Probing the close environment of young stellar objects with interferometry

The study of Young Stellar Objects (YSOs) is one of the most exciting topics that can be undertaken by long baseline optical interferometry. The magnitudes of these objects are at the edge of capabilities of current optical interferometers, limiting the studies to a few dozen, but are well within the capability of coming large aperture interferometers like the VLT Interferometer, the Keck Interferometer, the Large Binocular Telescope or 'OHANA. The milli-arcsecond spatial resolution reached by interferometry probes the very close environment of young stars, down to a tenth of an astronomical unit. In this paper, I review the different aspects of star formation that can be tackled by interferometry: circumstellar disks, multiplicity, jets. I present recent observations performed with operational infrared interferometers, IOTA, PTI and ISI, and I show why in the next future one will extend these studies with large aperture interferometers. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemical evolution of circumstellar matter around young stellar objects

Recent observational studies of the chemical composition of circumstellar matter around both high- and low-mass young stellar objects are reviewed. The molecular abundances are found to be a strong function of evolutionary state, but not of system mass or luminosity. The data are discussed with reference to recent theoretical models.     

Near-infrared imaging polarimetry of young stellar objects in ρ Ophiuchi

The results of a near-infrared ( J H K L _P) imaging linear polarimetry survey of 20 young stellar objects (YSOs) in ρ 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 ρ Oph YSOs with extended nebulae. A ρ∝ 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 μm.     

Molecular maser radio emission in the environment of young stellar objects (Review)

Es wird ein kurzer Überblick über die Eigenschaften von Molekülmasern in Sternentstehungsgebieten gegeben. Verschiedene Typen von Quellen, ihre Struktur und Veränderlichkeit werden diskutiert. Eine Erklärung der mit einer Zeitskala von einigen Tagen veränderlichen H₂O-Maser in der Cep-A-Maserquelle innerhalb des Modells der Energiediffusion in einer Maserwolke ist schwierig. Es wird deshalb eine andere Möglichkeit vorgeschlagen und analysiert. Ein H₂O-Maserausbruch kommt möglicherweise durch die Verstärkung eines starken nichtthermischen Radioaus-bruchs von einem magnetisch aktiven Zentralstern (z. B. einem T Tauri-Stern) durch einen anfangs ungesättigten H₂O-Maser zustande. Ein Beobachtungstest wird vorgeschlagen.     

Progress in spectral studies of the FBS blue stellar objects

Spectral observations of 10 FBS blue stellar objects (BSO) with the OHP 1.93-m and BAO 2.6.m telescopes are reported, and overall progress in all the spectral observations and classifications of FBS BSO over 1987–2000 and in the classification of these objects based on all the accessible sources-- in all, 753 out of 1103 objects-- is discussed. Representative slit spectra for the major types of objects are presented and compared with digitized low dispersion spectra from the DFBS. The nature of the FBS objects is examined in terms of advances in the spectral studies. Two-color diagrams are constructed from the SDSS data and are used to find the regions occupied by the various types for further identification of objects of unknown type. Translated from Astrofizika, Vol. 52, No. 1, pp. 85–97 (February 2009).     

Discs and the 10-μm silicate spectra of young stellar objects with non-photospheric continua

Dust emission in the non-photospheric 10-μm continua of HL Tau and Taurus-Elias 7 (Haro6-10, GV Tau) is distinguished from foreground silicate absorption using a simple disc model with radial power-law temperature and mass–density distributions based on the IR–submm model of T Tauri stars by Adams, Lada & Shu with foreground extinction. The resulting 10-μm absorption profiles are remarkably similar to those of the field star Taurus-Elias 16 obtained by Bowey, Adamson & Whittet. The fitted temperature indices are 0.44 (HL Tau) and 0.33 (Elias 7) in agreement with Boss's theoretical models of the 200–300 K region, but lower than those of IR–submm discs (0.5–0.61; Mannings & Emerson); a significant fraction of the modelled 10-μm emission of HL Tau is optically thin, whilst that of Elias 7 is optically thick. We suggest that HL Tau's optically thin component arises from silicate dust within low-density layers above an optically thick disc.     

