- ID:
- ivo://CDS.VizieR/J/A+A/641/A90
- Title:
- AB Dor A VLBI images
- Short Name:
- J/A+A/641/A90
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The fast rotator, pre-main sequence star AB Dor A is a strong and persistent radio emitter. The extraordinary coronal flaring activity is thought to be the origin of compact radio emission and other associated phenomena, such as large slingshot prominences. We aim to investigate the radio emission mechanism and the milliarcsecond radio structure around AB Dor A. We performed phase-referenced VLBI observations at 22.3GHz, 8.4GHz, and 1.4GHz over more than one decade using the Australian VLBI array. Our 8.4GHz images show a double core-halo morphology, similar at all epochs, with emission extending at heights between 5 and 18 stellar radii. Furthermore, the sequence of the 8.4GHz maps shows a clear variation of the source structure within the observing time. However, images at 1.4GHz and 22.3GHz are compatible with a compact source. The phase-reference position at 8.4GHz and 1.4GHz are coincident with those expected from the well-known milliarcsecond-precise astrometry of this star, meanwhile the 22.3GHz position is 4sigma off the prediction in the north-west direction. The origin of this offset is still unclear. We have considered several models to explain the morphology and evolution of the inner radio structure detected in AB Dor A. These models include emission from the stellar polar caps, a flaring, magnetically-driven loop structure, and the presence of helmet streamers. We also investigated a possible close companion to AB Dor A. Our results confirm the extraordinary coronal magnetic activity of this star, capable of producing compact radio structures at very large heights that have so far only been seen in binary interacting systems.
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- ID:
- ivo://CDS.VizieR/J/ApJ/660/1462
- Title:
- Abundances of ONC X-ray PMS stars
- Short Name:
- J/ApJ/660/1462
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Following the Chandra Orion Ultradeep Project (COUP) observation, we have studied the chemical composition of the hot plasma in a sample of 146 X-ray-bright pre-main-sequence stars in the Orion Nebula Cluster (ONC). We report measurements of individual element abundances for a subsample of 86 slightly absorbed and bright X-ray sources, using low-resolution X-ray spectra obtained from the Chandra ACIS instrument.
- ID:
- ivo://CDS.VizieR/J/A+A/561/A2
- Title:
- 36 accreting YSOs emission lines
- Short Name:
- J/A+A/561/A2
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present VLT/X-shooter observations of a sample of 36 accreting low-mass stellar and substellar objects (YSOs) in the Lupus star-forming region, spanning a range in mass from ~0.03 to ~1.2M_{sun}_, but mostly with 0.1M_{sun}_<M*<0.5M_{sun}_. Our aim is twofold: firstly, to analyse the relationship between excess-continuum and line emission accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (L_acc_), and in turn the accretion rate (dM/dt_acc_), was derived by modelling the excess emission from the UV to the near-infrared as the continuum emission of a slab of hydrogen. We computed the flux and luminosity (L_line_) of many emission lines of H, He, and CaII, observed simultaneously in the range from ~330nm to 2500nm.
- ID:
- ivo://CDS.VizieR/J/ApJ/710/597
- Title:
- Accretion in disks in Cep OB2
- Short Name:
- J/ApJ/710/597
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present accretion rates for a large number of solar-type stars in the Cep OB2 region, based on U-band observations. Our study comprises 95 members of the ~4Myr old cluster Tr 37 (including 20 "transition" objects (TOs)), as well as the only classical T Tauri star (CTTS) in the ~12Myr old cluster NGC 7160. The stars show different disk morphologies, with the majority of them having evolved and flattened disks. The typical accretion rates are about 1 order of magnitude lower than in regions aged 1-2Myr, and we find no strong correlation between disk morphology and accretion rates. Although half of the TOs are not accreting, the median accretion rates of normal CTTS and accreting "transition" disks are similar (~3x10^-9^ and 2x10^-9^M_{sun}_/yr, respectively). Comparison with other regions suggests that the TOs observed at different ages do not necessarily represent the same type of objects, which is consistent with the fact that the different processes that can lead to reduced IR excess/inner disk clearing (e.g., binarity, dust coagulation/settling, photoevaporation, giant planet formation) do not operate on the same timescales.
- ID:
- ivo://CDS.VizieR/J/A+A/579/A66
- Title:
- Accretion in {rho}-Ophiucus
- Short Name:
- J/A+A/579/A66
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present new VLT/X-Shooter optical and near-infrared spectra of a sample of 17 candidate young low-mass stars and brown dwarfs located in the {rho}-Ophiucus cluster. We derived the spectral type and extinction for all the targets, and then we determined their physical parameters. All the objects but one have M_*_<~0.6M_{sun}_, and eight have mass below or close to the hydrogen-burning limit. Using the intensity of various permitted emission lines present in their spectra, we determined the accretion luminosity and mass accretion rates ({dot}(M)_acc_) for all the objects. When compared with previous works targeting the same sample, we find that, in general, these objects are not as strongly accreting as previously reported, and we suggest that the reason is our more accurate estimate of the photospheric parameters. We also compare our findings with recent works in other slightly older star-forming regions, such as Lupus, to investigate possible differences in the accretion properties, but we find that the accretion properties for our targets have the same dependence on the stellar and substellar parameters as in the other regions. This leads us to conclude that we do not find evidence for a different dependence of {dot}(M)_acc_ with M_*_ when comparing low-mass stars and brown dwarfs. Moreover, we find a similar small (<~1dex) scatter in the {dot}(M)_acc_-M_*_ relation as in some of our recent works in other star-forming regions, and no significant differences in {dot}(M)_acc_ due to different ages or properties of the regions. The latter result suffers, however, from low statistics and sample selection biases in the current studies. The small scatter in the {dot}(M)_acc_-M_*_ correlation confirms that mass accretion rate measurements in the literature based on uncertain photospheric parameters and single accretion indicators, such as the H{alpha} width, can lead to a scatter that is unphysically large. Our studies show that only broadband spectroscopic surveys coupled with a detailed analysis of the photospheric and accretion properties allows us to properly study the evolution of disk accretion rates in star-forming regions.
