The Spectroscopically Determined Substellar Mass Function of the Orion Nebula Cluster

We present a spectroscopic study of candidate brown dwarf members of the Orion Nebula Cluster (ONC). We obtained new J- and/or K-band spectra of ~100 objects within the ONC which are expected to be substellar based on their K,(H-K) magnitudes and colors. Spectral classification in the near-infrared of young low mass objects is described, including the effects of surface gravity, veiling due to circumstellar material, and reddening. From our derived spectral types and existing near-infrared photometry we construct an HR diagram for the cluster. Masses are inferred for each object and used to derive the brown dwarf fraction and assess the mass function for the inner 5.'1 x 5.'1 of the ONC, down to ~0.02 solar masses. The derived logarithmic mass function rises to a peak at ~0.2 solar masses, similar to previous IMF determinations derived from purely photometric methods, but falls off more sharply at the hydrogen-burning limit before leveling through the substellar regime. We compare the mass function derived here for the inner ONC to those presented in recent literature for the sparsely populated Taurus cloud members and the rich cluster IC 348. We find good agreement between the shapes and peak values of the ONC and IC 348 mass distributions, but little similarity between the ONC and Taurus results.Comment: Accepted for Publication in Apj. Added Erratu

A Large-Area Search for Low Mass Objects in Upper Scorpius I: The Photometric Campaign and New Brown Dwarfs

We present a wide-field photometric survey covering ~200 deg^2 toward the Upper Scorpius OB association. Data taken in the R and I bands with the Quest-2 camera on the Palomar 48-inch telescope were combined with the 2MASS JHK survey and used to select candidate pre-main sequence stars. Follow-up spectroscopy with the Palomar 200-inch telescope of 62 candidate late-type members identified 43 stars that have surface gravity signatures consistent with association membership. From the optical/near-infrared photometry and derived spectral types we construct an HR diagram for the new members and find 30 likely new brown dwarfs, nearly doubling the known substellar population of the Upper Scorpius OB association. Continuation of our spectroscopic campaign should reveal hundreds on new stellar and substellar members.Comment: 36 pages including 14 figures and 2 tables. Accepted for publication in A

The Mass-Radius(-Rotation?) Relation for Low-Mass Stars

The fundamental properties of low-mass stars are not as well understood as those of their more massive counterparts. The best method for constraining these properties, especially masses and radii, is to study eclipsing binary systems, but only a small number of late-type (M0 or later) systems have been identified and well-characterized to date. We present the discovery and characterization of six new M dwarf eclipsing binary systems. The twelve stars in these eclipsing systems have masses spanning 0.38-0.59 Msun and orbital periods of 0.6--1.7 days, with typical uncertainties of ~0.3% in mass and 0.5--2.0% in radius. Combined with six known systems with high-precision measurements, our results reveal an intriguing trend in the low-mass regime. For stars with M=0.35-0.80 Msun, components in short-period binary systems (P<1 day; 12 stars) have radii which are inflated by up to 10% (mean=4.8+/-1.0%) with respect to evolutionary models for low-mass main-sequence stars, whereas components in longer-period systems (>1.5 days; 12 stars) tend to have smaller radii (mean=1.7+/-0.7%). This trend supports the hypothesis that short-period systems are inflated by the influence of the close companion, most likely because they are tidally locked into very high rotation speeds that enhance activity and inhibit convection. In summary, very close binary systems are not representative of typical M dwarfs, but our results for longer-period systems indicate that the evolutionary models are broadly valid in the M~0.35-0.80 Msun regime.Comment: Accepted to ApJ; 21 pages, 10 figures, 8 tables in emulateapj format. The full contents of Table 4 are included in the submission as tab4.tx

The Mass Function of Newly Formed Stars (Review)

The topic of the stellar "original mass function" has a nearly 50 year history,dating to the publication in 1955 of Salpeter's seminal paper. In this review I discuss the many more recent results that have emerged on the initial mass function (IMF), as it is now called, from studies over the last decade of resolved populations in star forming regions and young open clusters.Comment: 9 pages, 1 figure; to appear in "The Dense Instellar Medium in Galaxies -- 4'th Cologne-Bonn-Zermatt-Symposium" editted by S. Pfalzner, C. Kramer, C. Straubmeier and A. Heithausen, Springer-Verlag (2004

Searching for Young M Dwarfs with GALEX

The census of young moving groups in the solar neighborhood is significantly incomplete in the low-mass regime. We have developed a new selection process to find these missing members based on the GALEX All-Sky Imaging Survey. For stars with spectral types >K5 and younger than 300~Myr, we show that near-UV and far-UV emission is greatly enhanced above the quiescent photosphere, analogous to the enhanced X-ray emission of young low-mass stars seen by ROSAT but detectable to much larger distances with GALEX. By combining GALEX data with optical (HST Guide Star Catalog) and near-IR (2MASS) photometry, we identified an initial sample of 34 young M dwarf candidates in a 1000 sq.~deg.~region around the 10-Myr TW Hydra Association (TWA). Low-resolution spectroscopy of 30 of these found 16 which had H_alpha in emission, which were then followed-up at high resolution to search for spectroscopic evidence of youth and to measure radial velocities. Four objects have low surface gravities, photometric distances and space motions consistent with TWA, but the non-detection of Li indicates they may be too old to belong to this moving group. One object (M3.5, 93 pc) appears to be the first known accreting low-mass member of the 15~Myr Lower Centaurus Crux OB association. Two objects exhibit all the characteristics of the known TWA members, and thus we designate them as TWA 31 (M4.2, 110 pc) and TWA 32 (M6.3, 53 pc). TWA 31 shows extremely broad (447 km/s) H_alpha emission, making it the sixth member of TWA found to have ongoing accretion. TWA 32 is resolved into a 0.6" binary in Keck laser guide star adaptive optics imaging. Our search should be sensitive down to spectral types of at least M4-M5 in TWA and thus the small numbers of new member is puzzling. This may indicate TWA has an atypical mass function or that the presence of Li may be too restrictive a criteria for selecting young low-mass stars.Comment: Accepted to Ap

Are There Age Spreads in Star Forming Regions?

