On the Submillimeter Opacity of Protoplanetary Disks

Solid particles with the composition of interstellar dust and power-law size distribution dn/da propto a^{-p} for a 3 lambda and 3 < p < 4 will have submm opacity spectral index beta(lambda) = dln(kappa)/dln(nu) approx (p-3) beta_{ism}, where beta_{ism} approx 1.7 is the opacity spectral index of interstellar dust material in the Rayleigh limit. For the power-law index p approx 3.5 that characterizes interstellar dust, and that appears likely for particles growing by agglomeration in protoplanetary disks, grain growth to sizes a > 3 mm will result in beta(1 mm) < ~1. Grain growth can naturally account for beta approx 1 observed for protoplanetary disks, provided that a_{max} > ~ 3 lambda.Comment: Submitted to ApJ. 17 pages, 6 figure

Mid-infrared interferometry of the massive young stellar object NGC 2264 IRS 1

The optically invisible infrared-source NGC 2264 IRS 1 is thought to be a massive young stellar object (~10 Msun). Although strong infrared excess clearly shows that the central object is surrounded by large amounts of circumstellar material, no information about the spatial distribution of this circumstellar material has been available until now. We used the ESO Very Large Telescope Interferometer to perform long-baseline interferometric observations of NGC 2264 IRS 1 in the mid-infrared regime. Our observations resolve the circumstellar material around NGC 2264 IRS 1, provide the first direct measurement of the angular size of the mid-infrared emission, and yield direct constraints on the spatial distribution of the dust. We use different approaches (a geometrical model, a temperature-gradient model, and radiative transfer models) to jointly model the observed interferometric visibilities and the spectral energy distribution. The derived visibility values between ~0.02 and ~0.3 show that the mid-infrared emission is clearly resolved. The characteristic size of the MIR-emission region is ~30-60 AU; this value is typical for other YSOs with similar or somewhat lower luminosities. A comparison of the sizes for the two position angles shows a significant elongation of the dust distribution. Simple spherical envelope models are therefore inconsistent with the data. The radiative transfer modeling of our data suggests that we observe a geometrically thin and optically thick circumstellar disk with a mass of about 0.1 Msun. Our modeling indicates that NGC 2264 IRS 1 is surrounded by a flat circumstellar disk that has properties similar to disks typically found around lower-mass young stellar objects. This result supports the assumption that massive young stellar objects form via accretion from circumstellar disks.Comment: 11 pages, accepted for publication in A&

Bispectrum speckle interferometry of the massive protostellar outflow source IRAS 23151+5912

We present bispectrum speckle interferometry of the massive protostellar object IRAS 23151+5912 in the near-infrared K' band. The reconstructed image shows the diffuse nebulosity north-east of two point-like sources in unprecedented detail. The comparison of our near-infrared image with mm continuum and CO molecular line maps shows that the brighter of the two point sources lies near the center of the mm peak, indicating that it is a high-mass protostar. The nebulosity coincides with the blue-shifted molecular outflow component. The most prominent feature in the nebulosity is a bow-shock-like arc. We assume that this feature is associated with a precessing jet which has created an inward-pointed cone in the swept-up material. We present numerical jet simulations that reproduce this and several other features observed in our speckle image of the nebulosity. Our data also reveal a linear structure connecting the central point source to the extended diffuse nebulosity. This feature may represent the innermost part of a jet that drives the strong molecular outflow (PA ~80 degr) from IRAS 23151+5912. With the aid of radiative transfer calculations, we demonstrate that, in general, the observed inner structures of the circumstellar material surrounding high-mass stars are strongly influenced by the orientation and symmetry of the bipolar cavity.Comment: accepted by Astronomy & Astrophysics; preprints with high-resolution images can be obtained from http://www.mpifr-bonn.mpg.de/staff/tpreibis/iras23151.htm

