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Register a new userStellar and Circumstellar Properties of Class I Protostars
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We present a study of the stellar and circumstellar properties of Class I sources using low-resolution (R~1000) near-infrared K- and L-band spectroscopy. We measure prominent spectral lines and features in 8 objects and use fits to standard star spectra to determine spectral types, visual extinctions, K-band excesses, and water ice optical depths. Four of the seven systems studied are close binary pairs; only one of these systems, Haro 6-10, was angularly resolvab le. For certain stars some properties found in our analysis differ substantially from published values; we analyze the origin of these differences. We determine extinction to each source using three different methods and compare and discuss the resulting values. One hypothesis that we were testing, that extinction dominates over the K-band excess in obscuration of the stellar photospheric absorption lines, appears not to be true. Accretion luminosities and mass accretion rates calculated for our targets are highly uncertain, in part the result of our inexact knowledge of extinction. For the six targets we were able to place on an H-R diagram, our age estimates, <2 Myr, are somewhat younger than those from comparable studies. Our results underscore the value of low-resolution spectroscopy in the study of protostars and their environments; however, the optimal approach to the study of Class I sources likely involves a combination of high- and low-resolution near-infrared, mid-infrared, and millimeter wavelength observations. Accurate and precise measurements of extinction in Class I protostars will be key to improving our understanding of these objects.
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We investigate the possibility of the growth of magnetorotational instability (MRI) in disks around Class 0 protostars. We construct a disk model and calculate the chemical reactions of neutral and charged atoms, molecules and dust grains to derive the abundance of each species and the ionization degree of the disk. Then, we estimate the diffusion coefficients of non-ideal magnetohydrodynamics effects such as ohmic dissipation, ambipolar diffusion and the Hall effect. Finally, we evaluate the linear growth rate of MRI in each area of the disk. We investigate the effect of changes in the strength and direction of the magnetic field in our disk model and we adopt four different dust models to investigate the effect of dust size distribution on the diffusion coefficients. Our results indicate that an MRI active region possibly exists with a weak magnetic field in a region far from the protostar where the Hall effect plays a role in the growth of MRI. On the other hand, in all models the disk is stable against MRI in the region within $<20$ au from the protostar on the equatorial plane. Since the size of the disks in the early stage of star formation is limited to $lesssim 10-$$20$ au, it is difficult to develop MRI-driven turbulence in such disks.
(Abridged) High dispersion spectrographs on large aperture telescopes have recently allowed observers to study the stellar and accretion properties of deeply embedded young stars, commonly referred to as Class I stars. We summarize these newly determined properties and compare them with observations of more optically revealed Class II (T Tauri) stars. Class I stars have spectral types and stellar luminosities similar to those of Class II stars, suggesting similar masses and ages. Estimates of stellar luminosity and age, however, are especially uncertain given the large extinctions, scattered light emission and continuum excesses typical of Class I stars. Several candidate Class I brown dwarfs are identified. Class I stars appear to rotate more rapidly than T Tauri stars, by roughly a factor of 2. Likewise, Class I disk accretion rates are only a factor of two larger than those of T Tauri stars, less than the mass infall rates predicted by envelope models by 1-2 orders of magnitude. In at least a few cases the discrepancy appears to be caused by T Tauri stars being misclassified as Class I stars because of their edge-on disk orientation. Stars where the envelope density and infall velocity have been determined directly and unambiguously imply that stellar mass is not acquired in a steady-state fashion, but instead through brief outbursts of enhanced accretion. If some Class I stars are in fact as old as T Tauri stars, replenishment may be necessary to sustain the long-lived envelopes, possibly via continued dynamical interactions with cloud material.
