The Nature of Class I Sources: Periodic Variables in Orion

We present a quantitative, empirically based argument that at least some Class I sources are low-mass, pre-main-sequence stars surrounded by spatially extended envelopes of dusty gas. The source luminosity arises principally from stellar gravitational contraction, as in optically visible pre-main-sequence stars that lack such envelopes. We base our argument on the fact that some Class I sources in Orion and other star-forming regions have been observed by Spitzer to be periodic variables in the mid-infrared, and with periods consistent with T Tauri rotation rates. Using a radiative transfer code, we construct a variety of dust envelopes surrounding rotating, spotted stars, to see if an envelope that produces a Class I SED at least broadly matches the observed modulations in luminosity. Acceptable envelopes can either be spherical or flattened, and may or may not have polar cavities. The key requirement is that they have a modest equatorial optical depth at the Spitzer waveband of 3.6 ${\mu}$m, typically ${\tau_{3.6}}$ ${\approx}$ 0.6. The total envelope mass, based on this limited study, is at most about 0.1 $\text{M}_{\odot}$, less than a typical stellar mass. Future studies should focus on the dynamics of the envelope, to determine whether material is actually falling onto the circumstellar disk.Comment: 17 pages, 16 figures. Additional light curve figures and associated data table referred to in Appendix B available as online dat

A Spectroscopic Survey of Subarcsecond Binaries in the Taurus-Auriga Dark Cloud with the Hubble Space Telescope

We report the results of a spectroscopic survey of 20 close T Tauri binaries in the Taurus-Auriga dark cloud where the separations between primaries and their secondaries are less than the typical size of a circumstellar disk around a young star. Analysis of low-resolution and medium-resolution STIS spectra yields the stellar luminosities, reddenings, ages, masses, mass accretion rates, IR excesses, and emission line luminosities for each star in each pair. We examine the ability of IR color excesses, H-alpha equivalent widths, [O I] emission, and veiling to distinguish between weak emission and classical T Tauri stars. Four pairs have one cTTs and one wTTs; the cTTs is the primary in three of these systems. This frequency of mixed pairs among the close T Tauri binaries is similar to the frequency of mixed pairs in wider young binaries. Extinctions within pairs are usually similar; however, the secondary is more heavily reddened than the primary in some systems, where it may be viewed through the primary's disk. Mass accretion rates of primaries and secondaries are strongly correlated, and H-alpha luminosities, IR excesses, and ages also correlate within pairs. Primaries tend to have somewhat larger accretion rates than their secondaries do, and are typically slightly older than their secondaries according to three different sets of modern pre-main-sequence evolutionary tracks. Age differences for XZ Tau and FS Tau, systems embedded in reflection nebulae, are striking; the secondary in each pair is less massive but more luminous than the primary. The stellar masses of the UY Aur and GG Tau binaries measured from their rotating molecular disks are about 30% larger than the masses inferred from the spectra and evolutionary tracks

Giant Impacts and Debris Disk Morphology

Certain debris disks have non-axisymmetric shapes in scattered light which are unexplained. The appearance of a disk depends on how its constituent Keplerian ellipses are arranged. The more the ellipses align apsidally, the more non-axisymmetric the disk. Apsidal alignment is automatic for fragments released from a catastrophic collision between solid bodies. We synthesize scattered light images, and thermal emission images, of such giant impact debris. Depending on the viewing geometry, and if and how the initial apsidal alignment is perturbed, the remains of a giant impact can appear in scattered light as a one-sided or two-sided "fork", a lopsided "needle", or a set of "double wings". Double wings are difficult to reproduce in other scenarios involving gravitational forcing or gas drag, which do not align orbits as well. We compare our images with observations and offer a scorecard assessing whether the scattered light asymmetries in HD 15115, HD 32297, HD 61005, HD 111520, HD 106906, beta Pic, and AU Mic are best explained by giant impacts, gravitational perturbations, or sculpting by the interstellar medium.Comment: Final ApJ-proofed version with updated references to summary Table 1. Animations available at https://github.com/joshwajones/jones_etal_animations

Mid-Infrared Observations of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds

