27 AU; P ~ 444 years). Nevertheless, we find evidence for a binary sequence in the color-magnitude diagrams, in agreement with the results of Steele & Jameson (1995) for higher mass stars. We compare the multiplicity statistics of the Pleiades very low-mass stars and brown dwarfs with that of G and K-type main sequence stars in the solar neighborhood (Duquennoy & Mayor 1991). We find that there is some evidence for a deficiency of wide binary systems (separation >27 AU) among the Pleiades very low-mass members. We briefly discuss how this result can fit with current scenarios of brown dwarf formation. We correct the Pleiades substellar mass function for the contamination of cluster non-members found in this work. We find a contamination level of 33% among the brown dwarf candidates identified by Bouvier et al. (1998). Assuming a power law IMF across the substellar boundary, we find a slope dN/dM ~ M^{-0.53}, implying that the number of objects per mass bin is still rising but the contribution to the total mass of the cluster is declining in the brown dwarf regime.

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arXiv1999

Keck Spectra of Brown Dwarf Candidates and a Precise Determination of the Lithium Depletion Boundary in the Alpha Persei Open Cluster

We have identified twenty-seven candidate very low mass members of the relatively young Alpha Persei open cluster from a six square degree CCD imaging survey. Based on their I magnitudes and the nominal age and distance to the cluster, these objects should have masses less than 0.1 Msunif they are cluster members. We have subsequently obtained intermediate resolution spectra of seventeen of these objects using the Keck II telescope and LRIS spectrograph. We have also obtained near-IR photometry for many of the stars. Our primary goal was to determine the location of the "lithium depletion boundary" and hence to derive a precise age for the cluster. We detect lithium with equivalent widths greater than or equal to 0.4 Åin five of the program objects. We have constructed a color-magnitude diagram for the faint end of the Alpha Persei main sequence. These data allow us to accurately determine the Alpha Persei single-star lithium depletion boundary at M(I$_C$) = 11.47, M(Bol) = 11.42, (R-I)$_{C0}$ = 2.12, spectral type M6.0. By reference to theoretical evolutionary models, this converts fairly directly into an age for the Alpha Persei cluster of 90 $\pm$ 10 Myr. At this age, the two faintest of our spectroscopically confirmed members should be sub-stellar (i.e., brown dwarfs) according to theoretical models.

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arXiv1999

Low-Mass Star Formation and the Initial Mass Function in the Rho Ophiuchi Cloud Core

We have obtained moderate-resolution (R=800-1200) K-band spectra for ~100 stars within and surrounding the cloud core of rho Oph. We have measured spectral types and continuum veilings and have combined this information with results from new deep imaging. The IMF peaks at about 0.4 M_sun and slowly declines to the hydrogen burning limit with a slope of ~-0.5 in logarithmic units (Salpeter is +1.35). Our lower limits on the numbers of substellar objects demonstrate that the IMF probably does not fall more steeply below the hydrogen burning limit, at least down to ~0.02 M_sun. We then make the first comparison of mass functions of stars and pre-stellar clumps (Motte, Andre, & Neri) measured in the same region. The similar behavior of the two mass functions in rho Oph supports the suggestion of Motte et al. and Testi & Sargent that the stellar mass function in young clusters is a direct product of the process of cloud fragmentation. After considering the effect of extinction on the SED classifications of the sample, we find that ~17% of the rho Oph stars are Class I, implying ~0.1 Myr for the lifetime of this stage. In spectra separated by two years, we observe simultaneous variability in the Br gamma emission and K-band continuum veiling for two stars, where the hydrogen emission is brighter in the more heavily veiled data. This behavior indicates that the disk may contribute significantly to continuous K-band emission, in contrast to the proposal that the infalling envelope always dominates. Our detection of strong 2 micron veiling (r_K=1-4) in several Class II and III stars, which should have disks but little envelope material, further supports this proposition.

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arXiv1999

Young Low-Mass Stars and Brown Dwarfs in IC 348

I present new results from a continuing program to identify and characterize the low-mass stellar and substellar populations in the young cluster IC 348 (1-10~Myr). Optical spectroscopy has revealed young objects with spectral types as late as M8.25. The intrinsic J-H and H-K colors of these sources are dwarf-like, whereas the R-I and I-J colors appear intermediate between the colors of dwarfs and giants. Furthermore, the spectra from 6500 to 9500 A are reproduced well with averages of standard dwarf and giant spectra, suggesting that such averages should be used in the classification of young late-type sources. An H-R diagram is constructed for the low-mass population in IC 348 (K6-M8). The presumably coeval components of the young quadruple system GG~Tau (White et al.) and the locus of stars in IC 348 are used as empirical isochrones to test the theoretical evolutionary models. For the models of Baraffe et al., an adjustment of the temperature scale to progressively warmer temperatures at later M types, intermediate between dwarfs and giants, brings all components of GG~Tau onto the same model isochrone and gives the population of IC 348 a constant age and age spread as a function of mass. When other observational constraints are considered, such as the dynamical masses of GM~Aur, DM~Tau, and GG~Tau~A, the models of Baraffe et al. are the most consistent with observations of young systems. With compatible temperature scales, the models of both D'Antona & Mazzitelli and Baraffe et al. suggest that the hydrogen burning mass limit occurs near M6 at ages of <10 Myr. Thus, several likely brown dwarfs are discovered in this study of IC 348, with masses down to ~20-30 M_J.

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arXiv1998

Near-Infrared Spectroscopy of Photodissociation Regions: The Orion Bar and Orion S

To test the PDR model spectra under the physical conditions present in Orion, we have obtained moderate-resolution (R=3000) J-, H-, and K-band long-slit spectra of the Orion bar and Orion S regions These observations provide conclusive evidence for the PDR origin (rather than shocks) of the H_2 emission in each of these regions and demonstrate significant departures in the observed chemical structure from that predicted by models of homogeneous PDRs.

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