We analyze high-resolution (Δv ≤ 10 km s−1) optical and infrared spectra covering the O i λ6300 and Ne ii 12.81 m lines from a sample of 31 disks in different evolutionary stages. Following work at ...optical wavelengths, we use Gaussian profiles to fit the Ne ii lines and classify them into high-velocity component (HVC) or low-velocity component (LVC) if the line centroid is more or less blueshifted than 30 km s−1 with respect to the stellar radial velocity, respectively. Unlike for the O i, where an HVC is often accompanied by an LVC, all 17 sources with an Ne ii detection have either an HVC or an LVC. Ne ii HVCs are preferentially detected toward high accretors ( M yr−1), while LVCs are found in sources with low , low O i luminosity, and large infrared spectral index (n13-31). Interestingly, the Ne ii and O i LVC luminosities display an opposite behavior with n13-31: as the inner dust disk depletes (higher n13-31), the Ne ii luminosity increases while the O i weakens. The Ne ii and O i HVC profiles are generally similar, with centroids and FWHMs showing the expected behavior from shocked gas in microjets. In contrast, the Ne ii LVC profiles are typically more blueshifted and narrower than the O i profiles. The FWHM and centroid versus disk inclination suggest that the Ne ii LVC predominantly traces unbound gas from a slow, wide-angle wind that has not lost completely the Keplerian signature from its launching region. We sketch an evolutionary scenario that could explain the combined O i and Ne ii results and includes screening of hard (∼1 keV) X-rays in inner, mostly molecular, MHD winds.
ABSTRACT
The past two decades have seen dramatic progress in our knowledge of the population of young stars of age $\lt \!200\,$ Myr that lie within $150\,$ pc of the Sun. These nearby, young stars, ...most of which are found in loose, comoving groups, provide the opportunity to explore (among many other things) the dissolution of stellar clusters and their diffusion into the field star population. Here, we exploit the combination of astrometric and photometric data from Gaia and photometric data from GALEX (UV) and 2MASS (near-IR) in an attempt to identify additional nearby, young, late-type stars. Specifically, we present a sample of 146 GALEX UV-selected late-type (predominantly K-type) field stars with Gaia-based distances $\lt \!125\,$ pc (based on Gaia Data Release 1) that have isochronal ages $\lt \!80\,$ Myr even if equal-components binaries. We investigate the spectroscopic and kinematic properties of this sample. Despite their young isochronal ages, only ∼10 per cent of stars among this sample can be confidently associated with established nearby, young moving groups (MGs). These candidate MG members include five stars newly identified in this study. The vast majority of our sample of 146 nearby young star candidates have anomalous kinematics relative to the known MGs. These stars may hence represent a previously unrecognized population of young stars that has recently mixed into the older field star population. We discuss the implications and caveats of such a hypothesis – including the intriguing fact that, in addition to their non-young-star-like kinematics, the majority of the UV-selected, isochronally young field stars within $50\,$ pc appear surprisingly X-ray faint.
The Ariel mission will characterise the chemical and thermal properties of the atmospheres of about a thousand exoplanets transiting their host star(s). The observation of such a large sample of ...planets will allow to deepen our understanding of planetary and atmospheric formation at the early stages, providing a truly representative picture of the chemical nature of exoplanets, and relating this directly to the type and chemical environment of the host star. Hence, the accurate and precise determination of the host star fundamental properties is essential to Ariel for drawing a comprehensive picture of the underlying essence of these planetary systems. We present here a structured approach for the characterisation of Ariel stars that accounts for the concepts of homogeneity and coherence among a large set of stellar parameters. We present here the studies and benchmark analyses we have been performing to determine robust stellar fundamental parameters, elemental abundances, activity indices, and stellar ages. In particular, we present results for the homogeneous estimation of the activity indices
S
and
log
(
R
HK
′
)
, and preliminary results for elemental abundances of Na, Al, Mg, Si, C, N. In addition, we analyse the variation of a planetary spectrum, obtained with Ariel, as a function of the uncertainty on the stellar effective temperature. Finally, we present our observational campaign for precisely and homogeneously characterising all Ariel stars in order to perform a meaningful choice of final targets before the mission launch.
