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Register a new userThe low-mass diskless population of Corona Australis
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Bel\\'en L\\'opez Mart\\'i
Publication date
2010
fields
Physics
and research's language is
English
Authors
Belen Lopez Marti
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We combine published optical and near-infrared photometry to identify new low-mass candidate members in an area of about 0.64 deg^2 in Corona Australis, using the S-parameter method. Five new candidate members of the region are selected, with estimated ages between 3 and 15 Myr, and masses between 0.05 and 0.15 M_Sun. Using Spitzer photometry, we confirm that these objects are not surrounded by optically thick disks. However, one of them is found to display excess at 24 micron, thus suggesting it harbours a disk with an inner hole. With an estimated mass of 0.07 M_Sun according to the SED fitting, this is one of the lowest-mass objects reported to possess a transitional disk. Including these new members, the fraction of disks is about 50% among the total Corona Australis population selected by the same criteria, lower than the 70% fraction reported earlier for this region. Even so, we find a ratio of transitional to primordial disks (45%) very similar to the value derived by other authors. This ratio is higher than for solar-type stars (5-10%), suggesting that disk evolution is faster in the latter, and/or that the transitional disk stage is not such a short-lived step in the case of very low-mass objects. However, this impression needs to be confirmed with better statistics.
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We uncover the H2 flows in the Corona Australis molecular cloud and in particular identify the flows from the Coronet cluster. Near-infrared H2 v=1--0 S(1), 2.12micron-line, narrow-band imaging survey of the R CrA cloud core was carried out. We identify the best candidate-driving source for each outflow by comparing the flow properties, available proper motions, and the known/estimated properties of the driving sources. We also adopted the thumbrule of outflow power as proportional to source luminosity and inversely proportional to the source age to reach a consensus. Results: Continuum-subtracted, narrow-band images reveal several new Molecular Hydrogen emission-line Objects (MHOs). Together with previously known MHOs and Herbig-Haro objects we catalog at least 14 individual flow components of which 11 appear to be driven by the RCrA aggregate members. The flows originating in the Coronet cluster have lengths of ~0.1-0.2 pc. Eight out of nine submillimeter cores mapped in the Coronet cluster region display embedded stars driving an outflow component. Roughly 80% of the youngest objects in the Coronet are associated with outflows. The MHO flows to the west of the Coronet display lobes moving to the west and vice-versa, resulting in nondetections of the counter lobe in our deep imaging. We speculate that these counterflows may be experiencing a stunting effect in penetrating the dense central core. Conclusions:Although this work has reduced the ambiguities for many flows in the Coronet region, one of the brightest H2 feature (MHO2014) and a few fainter features in the region remain unassociated with a clear driving source. The flows from Coronet, therefore, continue to be interesting targets for future studies.
The late stages of evolution of the primordial circumstellar disks surrounding young stars are poorly understood, yet vital to constrain theories of planet formation. We consider basic structural models for the disks around two ~10 Myr-old members of the nearby RCrA association, RX J1842.9-3532 and RX J1852.3-3700. We present new arcsecond-resolution maps of their 230 GHz continuum emission from the Submillimeter Array and unresolved CO(3-2) spectra from the Atacama Submillimeter Telescope Experiment. By combining these data with broadband fluxes from the literature and infrared fluxes and spectra from the catalog of the Formation and Evolution of Planetary Systems (FEPS) Legacy program on the Spitzer Space Telescope, we assemble a multiwavelength data set probing the gas and dust disks. Using the Monte Carlo radiative transfer code RADMC to model simultaneously the SED and millimeter continuum visibilities, we derive basic dust disk properties and identify an inner cavity of radius 16 AU in the disk around RX J1852.3-3700. We also identify an optically thin 5 AU cavity in the disk around RX J1842.9-3532, with a small amount of optically thick material close to the star. The molecular line observations suggest an intermediate disk inclination in RX J1842.9-3532, consistent with the continuum emission. In combination with the dust models, the molecular data allow us to derive a lower CO content than expected, suggesting that the process of gas clearing is likely underway in both systems, perhaps simultaneously with planet formation.
