Near-Infrared Photometric Variability of Stars Toward the Orion A Molecular Cloud

We present an analysis of J, H, and K time series photometry obtained with the southern 2MASS telescope over a 0.84 x 6 deg^2 region centered near the Orion Nebula Cluster. These data are used to establish the near-infrared variability properties of pre-main-sequence stars in Orion on time scales of 1-36 days, 2 months, and 2 years. A total of 1235 variable stars are identified, ~93% of which are associated with the Orion A molecular cloud. The variable stars exhibit a diversity of photometric behavior with time, including cyclic fluctuations, aperiodic day-to-day fluctuations, eclipses, slow drifts in brightness over one month, colorless variability, stars that become redder as they fade, and stars that become bluer as they fade. We examine rotational modulation of cool and hot star spots, variable obscuration from an inner circumstellar disk, and changes in the mass accretion rate and other properties in a circumstellar disk as possible origins of the variability. Cool spots can explain the variability characteristics in 56-77% of the stars, while the properties of the photometric fluctuations are more consistent with hot spots or extinction changes in at least 23% of the stars, and with variations in the disk mass accretion rate or inner disk radius in 1% of our sample. However, differences between the details of the observations and the details of variability predicted these models suggest either that another variability mechanism not considered here may be operative, or that the observed variability represents the net results of several of these phenomena. Analysis of the star count data indicates that the ONC is part of a larger area of enhanced stellar surface density which extends over a 0.4 x 2.4 deg^2 (3.4 x 20 pc^2) region containing 2700 stars brighter than K=14. (abridged version)Comment: 75 pages with 27 figures; to appear in AJ; see also http://www.astro.caltech.edu/~jmc/variables/orio

Periodic Photometric Variability in the Becklin-Neugebauer Object

The Becklin-Neugebauer (BN) object in the Orion Nebula Cluster (ONC) is a well-studied optically invisible, infrared-bright young stellar object, thought to be an intermediate-mass protostar. We report here that BN exhibited nearly-sinusoidal periodic variability at the near-infrared H- and Ks-bands during a one month observing campaign in 2000 March/April. The period was 8.28 days and the peak-to-peak amplitude ~0.2 mag. Plausible mechanisms for producing the observed variability characteristics are explored.Comment: Accepted by ApJ Letter

Constraining the Sub-AU-Scale Distribution of Hydrogen and Carbon Monoxide Gas around Young Stars with the Keck Interferometer

We present Keck Interferometer observations of T Tauri and Herbig Ae/Be stars with a spatial resolution of a few milliarcseconds and a spectral resolution of ~2000. Our observations span the K-band, and include the Br gamma transition of Hydrogen and the v=2-0 and v=3-1 transitions of carbon monoxide. For several targets we also present data from Keck/NIRSPEC that provide higher spectral resolution, but a seeing-limited spatial resolution, of the same spectral features. We analyze the Br gamma emission in the context of both disk and infall/outflow models, and conclude that the Br gamma emission traces gas at very small stellocentric radii, consistent with the magnetospheric scale. However some Br gamma-emitting gas also seems to be located at radii of >0.1 AU, perhaps tracing the inner regions of magnetically launched outflows. CO emission is detected from several objects, and we generate disk models that reproduce both the KI and NIRSPEC data well. We infer the CO spatial distribution to be coincident with the distribution of continuum emission in most cases. Furthermore the Br gamma emission in these objects is roughly coincident with both the CO and continuum emission. We present potential explanations for the spatial coincidence of continuum, Br gamma, and CO overtone emission, and explore the implications for the low occurrence rate of CO overtone emission in young stars. Finally, we provide additional discussion of V1685 Cyg, which is unusual among our sample in showing large differences in emitting region size and spatial position as a function of wavelength.Comment: Accepted for publication in MNRA

The spatial distribution of substellar objects in IC348 and the Orion Trapezium Cluster

Aims: Some theoretical scenarios suggest the formation of brown dwarfs as ejected stellar embryos in star-forming clusters. Such a formation mechanism can result in different spatial distributions of stars and substellar objects. We aim to investigate the spatial structure of stellar and substellar objects in two well sampled and nearby embedded clusters, namely IC348 and the Orion Trapezium Cluster (OTC) to test this hypothesis. Methods:Deep near-infrared K-band data complete enough to sample the substellar population in IC348 and OTC are obtained from the literature. The spatial distribution of the K-band point sources is analysed using the Minimum Spanning Tree (MST) method. The Q parameter and the spanning trees are evaluated for stellar and substellar objects as a function of cluster core radius R$_c$. Results: The stellar population in both IC348 and OTC display a clustered distribution whereas the substellar population is distributed homogeneously in space within twice the cluster core radius. Although the substellar objects do not appear to be bound by the cluster potential well, they are still within the limits of the cluster and not significantly displaced from their birth sites. Conclusions: The spatially homogeneous distribution of substellar objects is best explained by assuming higher initial velocities, distributed in a random manner and going through multiple interactions. The overall spatial coincidence of these objects with the cluster locations can be understood if these objects are nevertheless travelling slowly enough so as to feel the gravitational effect of the cluster. The observations support the formation of substellar objects as ``ejected stellar embryos''. Higher ejection velocities are necessary but net spatial displacements may not be necessary to explain the observational data.Comment: 4 pages. Accepted by A&A Letter