Gemini observations of disks and jets in young stellar objects and in active galaxies

We present first results from the Near-infrared Integral Field Spectrograph (NIFS) located at Gemini North. For the active galaxies Cygnus A and Perseus A we observe rotationally-supported accretion disks and adduce the existence of massive central black holes and estimate their masses. In Cygnus A we also see remarkable high-excitation ionization cones dominated by photoionization from the central engine. In the T-Tauri stars HV Tau C and DG Tau we see highly-collimated bipolar outflows in the [Fe II] λ 1.644 micron line, surrounded by a slower molecular bipolar outflow seen in the H₂ lines, in accordance with the model advocated by Pyo, T.-S., et al., Astrophys. J. 570, 724 (2002).     

The serendipitous discovery of a stellar ring at mid-infrared wavelengths

We present images and photometry of a stellar ring-like structure G052.4−00.0 discovered during visual inspection of the Spitzer GLIMPSE Atlas. Although G052.4−00.0 may correspond to one of a large variety of sources, it is argued that it is most likely to be associated with an early-type supergiant at a distance of ∼8 kpc from the Sun. Where this is the case, then the radius of the ring would be of the order of ≈0.3 pc, comparable to those of similar structures about other supergiant stars, and the extinction   A _V   would be of the order of ∼21 mag.     

Testing the circumstellar disc hypothesis: a search for H2 outflow signatures from massive young stellar objects with linearly distributed methanol masers

The results of a survey searching for outflows using near-infrared imaging are presented. Targets were chosen from a compiled list of massive young stellar objects associated with methanol masers in linear distributions. Presently, it is a widely held belief that these methanol masers are found in (and delineate) circumstellar accretion discs around massive stars. If this scenario is correct, one way to test the disc hypothesis is to search for outflows perpendicular to the methanol maser distributions. The main objective of the survey was to obtain wide-field near-infrared images of the sites of linearly distributed methanol masers using a narrow-band 2.12-μm filter. This filter is centred on the  H₂ v = 1–0 S(1)  line; a shock diagnostic that has been shown to successfully trace CO outflows from young stellar objects. 28 sources in total were imaged of which 18 sources display H₂ emission. Of these, only two sources showed emission found to be dominantly perpendicular to the methanol maser distribution. Surprisingly, the H₂ emission in these fields is not distributed randomly, but instead the majority of sources are found to have H₂ emission dominantly parallel to their distribution of methanol masers. These results seriously question the hypothesis that methanol masers exist in circumstellar discs. The possibility that linearly distributed methanol masers are instead directly associated with outflows is discussed.     

The stellar populations of starburst galaxies through near-infrared spectroscopy

We study the central (inner few hundred parsecs) stellar populations of four starburst galaxies (NGC 34, 1614, 3310 and 7714) in the near-infrared (NIR), from 0.8 to 2.4 μm, by fitting combinations of stellar population models of various ages and metallicities. The NIR spectra of these galaxies feature many absorption lines. For the first time, we fit simultaneously as much as 15 absorption features in the NIR. The observed spectra are best explained by stellar populations containing a sizable amount (20–56 per cent by mass) of ∼1-Gyr-old stellar population with thermally pulsing asymptotic giant branch stars. We found that the metallicity of the stars which dominates the light is solar. Metallicities substantially different from solar give a worse fit. Though the ages and metallicities we estimate using the NIR spectroscopy are in agreement with values from the literature based on the ultraviolet/optical, we find older ages and a larger age spread. This may be due to the fact that the optical is mostly sensitive to the last episode of star formation, while the NIR better maintains the record of previous stellar generations. Another interesting result is that the reddening estimated from the whole NIR spectrum is considerably lower than that based on emission lines. Finally, we find a good agreement of the free emission-line spectrum with photoionization models, using as input spectral energy distribution the synthetic composite template we derived as best fit.     

An infrared study of Be stars based on ISO SWS01 spectra

The Infrared Space Observatory （ISO） Short-Wavelength Spectrometer （SWS） spectra of 10 Be stars are presented. It can be seen that the Be stars show a diversity in their ISO SWS01 spectral classifications by Kraemer et al., from naked stars, stars associated with dust, stars with warm dust shells, stars with cool dust shells to very red sources. In addition, the Brα/HI（14-6） line flux ratio derived for the sample stars is compared with that of P Cyg, and it is found that the line ratio of Be stars which were investigated show not only lower values as suggested by Waters et al., but also larger values. Therefore, the line ratio cannot be used to judge whether a star is a Be star or not.     

