1,721,192 research outputs found
The study of young substellar objects with ALMA
No full textWe discuss the potential of ALMA for studying the formation of substellar objects. We first review briefly the various formation mechanisms proposed so far and stress the unique capability of ALMA to detect and study pre-brown dwarf cores and to confirm the core-collapse scenario to the lowest possible masses. We then discuss the properties of disks around substellar objects. We show how it will be possible to detect with ALMA most disks around objects with mass as low as few Jupiter masses, and to resolve spatially their emission in the more favorable cases
Accretion in the ρ-Ophiuchi pre-main sequence stars
No full textAims.The aim of this paper is to provide a measurement of the mass accretion rate in a large, complete sample of objects in the core of the star forming region ρ Oph.
Methods: .The sample includes most of the objects (104 out of 111) with evidence of a circumstellar disk from mid-infrared photometry; it covers a stellar mass range from about 0.03 to 3 M_⊙ and it is complete to a limiting mass of 0.05 M_⊙. We used J and K-band spectra to derive the mass accretion rate of each object from the intensity of the hydrogen recombination lines, Paβ or Brγ. For comparison, we also obtained similar spectra of 35 diskless objects.
Results: .The results show that emission in these lines is only seen in stars with disks, and can be used as an indicator of accretion. However, the converse does not hold, as about 50% of our disk objects do not have detectable line emission. The measured accretion rates show a strong correlation with the mass of the central object (dot M_acc ∝ M_star1.8±0.2) and a large spread, of two orders of magnitude at least, for any interval of M_star. A comparison with existing data for Taurus shows that the objects in the two regions have similar behaviour, at least for objects more massive than ∼ 0.1 M_⊙. The implications of these results are briefly discussed
The onset of cluster formation around Herbig Ae/Be stars
No full textThe large body of near infrared observations presented in Testi et al. ( te{Tea97}; te{Tea98}) are analysed with the aim of characterizing the young stellar clusters surrounding Herbig Ae/Be stars. The results confirm the tendency of early Be stars to be surrounded by dense clusters of lower mass ``companions'', while Ae stars are never found to be associated with conspicuous groups. The transition between the different environments appears to occur smoothly from Ae to Be stars without a sharp threshold. No correlation of the richness of the stellar groups detected is found with the galactic position or the age of the central Herbig Ae/Be star. The stellar volume densities estimated for the groups surrounding pre-main-sequence stars of intermediate mass show the transition from the low density aggregates of T Tauri stars and the dense clusters around massive stars. Only the most massive stars (10-20 M_sun) are found to be associated with dense ( ~ 10(3 pc(-3)) ) stellar clusters. This is exactly the mass regime at which the conventional accretion scenario for isolated star formation faces theoretical problems. Thus our findings strongly supports the idea that the formation of high-mass stars is influenced by dynamical interaction in a young cluster environment
Accretion in Protoplanetary Disks: The Imprint of Core Properties
No full textIn this Letter, we present a theoretical scenario to explain the steep correlation between disk accretion rates and stellar masses observed in pre-main-sequence stars. We show that the correlations and spread observed in the two best-studied regions, ρ Oph and Taurus, can be reproduced by a simple model of single-star formation from a rotating collapsing core and the viscous evolution of the circumstellar disk. In this model, the rate of rotation of the parent core sets the value of the ``centrifugal radius'' within which the infalling matter is loaded onto the surface of the disk. As a consequence, the disk accretion rate measured long after the dispersal of the parental core bears the imprint of the initial conditions of star formation. The observed trend results naturally if, at the onset of the collapse, cores of all masses rotate with the same distribution of angular velocities, measured in units of the breakup rotation rate
Large dust grains in the inner region of circumstellar disks
No full textContext. Simple geometrical ring models account well for near-infrared interferometric observations of dusty disks surrounding pre-main sequence stars of intermediate mass. Such models demonstrate that the dust distribution in these disks has an inner hole and puffed-up inner edge consistent with theoretical expectations.
Aims. In this paper, we reanalyze the available interferometric observations of six intermediate mass pre-main sequence stars (CQ Tau, VV Ser, MWC 480, MWC 758, V1295 Aql and AB Aur) in the framework of a more detailed physical model of the inner region of the dusty disk. Our aim is to verify whether the model will allow us to constrain the disk and dust properties.
Methods. Observed visibilities from the literature are compared with theoretical visibilities from our model. With the assumption that silicates are the most refractory dust species, our model computes self-consistently the shape and emission of the inner edge of the dusty disk (and hence its visibilities for given interferometer configurations). The only free parameters in our model are the inner disk orientation and the size of the dust grains.