The low-mass Initial Mass Function in the Orion Nebula cluster based on HST/NICMOS III imaging

We present deep HST/NICMOS Camera 3 F110W and F160W imaging of a 26'x33', corresponding to 3.1pcx3.8pc, non-contiguous field towards the Orion Nebula Cluster (ONC). The main aim is to determine the ratio of low--mass stars to brown dwarfs for the cluster as a function of radius out to a radial distance of 1.5pc. The sensitivity of the data outside the nebulous central region is F160W=21.0 mag, significantly deeper than previous studies of the region over a comparable area. We create an extinction limited sample and determine the ratio of low-mass stars (0.08-1Msun) to brown dwarfs (0.02-0.08Msun and 0.03-0.08Msun) for the cluster as a whole and for several annuli. The ratio found for the cluster within a radius of 1.5pc is R(02)=N(0.08-1Msun)/N(0.02-0.08Msun)=1.7+-0.2, and R(03)=N(0.08-1Msun)/N(0.03-0.08Msun)=2.4+-0.2, after correcting for field stars. The ratio for the central 0.3pcx0.3pc region down to 0.03Msun was previously found to be R(03)=3.3+0.8-0.7, suggesting the low-mass content of the cluster is mass segregated. We discuss the implications of a gradient in the ratio of stars to brown dwarfs in the ONC in the context of previous measurements of the cluster and for other nearby star forming regions. We further discuss the current evidence for variations in the low-mass IMF and primordial mass segregation.Comment: Accepted to A&

Near-Infrared Photometric Variability of Stars Toward the Chamaeleon I Molecular Cloud

We present the results of a J, H, and K_s photometric monitoring campaign of a 0.72 x 6 sq deg. area centered on the Chamaeleon I star forming region. Data were obtained on 15 separate nights over a 4 month time interval using the 2MASS South telescope. Out of a total of 34,539 sources brighter than the photometric completeness limits (J=16.0, H=15.2, K_s=14.8), 95 exhibit near-infrared variability in one or more bands. The variables can be grouped into a population of bright, red objects that are associated with the Chamaeleon I association, and a population of faint, blue variables that are dispersed over the full 6 deg of the survey and are likely field stars or older pre-main-sequence stars unrelated to the present-day Chamaeleon I molecular cloud. Ten new candidate members of Chamaeleon I, including 8 brown dwarf candidates, have been identified based on variability and/or near-infrared excess emission in the J-H vs. H-K_s color-color-diagram. We also provide a compendium of astrometry and J, H, and K_s photometry for previously identified members and candidate members of Chamaeleon I.Comment: To appear in AJ; see http://www.astro.caltech.edu/~jmc/variables/cham1

VIRIS: A Visual-Infrared Imaging System for the Lick Observatory 1-M Telescope

We describe a system in use at the Lick Observatory 1-m Nickel telescope for near-simultaneous imaging at optical and near-infrared wavelengths. The combined availability of a CCD and a NICMOS-3 camera makes the system well-suited for photometric monitoring from 0.5-2.2 microns of a variety of astrophysical objects. Our science program thus far has concentrated on studying variability trends in young stellar objects.Comment: 11 pages LaTex, 3 Postscript figure, Pub. Astr. Soc. Pac. 1998, in pres

Evidence for Mass-dependent Circumstellar Disk Evolution in the 5 Myr Old Upper Scorpius OB Association

We present 4.5, 8, and 16 µm photometry from the Spitzer Space Telescope for 204 stars in the Upper Scorpius OB association. The data are used to investigate the frequency and properties of circumstellar disks around stars with masses between ~0.1 and 20 M_☉ at an age of ~5 Myr. We identify 35 stars that have emission at 8 or 16 µm in excess of the, stellar photosphere. The lower mass stars (~0.1–1.2M_☉) appear surrounded by primordial optically thick disks based on, the excess emission characteristics. Starsmoremassive than ~1.8M_☉ have lower fractional excess luminosities suggesting, that the inner ~10 AU of the disk has been largely depleted of primordial material. None of the G and F stars (~1.2–1.8 M_☉) in our sample have an infrared excess at wavelengths ≤16 µm. These results indicate that the mechanisms for, dispersing primordial optically thick disks operate less efficiently, on average, for low-mass stars, and that longer timescales are available for the buildup of planetary systems in the terrestrial zone for stars with masses ≾1 M_☉