- ID:
- ivo://CDS.VizieR/J/A+A/548/A56
- Title:
- X-shooter spectra of 12 young stellar objects
- Short Name:
- J/A+A/548/A56
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present high-quality, medium-resolution X-shooter/VLT spectra in the range 300-2500nm for a sample of 12 very low mass stars in the {sigma} Orionis cluster. The sample includes eight stars with evidence of disks from Spitzer and four without disks, with masses ranging from 0.08 to 0.3M_{sun}_. The aim of this first paper is to investigate the reliability of the many accretion tracers currently used to measure the mass accretion rate in low-mass young stars and the accuracy of the correlations between these secondary tracers (mainly accretion line luminosities) found in the literature. We use our spectra to measure the accretion luminosity from the continuum excess emission in the UV and visual; the derived mass accretion rates range from 10^-9^M_{sun}_/yr down to 5x10^-11^M_{sun}_/yr, allowing us to investigate the behavior of the accretion-driven emission lines in very low mass accretion rate regimes. We compute the luminosity of ten accretion-driven emission lines from the UV to the near-IR, which are all obtained simultaneously. In general, most of the secondary tracers correlate well with the accretion luminosity derived from the continuum excess emission. We recompute the relationships between the accretion luminosities and the line luminosities, and we confirm the validity of the correlations given in the literature, with the possible exception of H{alpha}. Metallic lines, such as the CaII IR triplet or the NaI line at 589.3nm, show a larger dispersion. When looking at individual objects, we find that the hydrogen recombination lines, from the UV to the near-IR, give good and consistent measurements of L_acc_ that often better agree than the uncertainties introduced by the adopted correlations. The average L_acc_ derived from several hydrogen lines, measured simultaneously, have a much reduced error. This suggests that some of the spread in the literature correlations may be due to the use of nonsimultaneous observations of lines and continuum. Three stars in our sample deviate from this behavior, and we discuss them individually.
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- ID:
- ivo://CDS.VizieR/J/A+A/551/A107
- Title:
- X-shooter spectra of 24 young stellar objects
- Short Name:
- J/A+A/551/A107
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The lack of knowledge of photospheric parameters and the level of chromospheric activity in young low-mass pre-main sequence stars introduces uncertainties when measuring mass accretion rates in accreting (Class II) young stellar objects. A detailed investigation of the effect of chromospheric emission on the estimates of mass accretion rate in young low-mass stars is still missing. This can be undertaken using samples of young diskless (Class III) K and M-type stars. Our is to measure the chromospheric activity of Class III pre main sequence stars to determine its effect on the estimates of the accretion luminosity (Lacc) and mass accretion rate (dMacc/dt) in young stellar objects with disks. Using VLT/X-shooter spectra, we analyzed a sample of 24 nonaccreting young stellar objects of spectral type between K5 and M9.5. We identified the main emission lines normally used as tracers of accretion in Class II objects, and we determined their fluxes in order to estimate the contribution of the chromospheric activity to the line luminosity. We have used the relationships between line luminosity and accretion luminosity derived in the literature for Class II objects to evaluate the impact of chromospheric activity on the accretion rate measurements. We find that the typical chromospheric activity would bias the derived accretion luminosity by L_acc,noise_<10^-3^L_{sun}_, with a strong dependence on the Teff of the objects. The noise on Macc depends on stellar mass and age, and the typical values of log(M_acc,noise_) range between ~-9.2 to -11.6M_{sun}_/yr. Values of L_acc_<10^-3^L_{sun}_ obtained in accreting low-mass pre main sequence stars through line luminosity should be treated with caution because the line emission may be dominated by the contribution of chromospheric activity.
- ID:
- ivo://CDS.VizieR/J/A+A/602/A33
- Title:
- X-Shooter spectroscopy of YSOs in Lupus
- Short Name:
- J/A+A/602/A33
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A homogeneous determination of basic stellar parameters of young stellar object (YSO) candidates is needed to confirm their pre-main sequence evolutionary stage, membership to star forming regions (SFRs), and to get reliable values of the quantities related to chromospheric activity and accretion. We used the code ROTFIT and synthetic BT-Settl spectra for the determination of the atmospheric parameters (Teff and logg), the veiling (r), the radial (RV) and projected rotational velocity (vsini), from X-Shooter spectra of 102 YSO candidates (95 of infrared Class II and seven Class III) in the Lupus SFR. The spectral subtraction of inactive templates, rotationally broadened to match the vsini of the targets, enabled us to measure the line fluxes for several diagnostics of both chromospheric activity and accretion, such as H{alpha}, H{beta}, CaII, and NaI lines. We have shown that 13 candidates can be rejected as Lupus members based on their discrepant RV with respect to Lupus and/or the very low logg values. At least 11 of them are background giants, two of which turned out to be lithium-rich giants. Regarding the members, we found that all ClassIII sources have H{alpha} fluxes compatible with a pure chromospheric activity, while objects with disks lie mostly above the boundary between chromospheres and accretion. YSOs with transitional disks displays both high and low H{alpha} fluxes. We found that the line fluxes per unit surface are tightly correlated with the accretion luminosity (L_acc_) derived from the Balmer continuum excess. This rules out that the relationships between L_acc_ and line luminosities found in previous works are simply due to calibration effects. We also found that the CaII-IRT flux ratio is always small, indicating an optically thick emission source. The latter can be identified with the accretion shock near the stellar photosphere. The Balmer decrement reaches instead, for several accretors, high values typical of optically thin emission, suggesting that the Balmer emission originates in different parts of the accretion funnels with a smaller optical depth.