Our general understanding of multiple star and planet formation is primarily based on observations of young multiple systems in low density regions like Tau-Aur and Oph. Since many, if not most, of the stars are born in clusters, observational constraints from young binaries in those environments are fundamental for understanding both the formation of multiple systems and planets in multiple systems throughout the Galaxy. We build upon the largest survey for young binaries in the Orion Nebula Cluster (ONC) which is based on Hubble Space Telescope observations to derive both stellar and circumstellar properties of newborn binary systems in this cluster environment. We present Adaptive Optics spatially-resolved JHKL-band photometry and K-band R$sim$,5000 spectra for a sample of 8 ONC binary systems from this database. We characterize the stellar properties of binary components and obtain a census of protoplanetary disks through K-L color excess. For a combined sample of ONC binaries including 7 additional systems with NIR spectroscopy from the literature, we derive mass ratio and relative age distributions. We compare the stellar and circumstellar properties of binaries in ONC with those in Tau-Aur and Oph from samples of binaries with stellar properties derived for each component from spectra and/or visual photometry and with a disk census obtained through K-L color excess. The mass ratio distribution of ONC binaries is found to be indistinguishable from that of Tau-Aur and, to some extent, to that of Oph in the separation range 85-560,AU and for primary mass in the range 0.15 to 0.8,M$_{sun}$.A trend toward a lower mass ratio with larger separation is suggested in ONC binaries which is not seen in Tau-Aur binaries.The components of ONC binaries are found to be significantly more coeval than the overall ONC population and as coeval as components of binaries in Tau-Aur and Oph[...]
We present near-infrared K-band spectra for a sample of 7 Class 0 protostars in the Perseus and Orion star-forming regions. We detect Br gamma, CO overtone, and H2 emission, features that probe the near circumstellar environment of the protostar and reveal evidence of magnetospheric accretion, a hot inner disk atmosphere, and outflows, respectively. Comparing the properties of these features with those of Class I sources from the literature, we find that their Br gamma and CO emission are generally consistent in strength and velocity width. The Br gamma line profiles are broad and centrally peaked, with FWHMs of 200 km/s and wings extending to 300 km/s. The line ratios of our H2 emission features, which are spatially extended for some sources, are consistent with shock excitation and indicate the presence of strong jets or a disk wind. Within our small sample, the frequency of CO band emission (67%) is high relative to that of Class I samples (15%), indicating that Class 0s have high inner disk accretion rates, similar to those of the most actively accreting Class I sources. Collectively, our results suggest that Class 0 sources have similar accretion mechanisms to the more evolved classes, with strong organized stellar magnetic fields established at the earliest observable stage of evolution.
With the Stratospheric Observatory for Infrared Astronomy (SOFIA) routinely operating science flights, we demonstrate that observations with the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST) can provide reliable estimates of the internal luminosities, $L_{rm int}$, of protostars. We have developed a technique to estimate $L_{rm int}$ using a pair of FORCAST filters: one short-wavelength filter centered within 19.7-25.3 $mu$m, and one long-wavelength filter within 31.5-37.1 $mu$m. These $L_{rm int}$ estimates are reliable to within 30-40% for 67% of protostars and to within a factor of 2.3-2.6 for 99% of protostars. The filter pair comprised of F25.3$mu$m and F37.1$mu$m achieves the best sensitivity and most constrained results. We evaluate several assumptions that could lead to systematic uncertainties. The OH5 dust opacity matches observational constraints for protostellar environments best, though not perfectly; we find that any improved dust model will have a small impact of 5-10% on the $L_{rm int}$ estimates. For protostellar envelopes, the TSC84 model yields masses that are twice those of the Ulrich model, but we conclude this mass difference does not significantly impact results at the mid-infrared wavelengths probed by FORCAST. Thus, FORCAST is a powerful instrument for luminosity studies targeting newly discovered protostars or suspected protostars lacking detections longward of 24 $mu$m. Furthermore, with its dynamic range and greater angular resolution, FORCAST may be used to characterize protostars that were either saturated or merged with other sources in previous surveys using the Spitzer Space Telescope or Herschel Space Observatory.
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