We have obtained new mid-infrared observations of 65 Class I/Flat-Spectrum (F.S.) objects in the Perseus, Taurus, Chamaeleon I/II, Rho Ophiuchi, and Serpens dark clouds. We detected 45/48 (94%) of the single sources, 16/16 (100%) of the primary components, and 12/16 (75%) of the secondary/triple components of the binary/multiple objects surveyed. The composite spectral energy distributions (SEDs) for all of our sample sources are either Class I or F.S., and, in 15/16 multiple systems, at least one of the individual components displays a Class I or F.S. spectral index. However, the occurrence of mixed pairings, such as F.S. with Class I, F.S. with Class II, and, in one case, F.S. with Class III, is surprisingly frequent. Such behaviour is not consistent with that of multiple systems among T Tauri stars (TTS), where the companion of a classical TTS also tends to be a classical TTS, although other mixed pairings have been previously observed among Class II objects. Based on an analysis of the spectral indices of the individual binary components, there appears to be a higher proportion of mixed Class I/F.S. systems (65-80%) than that of mixed Classical/Weak-Lined TTS (25-40%), demonstrating that the envelopes of Class I/ F.S. systems are rapidly evolving during this evolutionary phase. We report the discovery of a steep spectral index secondary companion to ISO-ChaI 97, detected for the first time via our mid-infrared observations. In our previous near- infrared imaging survey of binary/multiple Class I/F.S. sources, ISO-ChaI 97 appeared to be single. With a spectral index of Alpha >= 3.9, the secondary component of this system is a member of a rare class of very steep spectral index objects, those with Alpha > 3. Only three such objects have previously been reported, all of which are either Class 0 or Class I.Comment: 31 pages, 4 figures, 6 table

The Role of Mass and Environment in Multiple Star Formation: A 2MASS Survey of Wide Multiplicity in Three Young Associations

We present the results of a search for wide binary systems among 783 members of three nearby young associations: Taurus-Auriga, Chamaeleon I, and two subgroups of Upper Scorpius. Near-infrared (JHK) imagery from 2MASS was analyzed to search for wide (1-30"; ~150-4500 AU) companions to known association members, using color-magnitude cuts to reject likely background stars. We identify a total of 131 candidate binary companions with colors consistent with physical association, of which 39 have not been identified previously in the literature. Our results suggest that the wide binary frequency is a function of both mass and environment, with significantly higher frequencies among high-mass stars than lower-mass stars and in the T associations than in the OB association. We discuss the implications for wide binary formation and conclude that the environmental dependence is not a direct result of stellar density or total association mass, but instead might depend on another environmental parameter like the gas temperature. We also analyze the mass ratio distribution as a function of mass and find that it agrees with the distribution for field stars to within the statistical uncertainties. The binary populations in these associations generally follow the empirical mass-maximum separation relation observed for field binaries, but we have found one candidate low-mass system (USco-160611.9-193532; Mtot~0.4 Msun) which has a projected separation (10.8"; 1550 AU) much larger than the suggested limit for its mass. (Abridged)Comment: Accepted to ApJ; 27 pages in emulateapj format. The full version of table 2 can be downloaded via http://www.astro.caltech.edu/~alk/tab2.pdf (PDF) or http://www.astro.caltech.edu/~alk/tab2.txt (text

Observational Constraints on the Formation and Evolution of Binary Stars

We present a high spatial resolution UV to NIR survey of 44 young binary stars in Taurus with separations of 10-1000 AU. The primary results include: (1) The relative ages of binary star components are more similar than the relative ages of randomly paired single stars, supporting coeval formation. (2) Only one of the companion masses is substellar, and hence the apparent overabundance of T Tauri star companions relative to main-sequence star companions can not be explained by a wealth of substellar secondaries that would have been missed in main-sequence surveys. (3) Roughly 10% of T Tauri binary star components have very red NIR colors (K-L > 1.4) and unusually high mass accretion rates. This phenomenon does not appear to be restricted to binary systems, however, since a comparable fraction of single T Tauri stars exhibit the same properties. (4) Although the disk lifetimes of single stars are roughly equal to their stellar ages, the disk lifetimes of binary stars are an order of magnitude less than their ages. (5) The accretion rates for both single and binary T Tauri stars appear to be moderately mass dependent. (6) Although most classical T Tauri star binaries retain both a circumprimary and a circumsecondary disk, there are several systems with only a circumprimary disk. Together with the relative accretion rates, this suggests that circumprimary disks survive longer, on average, than circumsecondary disks. (7) The disk lifetimes, mass ratios, and relative accretion signatures of the closest binaries (10-100 AU) suggest that they are being replenished from a circumbinary reservoir with low angular momentum. Overall, these results support fragmentation as the dominant binary star formation mechanism.Comment: 67 pages including 11 figures, LaTeX2e, accepted for publication in Ap