Aims. The study of the morphology of coronal mass ejections (CMEs) is an auspicious approach to understanding how magnetic fields are structured within CMEs. Although earlier studies have suggested an asymmetry in the width of CMEs in orthogonal directions, this has not been inspected using multi-viewpoint observations. Methods. We inspect the early evolution (below ten solar radii) of the morphology of a dozen CMEs occurring under specific conditions of observing spacecraft location and CME trajectory, favorable to reduce uncertainties typically involved in the 3D reconstruction used here. These events are carefully reconstructed by means of a forward modeling tool using simultaneous observations of STEREO EUVI and SDO/AIA as input when originating low in the corona, and followed up in the outer fields of view of the STEREO and the SOHO coronagraphs. We then examine the height evolution of the morphological parameters arising from the reconstructions. Results. The multi-viewpoint analysis of this set of CMEs revealed that their initial expansion --below three solar radii-- is considerably asymmetric and non-self-similar. Both angular widths, namely along the main axes of CMEs ($AW_L$) and in the orthogonal direction ($AW_D$, representative of the flux rope diameter), exhibit much steeper change rates below this height, with the growth rate of $AW_L$ found to be larger than that of $AW_D$, also below that height. Angular widths along the main axes of CMEs are on average $approx$1.8 times larger than widths in the orthogonal direction $AW_D$. The ratios of the two expansion speeds, namely in the directions of CMEs main axes and in their orthogonal, are nearly constant in time after $sim$4 solar radii, with an average ratio $approx$1.6. Heights at which the width change rate is defined to stabilize are greater for $AW_L$ than for $AW_D$.
Corona-Australis is one of the nearest regions to the Sun with recent and ongoing star formation, but the current picture of its stellar (and substellar) content is not complete yet. We take advantage of the second data release of the Gaia space mission to revisit the stellar census and search for additional members of the young stellar association in Corona-Australis. We applied a probabilistic method to infer membership probabilities based on a multidimensional astrometric and photometric data set over a field of 128 deg$^{2}$ around the dark clouds of the region. We identify 313 high-probability candidate members to the Corona-Australis association, 262 of which had never been reported as members before. Our sample of members covers the magnitude range between $Ggtrsim5$ mag and $Glesssim20$ mag, and it reveals the existence of two kinematically and spatially distinct subgroups. There is a distributed `off-cloud population of stars located in the north of the dark clouds that is twice as numerous as the historically known `on-cloud population that is concentrated around the densest cores. By comparing the location of the stars in the HR-diagram with evolutionary models, we show that these two populations are younger than 10 Myr. Based on their infrared excess emission, we identify 28 Class II and 215 Class III stars among the sources with available infrared photometry, and we conclude that the frequency of Class II stars (i.e. `disc-bearing stars) in the on-cloud region is twice as large as compared to the off-cloud population. The distance derived for the Corona-Australis region based on this updated census is $d=149.4^{+0.4}_{-0.4}$ pc, which exceeds previous estimates by about 20 pc.In this paper we provide the most complete census of stars in Corona-Australis available to date that can be confirmed with Gaia data.
At a distance of about 130 pc, the Corona Australis molecular cloud complex is one of the nearest regions with ongoing and/or recent star formation. It is a region with highly variable extinction of up to AV~45 mag, containing, at its core, the Coronet protostar cluster. There are now 55 known optically detected members, starting at late B spectral types. At the opposite end of the mass spectrum, there are two confirmed brown dwarf members and seven more candidate brown dwarfs. The CrA region has been most widely surveyed at infrared wavelengths, in X-rays, and in the millimeter continuum, while follow-up observations from centimeter radio to X-rays have focused on the Coronet cluster.
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