A luminosity spread at a given effective temperature is ubiquitously seen in the Hertzsprung-Russell (HR) diagrams of young star forming regions and often interpreted in terms of a prolonged period (>=10 Myr) of star formation. I review the evidence that the observed luminosity spreads are genuine and not caused by astrophysical sources of scatter. I then address whether the luminosity spreads necessarily imply large age spreads, by comparing HR diagram ages with ages from independent clocks such as stellar rotation rate, the presence of circumstellar material and lithium depletion. I argue that whilst there probably is a true luminosity dispersion, there is little evidence to support age spreads larger than a few Myr. This paradox could be resolved by brief periods of rapid accretion during the class I pre main-sequence phase.Comment: To appear in the proceedings of JENAM10: Star Clusters in the Era of Large Surveys, 8 page

Near and Mid-IR Photometry of the Pleiades, and a New List of Substellar Candidate Members

We make use of new near and mid-IR photometry of the Pleiades cluster in order to help identify proposed cluster members. We also use the new photometry with previously published photometry to define the single-star main sequence locus at the age of the Pleiades in a variety of color-magnitude planes. The new near and mid-IR photometry extend effectively two magnitudes deeper than the 2MASS All-Sky Point Source catalog, and hence allow us to select a new set of candidate very low mass and sub-stellar mass members of the Pleiades in the central square degree of the cluster. We identify 42 new candidate members fainter than Ks =14 (corresponding to 0.1 Mo). These candidate members should eventually allow a better estimate of the cluster mass function to be made down to of order 0.04 solar masses. We also use new IRAC data, in particular the images obtained at 8 um, in order to comment briefly on interstellar dust in and near the Pleiades. We confirm, as expected, that -- with one exception -- a sample of low mass stars recently identified as having 24 um excesses due to debris disks do not have significant excesses at IRAC wavelengths. However, evidence is also presented that several of the Pleiades high mass stars are found to be impacting with local condensations of the molecular cloud that is passing through the Pleiades at the current epoch.Comment: Accepted to ApJS; data tables and embedded-figure version available at http://spider.ipac.caltech.edu/staff/stauffer/pleiades07

Thirty New Low-Mass Spectroscopic Binaries

As part of our search for young M dwarfs within 25 pc, we acquired high-resolution spectra of 185 low-mass stars compiled by the NStars project that have strong X-ray emission. By cross-correlating these spectra with radial velocity standard stars, we are sensitive to finding multi-lined spectroscopic binaries. We find a low-mass spectroscopic binary fraction of 16% consisting of 27 SB2s, 2 SB3s and 1 SB4, increasing the number of known low-mass SBs by 50% and proving that strong X-ray emission is an extremely efficient way to find M-dwarf SBs. WASP photometry of 23 of these systems revealed two low-mass EBs, bringing the count of known M dwarf EBs to 15. BD -22 5866, the SB4, is fully described in Shkolnik et al. 2008 and CCDM J04404+3127 B consists of a two mid-M stars orbiting each other every 2.048 days. WASP also provided rotation periods for 12 systems, and in the cases where the synchronization time scales are short, we used P_rot to determine the true orbital parameters. For those with no P_rot, we use differential radial velocities to set upper limits on orbital periods and semi-major axes. More than half of our sample has near-equal-mass components (q > 0.8). This is expected since our sample is biased towards tight orbits where saturated X-ray emission is due to tidal spin-up rather than stellar youth. Increasing the samples of M dwarf SBs and EBs is extremely valuable in setting constraints on current theories of stellar multiplicity and evolution scenarios for low-mass multiple systems.Comment: Accepted to Ap

Fifty Years of IMF Variation: The Intermediate-Mass Stars

I track the history of star count estimates of the Milky Way field star and open cluster IMFs, concentrating on the neglected mass range from 1 to 15 M${_\odot}$. The prevalent belief in a universal IMF appears to be without basis for this mass range. Two recent estimates of the field star IMF using different methods and samples give values of the average logarithmic slope $\Gamma$ between -1.7 and -2.1 in the mass range 1.1 to 4 M${_\odot}$. Two older estimates between 2 and 15 M${_\odot}$ disagree severely; the field IMF in this range is essentially unknown from star counts. Variations in $\Gamma$ among open cluster IMFs in this mass range have not decreased despite numerous detailed studies, even for studies using homogeneous data and reduction procedures and including only clusters with a significant mass range. These cluster variations \textit{might} be due to the combined effects of sampling, systematic errors, stellar evolution uncertainties, dynamical evolution, and unresolved binaries. If so, then the cluster data are consistent with a universal IMF, but are also consistent with sizeable variations. The cluster data do not allow an estimate of an average IMF or $\Gamma$ because the average depends on the choice of weighting procedure and other effects. If the spread in cluster IMFs is in excess of the effects listed above, real IMF variations must occur that do not depend much on physical conditions explored so far. The complexity of the star formation process seen in observations and simulations suggests that large realization-to-realization differences might be expected, in which case an individual cluster IMF would be in part the product of evolutionary contingency in star formation, and the function of interest is the probability distribution of IMF parameters.Comment: 18 pages, including 4 figures: invited talk presented at the conference on "IMF@50: The Stellar Initial Mass Function Fifty Years Later" held at Abbazia di Spineto, Siena, Italy, May 2004; to be published by Kluwer Academic Publishers, edited by E. Corbelli, F. Palla, and H. Zinnecke