Radio and X-ray variability of Young Stellar Objects in the Coronet Cluster

The Coronet Cluster in the nearby R CrA dark cloud offers the rare opportunity to study at least four "class I" protostellar sources as well as one candidate "class 0" source, a Herbig Ae star, and a candidate brown dwarf within a few square arcminutes - most of them detected at radio- and X-ray wavelengths. These sources were observed with the Very Large Array (VLA) at 3.5cm on nine occasions in 1998, spread over nearly four months. The source IRS 5, earlier shown to emit circularly polarized radio emission, was observed to undergo a flux increase accompanied by changes in its polarization properties. Comparison with VLA measurements taken in January 1997 allows for some analysis of longer-term variability. In addition to this radio monitoring, we analyze archival Chandra and XMM-Newton X-ray data of these sources. Three class I protostars are bright enough for X-ray spectroscopy, and we perform a variability analysis for these sources, covering a total of 154 ksec spread over more than two and a half years. Also in X-rays, IRS 5 shows the most pronounced variability, whilst the other two class I protostars IRS 1 and IRS 2 have more stable emission. X-ray data is also analyzed for the recently identified candidate class 0 source IRS 7E, which shows strong variability as well as for the Herbig Ae star R CrA for which we find extremely hot X-ray-emitting plasma. For IRS 1,2 and 5, the hydrogen column densities derived from the X-ray spectra are at about half the values derived with near-infrared techniques, a situation similar to what has been observed towards some other young stellar objects.Comment: 17 pages, 11 figures, accepted for publication in A&

First Detection of Millimeter Dust Emission from Brown Dwarf Disks

We report results from the first deep millimeter continuum survey targeting Brown Dwarfs (BDs). The survey led to the first detection of cold dust in the disks around two young BDs (CFHT-BD-Tau 4 and IC348 613), with deep JCMT and IRAM observations reaching flux levels of a few mJy. The dust masses are estimated to be a few Earth masses assuming the same dust opacities as usually applied to TTauri stars.Comment: 5 pages, accepted for ApJ

Simultaneous X-ray, radio, near-infrared, and optical monitoring of Young Stellar Objects in the Coronet cluster

Multi-wavelength (X-ray to radio) monitoring of Young Stellar Objects (YSOs) can provide important information about physical processes at the stellar surface, in the stellar corona, and/or in the inner circumstellar disk regions. While coronal processes should mainly cause variations in the X-ray and radio bands, accretion processes may be traced by time-correlated variability in the X-ray and optical/infrared bands. Several multi-wavelength studies have been successfully performed for field stars and approx. 1-10 Myr old T Tauri stars, but so far no such study succeeded in detecting simultaneous X-ray to radio variability in extremely young objects like class I and class 0 protostars. Here we present the first simultaneous X-ray, radio, near-infrared, and optical monitoring of YSOs, targeting the Coronet cluster in the Corona Australis star-forming region, which harbors at least one class 0 protostar, several class I objects, numerous T Tauri stars, and a few Herbig AeBe stars. [...] Seven objects are detected simultaneously in the X-ray, radio, and optical/infrared bands; they constitute our core sample. While most of these sources exhibit clear variability in the X-ray regime and several also display optical/infrared variability, none of them shows significant radio variability on the timescales probed. We also do not find any case of clearly time-correlated optical/infrared and X-ray variability. [...] The absence of time-correlated multi-wavelength variability suggests that there is no direct link between the X-ray and optical/infrared emission and supports the notion that accretion is not an important source for the X-ray emission of these YSOs. No significant radio variability was found on timescales of days.Comment: 11 pages, 11 figures, accepted for publication in A&A (06 Dec 2006