- ID:
- ivo://CDS.VizieR/J/AJ/155/109
- Title:
- Young binaries in Ophiuchus&Upper Centaurus-Lupus
- Short Name:
- J/AJ/155/109
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present measurements of the orbital positions and flux ratios of 17 binary and triple systems in the Ophiuchus star-forming region and the Upper Centaurus-Lupus cluster based on adaptive optics imaging at the Keck Observatory. We report the detection of visual companions in MML 50 and MML 53 for the first time, as well as the possible detection of a third component in WSB 21. For six systems in our sample, our measurements provide a second orbital position following their initial discoveries over a decade ago. For eight systems with sufficient orbital coverage, we analyze the range of orbital solutions that fit the data. Ultimately, these observations will help provide the groundwork toward measuring precise masses for these pre-main-sequence stars and understanding the distribution of orbital parameters in young multiple systems.
- ID:
- ivo://CDS.VizieR/J/A+A/657/A129
- Title:
- 13 young brown dwarfs SINFONI spectra
- Short Name:
- J/A+A/657/A129
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- Studies of the low-mass population statistics in young clusters are the foundation for our understanding of the formation of low-mass stars and brown dwarfs. Robust low-mass populations can be obtained through near-infrared spectroscopy, which provides confirmation of the cool and young nature of member candidates. However, the spectroscopic analysis of these objects is often not performed in a uniform manner, and the assessment of youth generally relies on the visual inspection of youth features whose behavior is not well understood. We aim at building a method that efficiently identifies young low-mass stars and brown dwarfs from low-resolution near-infrared spectra, by studying gravity-sensitive features and their evolution with age. We built a dataset composed of all publicly available (~2800) near-infrared spectra of dwarfs with spectral types between M0 and L3. First, we investigate methods for the derivation of the spectral type and extinction using comparison to spectral templates, and various spectral indices. Then, we examine gravity-sensitive spectral indices and apply machine learning methods, in order to efficiently separate young (<~10Myr) objects from the field. Using a set of six spectral indices for spectral typing, including two newly defined ones (TLI-J and TLI-K), we are able to achieve a precision below 1 spectral subtype across the entire spectral type range. We define a new gravity-sensitive spectral index (TLI-g) that consistently separates young from field objects, showing a performance superior to other indices from the literature. Even better separation between the two classes can be achieved through machine learning methods which use the entire NIR spectra as an input. Moreover, we show that the H- and K-bands alone are enough for this purpose. Finally, we evaluate the relative importance of different spectral regions for gravity classification as returned by the machine learning models. We find that the H-band broad-band shape is the most relevant feature, followed by the FeH absorption bands at 1.2um and 1.24um and the KI doublet at 1.24.
- ID:
- ivo://CDS.VizieR/J/A+A/599/A70
- Title:
- Young moving group M-dwarf multiplicity
- Short Name:
- J/A+A/599/A70
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The solar galactic neighbourhood contains a number of young co-moving associations of stars (so-called 'young moving groups') with ages of ~10-150 Myr, which are prime targets for a range of scientific studies, including direct imaging planet searches. The late-type stellar population of such groups still remain in their pre-main sequence phase, and are thus well suited for purposes such as isochronal dating. Close binaries are particularly useful in this regard, since they allow for a model-independent dynamical mass determination. Here we present a dedicated effort to identify new close binaries in nearby young moving groups, through high-resolution imaging with the AstraLux Sur Lucky Imaging camera. We surveyed 181 targets, resulting in the detection of 61 companions or candidates, of which 38 are new discoveries.