- ID:
- ivo://CDS.VizieR/J/A+A/586/A47
- Title:
- Accretion process in NGC 2264
- Short Name:
- J/A+A/586/A47
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- NGC 2264 is a young stellar cluster (~3Myr) with hundreds of low-mass accreting stars that allow a detailed analysis of the accretion process taking place in the pre-main sequence. Our goal is to relate the photometric and spectroscopic variability of classical T Tauri stars to the physical processes acting in the stellar and circumstellar environment, within a few stellar radii from the star. NGC 2264 was the target of a multiwavelength observational campaign with CoRoT, MOST, Spitzer, and Chandra satellites and photometric and spectroscopic observations from the ground. We classified the CoRoT light curves of accreting systems according to their morphology and compared our classification to several accretion diagnostics and disk parameters.
- ID:
- ivo://CDS.VizieR/J/A+A/599/A23
- Title:
- Accretion-rotation connection in NGC 2264
- Short Name:
- J/A+A/599/A23
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The low spin rates measured for solar-type stars at an age of a few Myr (~10% of the break-up velocity) indicate that some mechanism of angular momentum regulation must be at play in the early pre-main sequence. This may be associated with magnetospheric accretion and star-disk interaction, as suggested by observations that disk-bearing objects (CTTS) are slower rotators than diskless sources (WTTS) in young star clusters. We characterize the rotation properties for members of the star-forming region NGC 2264 (~3Myr) as a function of mass, and investigate the accretion-rotation connection at an age where about 50% of the stars have already lost their disks. We examined a sample of 500 cluster members (40% with disks, 60% without disks), distributed in mass between ~0.15 and 2M_{sun}_, whose photometric variations were monitored in the optical for 38 consecutive days with the CoRoT space observatory. Light curves were analyzed for periodicity using three different techniques: the Lomb-Scargle periodogram, the autocorrelation function and the string-length method. Periods were searched in the range between 0.17-days (i.e., 4h, twice the data sampling adopted) and 19-days (half the total time span). Period detections were confirmed using a variety of statistical tools (false alarm probability, Q-statistics), as well as visual inspection of the direct and phase-folded light curves.
- ID:
- ivo://CDS.VizieR/J/A+A/455/903
- Title:
- ACIS-I observations of NGC 2264
- Short Name:
- J/A+A/455/903
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- This paper's goal is to improve the member census of the NGC 2264 star-forming region and study the origin of X-ray activity in young PMS stars. We analyze a deep, 100ks long, Chandra ACIS observation covering a 17'x17' field in NGC 2264. The preferential detection in X-rays of low-mass PMS stars gives strong indications of their membership. We study X-ray activity as a function of stellar and circumstellar characteristics by correlating the X-ray luminosities, temperatures, and absorptions with optical and near-infrared data from the literature.
- ID:
- ivo://CDS.VizieR/J/A+A/575/A4
- Title:
- Activity and accretion in {gamma} Vel and Cha I
- Short Name:
- J/A+A/575/A4
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We use the fundamental parameters (effective temperature, surface gravity, lithium abundance, and radial velocity) delivered by the GES consortium in the first internal data release to select the members of Gamma Vel and Cha I among the UVES and GIRAFFE spectroscopic observations. A total of 140 Gamma Vel members and 74 Cha I members were studied. The procedure adopted by the GES to derive stellar fundamental parameters provided also measures of the projected rotational velocity (vsini). We calculated stellar luminosities through spectral energy distributions, while stellar masses were derived by comparison with evolutionary tracks. The spectral subtraction of low-activity and slowly rotating templates, which are rotationally broadened to match the vsini of the targets, enabled us to measure the equivalent widths (EWs) and the fluxes in the H{alpha} and H{beta} lines. The H{alpha} line was also used for identifying accreting objects, on the basis of its equivalent width and the width at the 10% of the line peak (10%W), and for evaluating the mass accretion rate (M_acc_). The distribution of vsini for the members of Gamma Vel displays a peak at about 10km/s with a tail toward faster rotators. There is also some indication of a different vsini distribution for the members of its two kinematical populations. Most of these stars have H{alpha} fluxes corresponding to a saturated activity regime. We find a similar distribution, but with a narrower peak, for Cha I. Only a handful of stars in Gamma Vel display signatures of accretion, while many more accretors were detected in the younger Cha I, where the highest H{alpha} fluxes are mostly due to accretion, rather than to chromospheric activity. Accreting and active stars occupy two different regions in a T_eff-flux diagram and we propose a criterion for distinguishing them. We derive M_acc in the ranges 10^-11^-10^-9^M_{sun}_/yr and 10^-10^-10^-7^M_{sun}_/yr for Gamma Vel and Cha I accretors, respectively.
- ID:
- ivo://CDS.VizieR/J/ApJS/125/161
- Title:
- A database of dense cores mapped in ammonia
- Short Name:
- J/ApJS/125/161
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a database of 264 cores mapped in the (J,K)=(1,1) and (2,2) lines of NH_3_. We list the core gas properties-peak positions, total ammonia column densities, intrinsic line widths, kinetic temperatures, volume densities, core sizes, aspect ratios, and velocity gradients, as well as the properties of associated young stellar objects (YSOs)-associated IRAS sources along with their luminosities and core-YSO distances, outflow velocities, and SIMBAD and cluster associations. We also present the results of our statistical analysis and enumerate important pairwise correlations among the various gas and YSO properties.