Very Low-Mass Objects in the Coronet Cluster: The Realm of the Transition Disks

We present optical and IR spectra of a set of low-mass stars and brown dwarfs in the Coronet cluster (aged ~1Myr), obtained with the multifiber spectrograph FLAMES/VLT and IRS/Spitzer. The optical spectra reveal spectral types between M1 and M7.5, confirm the youth of the objects (via Li 6708 A absorption), and show the presence of accretion (via Halpha) and shocks (via forbidden line emission). The IRS spectra, together with IR photometry from the IRAC/MIPS instruments on Spitzer and 2MASS, confirm the presence of IR excesses characteristic of disks around ~70% of the objects. Half of the disks do not exhibit any silicate emission, or present flat features characteristic of large grains. The rest of the disks show silicate emission typical of amorphous and crystalline silicate grains a few microns in size. About 50% of the objects with disks do not show near-IR excess emission, having "transitional" disks, according to their classical definition. This is a very high fraction for such a young cluster. The large number of "transitional" disks suggests lifetimes comparable to the lifetimes of typical optically thick disks. Therefore, these disks may not be in a short-lived phase, intermediate between Class II and Class III objects. The median spectral energy distribution of the disks in the Coronet cluster is also closer to a flat disk than observed for the disks around solar-type stars in regions with similar age. The differences in the disk morphology and evolution in the Coronet cluster could be related to fact that these objects have very late spectral types compared to the solar-type stars in other cluster studies. Finally, the optical spectroscopy reveals that one of the X-ray sources is produced by a Herbig Haro object in the cloud.Comment: 51 pages, 13 figures, 10 table

Spectroscopic identification of DENIS-selected brown dwarf candidates in the Upper Scorpius OB association

We present low-resolution (R=900) optical (576.1--1,051.1 nm) spectroscopic observations of 40 candidate very low-mass members in the Upper Scorpius OB association. These objects were selected using the $I$, $J$ and $K$ photometry available in the DENIS database. We have derived spectral types and we have measured H$\alpha$ and NaI doublet (at 818.3 and 819.5 nm) equivalent widths. We assess the youth of the objects by comparing them to their older counterparts of similar spectral type in the Pleiades cluster and the field. Our analysis indicates that 28 of our targets are young very low-mass objects, and thus they are strong candidate members of the OB association. The other 12 DENIS sources are foreground M dwarfs or background red giants. Our sample of spectroscopic candidate members includes 18 objects with spectral types in the range M6.5 and M9, which are likely young brown dwarfs. We classify these candidates as accreting/non accreting using the scheme proposed by Barrado y Navascu\'es & Mart\'\i n (2003). We find 5 substellar-mass candidate cluster members that are still undergoing mass accretion, indicating that the timescale for accretion onto brown dwarfs can be as long as 5 Myr in some cases.Comment: Accepted for publication in The Astronomical Journal, January 200

The Origin of T Tauri X-ray Emission: New Insights from the Chandra Orion Ultradeep Project

We use the data of the Chandra Orion Ultradeep Project (COUP) to study the nearly 600 X-ray sources that can be reliably identified with optically well characterized T Tauri stars (TTS) in the Orion Nebula Cluster. We detect X-ray emission from more than 97% of the optically visible late-type (spectral types F to M) cluster stars. This proofs that there is no ``X-ray quiet'' population of late-type stars with suppressed magnetic activity. All TTS with known rotation periods lie in the saturated or super-saturated regime of the relation between activity and Rossby numbers seen for main-sequence (MS) stars, but the TTS show a much larger scatter in X-ray activity than seen for the MS stars. Strong near-linear relations between X-ray luminosities, bolometric luminosities and mass are present. We also find that the fractional X-ray luminosity rises slowly with mass over the 0.1 - 2 M_sun range. The plasma temperatures determined from the X-ray spectra of the TTS are much hotter than in MS stars, but seem to follow a general solar-stellar correlation between plasma temperature and activity level. The large scatter about the relations between X-ray activity and stellar parameters seems to be related to the influence of accretion on the X-ray emission. While the X-ray activity of the non-accreting TTS is consistent with that of rapidly rotating MS stars, the accreting stars are less X-ray active (by a factor of ~2-3 on average) and produce much less well defined correlations than the non-accretors. We discuss possible reasons for the suppression of X-ray emission by accretion and the implications of our findings on long-standing questions related to the origin of the X-ray emission from young stars.Comment: accepted for ApJS, COUP Special Issu