- ID:
- ivo://CDS.VizieR/J/MNRAS/410/190
- Title:
- Young runaway stars within 3kpc
- Short Name:
- J/MNRAS/410/190
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Traditionally runaway stars are O and B type stars with large peculiar velocities. We want to extend this definition to young stars (up to ~50Myr) of any spectral type and identify those present in the Hipparcos catalogue applying different selection criteria such as peculiar space velocities or peculiar one-dimensional velocities. Runaway stars are important to study the evolution of multiple star systems or star clusters as well as to identify origins of neutron stars. We compile distances, proper motions, spectral types, luminosity classes, V magnitudes and B-V colours and utilise evolutionary models from different authors to obtain star ages and study a sample of 7663 young Hipparcos stars within 3kpc from the Sun. Radial velocities are obtained from the literature. We investigate the distributions of the peculiar spatial velocity, the peculiar radial velocity as well as the peculiar tangential velocity and its one-dimensional components and obtain runaway star probabilities for each star in the sample. In addition, we look for stars that are situated outside any OB association or OB cluster and the Galactic plane as well as stars of which the velocity vector points away from the median velocity vector of neighbouring stars or the surrounding local OB association/cluster although the absolute velocity might be small. We find a total of 2547 runaway star candidates (with a contamination of normal Population I stars of 20 per cent at most). Thus, after subtraction of those 20 per cent, the runaway frequency among young stars is about 27 per cent. We compile a catalogue of runaway stars which will be available via VizieR.
- ID:
- ivo://CDS.VizieR/J/MNRAS/477/298
- Title:
- Young star clusters in nearby molecular clouds
- Short Name:
- J/MNRAS/477/298
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The SFiNCs (Star Formation in Nearby Clouds) project is an X-ray/infrared study of the young stellar populations in 22 star-forming regions with distances <~1kpc designed to extend our earlier MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray) survey of more distant clusters. Our central goal is to give empirical constraints on cluster formation mechanisms. Using parametric mixture models applied homogeneously to the catalogue of SFiNCs young stars, we identify 52 SFiNCs clusters and 19 unclustered stellar structures. The procedure gives cluster properties including location, population, morphology, association with molecular clouds, absorption, age (Age_JX_), and infrared spectral energy distribution (SED) slope. Absorption, SED slope, and Age_JX_ are age indicators. SFiNCs clusters are examined individually, and collectively with MYStIX clusters, to give the following results. (1) SFiNCs is dominated by smaller, younger, and more heavily obscured clusters than MYStIX. (2) SFiNCs cloud-associated clusters have the high ellipticities aligned with their host molecular filaments indicating morphology inherited from their parental clouds. (3) The effect of cluster expansion is evident from the radius-age, radius-absorption, and radius-SED correlations. Core radii increase dramatically from ~0.08 to ~0.9pc over the age range 1-3.5Myr. Inferred gas removal time-scales are longer than 1Myr. (4) Rich, spatially distributed stellar populations are present in SFiNCs clouds representing early generations of star formation. An appendix compares the performance of the mixture models and non-parametric minimum spanning tree to identify clusters. This work is a foundation for future SFiNCs/MYStIX studies including disc longevity, age gradients, and dynamical modelling.
- ID:
- ivo://CDS.VizieR/J/AJ/130/188
- Title:
- Young stars in Trumpler 37 and NGC 7160
- Short Name:
- J/AJ/130/188
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of MMT observations of young stars for our study of protoplanetary disks at ages 1-10Myr in two young clusters located in the Cepheus OB2 region: Trumpler 37 (embedded in the HII region IC 1396) and NGC 7160. Using low-resolution optical spectra from the Hectospec multifiber spectrograph, we have tripled the number of known low-mass cluster members, identifying 130 new members in Tr 37 and 30 in NGC 7160. We use indicators of youth (Li absorption at 6707{AA}) and accretion/chromospheric activity (H{alpha} emission) to identify and classify the low-mass cluster members. We derive spectral types for all the low-mass candidates and calculate the individual extinctions and the average over the clusters.
- ID:
- ivo://CDS.VizieR/J/A+AS/124/449
- Title:
- Young stars south of Taurus-Auriga
- Short Name:
- J/A+AS/124/449
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present results of intermediate resolution spectroscopy of 131 optical counterparts to 115 ROSAT All-Sky Survey X-ray sources south of the Taurus-Auriga dark cloud complex. These objects have been selected as candidate young stars from a total of 1084 ROSAT sources in a ~300 square degree area. We identify 30 objects as low-mass PMS stars on the basis of the LiI{lambda}6708{AA} doublet in their spectrum, a signature of their young age. All these stars have a spectral type later than F7 and show spectral characteristics typical of weak-line and post-T Tauri stars. The presence of young objects several parsecs away from the regions of ongoing star formation is discussed in the light of the current models of T Tauri dispersal.