Imaging the asymmetric dust shell around CI Cam with long baseline optical interferometry

We present the first high angular resolution observation of the B[e] star/X-ray transient object CI Cam, performed with the two-telescope Infrared Optical Telescope Array (IOTA), its upgraded three-telescope version (IOTA3T) and the Palomar Testbed Interferometer (PTI). Visibilities and closure phases were obtained using the IONIC-3 integrated optics beam combiner. CI Cam was observed in the near-infrared H and K spectral bands, wavelengths well suited to measure the size and study the geometry of the hot dust surrounding CI Cam. The analysis of the visibility data over an 8 yr period from soon after the 1998 outburst to 2006 shows that the dust visibility has not changed over the years. The visibility data show that CI Cam is elongated which confirms the disc-shape of the circumstellar environment and totally rules out the hypothesis of a spherical dust shell. Closure phase measurements show direct evidence of asymmetries in the circumstellar environment of CI Cam and we conclude that the dust surrounding CI Cam lies in an inhomogeneous disc seen at an angle. The near-infrared dust emission appears as an elliptical skewed Gaussian ring with a major axis   a = 7.58 ± 0.24 mas  , an axis ratio   r = 0.39 ± 0.03  and a position angle  θ= 35°± 2°  .     

A Spitzer survey of young stellar objects in the Rosette Molecular Cloud

We present results from a survey of the Rosette Molecular Cloud (RMC) using both the Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) onboard the Spitzer Space Telescope . We have mapped a region of active star formation covering an area approximately 1° by 1.5° including several previously known clusters. Spectral energy distributions (SEDs) fitted to our data combined with that from Two Micron All Sky Survey (2MASS) are used to identify young stellar objects (YSOs) with infrared (IR) excesses. We find that roughly 50 per cent of the sources are forming in clustered environments and identify seven clusters of IR excess sources including four that were previously unknown. We investigate evidence for triggering of star formation due to the ionization front, identified in Brackett-α emission, associated with the young open cluster NGC 2244. Although the position of several of the clusters of IR excess sources are coincident with the ionization front, the bulk of the youngest YSOs are located far from the ionization front, in clusters located along the mid-plane of the cloud. We conclude that although triggering from the H  ii nebula is a possible origin for some of the recent star formation, the majority of the active star formation is occurring in already dense regions of the cloud not compressed by the expansion of the H  ii region.     

Accretion,jets and winds: High‐energy emission from young stellar objects – Doctoral Thesis Award Lecture 2010

This article summarizes the processes of high‐energy emission in young stellar objects. Stars of spectral type A and B are called Herbig Ae/Be (HAeBe) stars in this stage, all later spectral types are termed classical T Tauri stars (CTTS). Both types are studied by high‐resolution X‐ray and UV spectroscopy and modeling. Three mechanisms contribute to the highenergy emission from CTTS: 1) CTTS have active coronae similar to main‐sequence stars, 2) the accreted material passes through an accretion shock at the stellar surface, which heats it to a few MK, and 3) some CTTS drive powerful outflows. Shocks within these jets can heat the plasma to X‐ray emitting temperatures. Coronae are already well characterized in the literature; for the latter two scenarios models are shown. The magnetic field suppresses motion perpendicular to the field lines in the accretion shock, thus justifying a 1D geometry. The radiative loss is calculated as optically thin emission. A mixture of shocked and coronal gas is fitted to X‐ray observations of accreting CTTS. Specifically, the model explains the peculiar line‐ratios in the He‐like triplets of Ne IX and O VII. All stars require only small mass accretion rates to power the X‐ray emission. In contrast, the HAeBe HD 163296 has line ratios similar to coronal sources, indicating that neither a high density nor a strong UV‐field is present in the region of the X‐ray emission. This could be caused by a shock in its jet. Similar emission is found in the deeply absorbed CTTS DG Tau. Shock velocities between 400 and 500 km s^–1 are required to explain the observed spectrum (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)