Results.In all objects with the exception of AB Aur, our self-consistent models reproduce both the interferometric results and the near-infrared spectral energy distribution. In four cases, grains larger than ~1.2 μm, and possibly much larger are either required by or consistent with the observations. The inclination of the inner disk is found to be always larger than ~30°, and in at least two objects much larger
A search for clustering around Herbig Ae/Be stars. II. Atlas of the observed sources
No full textWe present large field infrared images of a sample of 45 Herbig Ae/Be stars. Stellar parameters, such as age and luminosity, have been derived for all of them in a consistent way. The images have been used to identify stellar groups or clusters associated with the Herbig Ae/Be star. The results presented in this paper form the database for a study of clustering around intermediate mass stars (Testi et al. 1998)
Exploring brown dwarf disks
No full textWe discuss the spectral energy distribution of three very low mass
objects in Chamaeleon I for which ground-based spectroscopy and photometry
as well as ISO measurements in the mid-infrared are available
(Comerón et al. [CITE]; Persi et al. [CITE]). One of these stars
(Cha Hα1) is a bona-fide brown dwarf, with mass 0.04-0.05 .
We show that the observed emission is very well described
by models of circumstellar disks identical to those associated to T Tauri stars,
scaled down to keep the ratio of the disk-to-star mass constant
and to the appropriate stellar parameters.
This result provides a first indication that the formation mechanism
of T Tauri stars (via core contraction and formation of an
accretion disk)
extends to objects in the brown dwarf mass range
Diamonds in HD 97048: A Closer Look
No full textWe present adaptive optics high angular resolution (~0.1") spectroscopic observations in the 3 μm region of the Herbig Ae/Be star HD 97048. For the first time, we spatially resolve the emission in the diamond features at 3.43 and 3.53 μm and in the adjacent continuum. Using both the intensity profiles along the slit and reconstructed two-dimensional images of the object, we derive FWHM sizes consistent with the predictions for a circumstellar disk seen pole-on. The diamond emission originates in the inner region (R<~15 AU) of the disk
Accretion rates in Herbig Ae stars
No full textAims.Accretion rates from disks around pre-main sequence stars are of importance for our understanding of planetary formation and disk evolution. We provide in this paper estimates of the mass accretion rates in the disks around a large sample of Herbig Ae stars.
Methods: .We obtained medium resolution 2 μm spectra and used the results to compute values of dot M_acc from the measured luminosity of the Brγ emission line, using a well established correlation between L(Brγ) and the accretion luminosity L_acc.
Results: .We find that 80% of the stars, all of which have evidence of an associated circumstellar disk, are accreting matter, with rates 3× 10-9 ⪉ dot M_acc ⪉ 10-6 M_⊙/yr; for 7 objects, 6 of which are located on the ZAMS in the HR diagram, we do not detect any line emission. Few HAe stars (25%) have dot M_acc>10-7 M_⊙/yr.
Conclusions: .In most HAe stars the accretion rate is sufficiently low that the gas in the inner disk, inside the dust evaporation radius, is optically thin and does not prevent the formation of a puffed-up rim, where dust is directly exposed to the stellar radiation. When compared to the dot M_acc values found for lower-mass stars in the star forming regions Taurus and Ophiuchus, HAe stars have on average higher accretion rates than solar-mass stars; however, there is a lack of very strong accretors among them, probably due to the fact that they are on average older
Dust grain growth in ρ-Ophiuchi protoplanetary disks
No full textWe present new ATCA observations at 3.3 mm of 27 young stellar objects in the ρ-Oph young cluster. 25 of these sources have been detected. We analyze the sub-millimeter and millimeter SED for a subsample of 17 isolated class II protoplanetary disks and derive constraints on the grain growth and total dust mass in the disk outer regions. All the disks in our sample show a mm slope of the SED which is significantly shallower than the one observed for the ISM at these long wavelengths. This indicates that 1) class II disks in Ophiuchus host grains grown to mm/cm-sizes in their outer regions; 2) formation of mm/cm-sized pebbles is a fast process and 3) a mechanism halting or slowing down the inward radial drift of solid particles is required to explain the data. These findings are consistent with previous results in other star forming regions. We compare the dust properties of this sample with those of a uniformly selected sample in Taurus-Auriga and find no statistical evidence of any difference in terms of grain growth between the two regions. Finally, in our sample the mm slope of the SED is not found to correlate with indicators of grain growth to micron sizes in the surface layers of the inner disk
- …