ABSTRACT We present results from a spectroscopic Spitzer and Herschel mid-to-far-infrared study of the circumbinary disk orbiting the evolved (age ∼12-23 Myr) close binary T Tauri system V4046 Sgr. ...Spitzer InfraRed Spectrograph spectra show emission lines of Ne ii, H2 S(1), CO2, and HCN, while Herschel Photodetector Array Camera and Spectrometer and Spectral and Photometric Imaging Receiver spectra reveal emission from O i, OH, and tentative detections of H2O and high-J transitions of CO. We measure Ne iii/Ne ii 0.13, which is comparable to other X-ray/EUV luminous T Tauri stars that lack jets. We use the H2 S(1) line luminosity to estimate the gas mass in the relatively warm surface layers of the inner disk. The presence of O i emission suggests that CO, H2O, and/or OH is being photodissociated, and the lack of C i emission suggests any excess C may be locked up in HCN, CN, and other organic molecules. Modeling of silicate dust grain emission features in the mid-infrared indicates that the inner disk is composed mainly of large (r ∼ 5 m) amorphous pyroxene and olivine grains (∼86% by mass) with a relatively large proportion of crystalline silicates. These results are consistent with other lines of evidence indicating that planet building is ongoing in regions of the disk within ∼30 AU of the central, close binary.
The Galex Nearby Young Star Survey (GALNYSS) has yielded a sample of ∼2000 UV-selected objects that are candidate nearby ( D 150 pc ), young (age ∼ 10-100 Myr), late-type stars. Here, we evaluate the ...distances and ages of the subsample of (19) GALNYSS stars with Gaia Data Release 1 (DR1) parallax distances D ≤ 120 pc . The overall youth of these 19 mid-K to early-M stars is readily apparent from their positions relative to the loci of main-sequence stars and giants in Gaia-based color-magnitude and color-color diagrams constructed for all stars detected by Galex and the Wide-field Infrared Space Explorer for which parallax measurements are included in DR1. The isochronal ages of all 19 stars lie in the range ∼10-100 Myr. Comparison with Li-based age estimates indicates a handful of these stars may be young main-sequence binaries rather than pre-main sequence stars. Nine of the 19 objects have not previously been considered as nearby, young stars, and all but one of these are found at declinations north of +30°. The Gaia DR1 results presented here indicate that the GALNYSS sample includes several hundred nearby, young stars, a substantial fraction of which have not been previously recognized as having ages 100 Myr .
We know that most stars form in clusters that very quickly disperse, leaving their members isolated. However, the process driving the early evolution of star clusters and their dissipation is not yet ...clear. Recent results obtained with the large spectroscopic survey Gaia-ESO are providing new interesting clues that can shed more light but also raise new questions.
Abstract
A significant fraction of apparently isochronally young, chromospherically active (UV-luminous) K and early-M stars appear to have anomalous kinematics and display weak X-ray emission. We ...demonstrate that the combination of UV excesses, X-ray fluxes, and newly available rotation rates from the TESS mission can potentially serve to assess the youth (or lack thereof) of these candidate pre-main sequence field stars. This approach holds promise for purposes of distinguishing between “bona fide” young, low-mass stars and “imposters”—overluminous, UV-bright ZAMS or evolved main sequence stars—and could shed light on why the latter might masquerade as the former.
VISIONS: the VISTA Star Formation Atlas Meingast, Stefan; Alves, João; Bouy, Hervé ...