Near-Infrared Imaging Polarimetry of Young Stellar Objects in rho-Ophiuchi

The results of a near-infrared (J H K LP) imaging linear polarimetry survey of 20 young stellar objects (YSOs) in rho Ophiuchi are presented. The majority of the sources are unresolved, with K-band polarizations, P_K < 6 per cent. Several objects are associated with extended reflection nebulae. These objects have centrosymmetric vector patterns with polarization discs over their cores; maximum polarizations of P_K > 20 per cent are seen over their envelopes. Correlations are observed between the degree of core polarization and the evolutionary status inferred from the spectral energy distribution. K-band core polarizations >6 per cent are only observed in Class I YSOs. A 3D Monte Carlo model with oblate grains aligned with a magnetic field is used to investigate the flux distributions and polarization structures of three of the rho Oph YSOs with extended nebulae. A rho proportional to r^(-1.5) power law for the density is applied throughout the envelopes. The large-scale centrosymmetric polarization structures are due to scattering. However, the polarization structure in the bright core of the nebula appears to require dichroic extinction by aligned non-spherical dust grains. The position angle indicates a toroidal magnetic field in the inner part of the envelope. Since the measured polarizations attributed to dichroic extinction are usually <10 per cent, the grains must either be nearly spherical or very weakly aligned. The higher polarizations observed in the outer parts of the reflection nebulae require that the dust grains responsible for scattering have maximum grain sizes <=1.05 microns.Comment: 26 pages. Accepted by MNRAS. Available as online early versio

Kinematic evidence for an embedded protoplanet in a circumstellar disc

Discs of gas and dust surrounding young stars are the birthplace of planets. However, direct detection of protoplanets forming within discs has proved elusive to date. We present the detection of a large, localized deviation from Keplerian velocity in the protoplanetary disc surrounding the young star HD163296. The observed velocity pattern is consistent with the dynamical effect of a two Jupiter-mass planet orbiting at a radius $\approx$ 260au from the star.Comment: Accepted for publication to ApJL, 8 pages, 5 figure

Stellar Multiplicity and the IMF: Most Stars Are Single

In this short communication I compare recent findings suggesting a low binary star fraction for late type stars with knowledge concerning the forms of the stellar initial and present day mass functions for masses down to the hydrogen burning limit. This comparison indicates that most stellar systems formed in the galaxy are likely single and not binary as has been often asserted. Indeed, in the current epoch two-thirds of all main sequence stellar systems in the Galactic disk are composed of single stars. Some implications of this realization for understanding the star and planet formation process are briefly mentioned.Comment: Accepted for publication in the Astrophysical Journal Letters, 5 pages, 2 figures. Complete paper can be also obtained at http://www.cfa.harvard.edu/~clada/pubs_html/binaries.htm

A Subarcsecond Companion to the T Tauri Star AS 353B

Adaptive optics imaging of the bright visual T Tauri binary AS 353 with the Subaru Telescope shows that it is a hierarchical triple system. The secondary component, located 5.6" south of AS 353A, is resolved into a subarcsecond binary, AS 353Ba and Bb, separated by 0.24". Resolved spectroscopy of the two close components shows that both have nearly identical spectral types of about M1.5. Whereas AS 353A and Ba show clear evidence for an infrared excess, AS 353Bb does not. We discuss the possible role of multiplicity in launching the large Herbig-Haro flow associated with AS 353A.Comment: AASTeXv5.0, 21 pages, 5 figures, Astronomical Journal, in pres