Astronomy and astrophysics (Berlin),
05/2023, Letnik:
673
Journal Article
Recenzirano
Odprti dostop
VISIONS is an ESO public survey of five nearby (
d
< 500 pc) star-forming molecular cloud complexes that are canonically associated with the constellations of Chamaeleon, Corona Australis, Lupus, ...Ophiuchus, and Orion. The survey was carried out with the Visible and Infrared Survey Telescope for Astronomy (VISTA), using the VISTA Infrared Camera (VIRCAM), and collected data in the near-infrared passbands
J
(1.25 μm), H (1.65 μm), and
K
S
(2.15 μm). With a total on-sky exposure time of 49.4h VISIONS covers an area of 650 deg
2
, it is designed to build an infrared legacy archive with a structure and content similar to the Two Micron All Sky Survey (2MASS) for the screened star-forming regions. Taking place between April 2017 and March 2022, the observations yielded approximately 1.15 million images, which comprise 19 TB of raw data. The observations undertaken within the survey are grouped into three different subsurveys. First, the wide subsurvey comprises shallow, large-scale observations and it has revisited the star-forming complexes six times over the course of its execution. Second, the deep subsurvey of dedicated high-sensitivity observations has collected data on areas with the largest amounts of dust extinction. Third, the control subsurvey includes observations of areas of low-to-negligible dust extinction. Using this strategy, the VISIONS observation program offers multi-epoch position measurements, with the ability to access deeply embedded objects, and it provides a baseline for statistical comparisons and sample completeness – all at the same time. In particular, VISIONS is designed to measure the proper motions of point sources, with a precision of 1 mas yr
−1
or better, when complemented with data from the VISTA Hemisphere Survey (VHS). In this way, VISIONS can provide proper motions of complete ensembles of embedded and low-mass objects, including sources inaccessible to the optical ESA
Gaia
mission. VISIONS will enable the community to address a variety of research topics from a more informed perspective, including the 3D distribution and motion of embedded stars and the nearby interstellar medium, the identification and characterization of young stellar objects, the formation and evolution of embedded stellar clusters and their initial mass function, as well as the characteristics of interstellar dust and the reddening law.
Red giant stars are perhaps the most important type of stars for Galactic and extra-galactic archaeology: they are luminous, occur in all stellar populations, and their surface temperatures allow ...precise abundance determinations for many different chemical elements. Yet, the full star formation and enrichment history of a galaxy can be traced directly only if two key observables can be determined for large stellar samples: age and chemical composition. While spectroscopy is a powerful method to analyse the detailed abundances of stars, stellar ages are the missing link in the chain, since they are not a direct observable. However, spectroscopy should be able to estimate stellar masses, which for red giants directly infer ages provided their chemical composition is known. Here we establish a new empirical relation between the shape of the hydrogen line in the observed spectra of red giants and stellar mass determined from asteroseismology. The relation allows determining stellar masses and ages with an accuracy of 10−15%. The method can be used with confidence for stars in the following range of stellar parameters: 4000 < Teff < 5000 K, 0.5 < log g< 3.5, −2.0 < Fe/H < 0.3, and luminosities log L/LSun < 2.5. Our analysis provides observational evidence that the Hα spectral characteristics of red giant stars are tightly correlated with their mass and therefore their age. We also show that the method samples well all stellar populations with ages above 1 Gyr. Targeting bright giants, the method allows obtaining simultaneous age and chemical abundance information far deeper than would be possible with asteroseismology, extending the possible survey volume to remote regions of the Milky Way and even to neighbouring galaxies such as Andromeda or the Magellanic Clouds even with current instrumentation, such as the VLT and Keck facilities.
The survival time of a star cluster depends on its total mass, density, and thus size, as well as on the environment in which it was born and in which lies. Its dynamical evolution is influenced by ...various factors such as gravitational effects of the Galactic bar, spiral structures, and molecular clouds. Overall, the factors that determine the longevity of a cluster are complex and not fully understood. This study aims to investigate if open clusters and field stars respond differently to the perturbations that cause radial migration. In particular, we aim at understanding the nature of the oldest surviving clusters. We compared the time evolution of the kinematic properties of two Gaia DR3 samples: the first sample is composed of \(\sim\)40 open clusters and the second one of \(\sim\)66,000 MSTO field stars. Both selected samples are composed of stars selected with the same quality criterion, belonging to the thin disc, in a similar metallicity range, located in the same Galactocentric region 7.5-9 kpc and with ages >1 Gyr. We performed a statistical analysis comparing the properties of the samples of field stars and of open clusters. A qualitative comparison of kinematic and orbital properties reveals that clusters younger than 2-3 Gyr are more resistant to perturbations than field stars and they move along quasi-circular orbits. Conversely, clusters older than approximately 3 Gyr have more eccentric and inclined orbits than isolated stars in the same age range. Such orbits lead them to reach higher elevations on the Galactic plane, maximising their probability to survive several Gyr longer. A formal statistical analysis reveals that there are differences among the time evolution of most of the kinematic and orbital properties of field stars and open clusters. Our results suggest that oldest survived clusters are usually more massive and move on orbits with higher eccentricity.
PDF
