This paper presents state-of-the-art spectral energy distributions (SEDs) of four Herbig Ae stars, based in part on new data in the mid and far-infrared and at millimeter wavelengths. The SEDs are ...discussed in the context of circumstellar disk models. We show that models of irradiated disks provide a good fit to the observations over the whole range of wavelengths. We offer a possible solution to the long-standing puzzle caused by the excess emission of Herbig Ae stars, where a large fraction of the stellar luminosity is re-radiated between ~1.25 and 7 μm, with a peak at about 3 μm. We suggest that this general behaviour can be caused by dust evaporation in disks where the gas component is optically thin to the stellar radiation, as expected if the accretion rate is very low. The creation of a puffed-up inner wall of optically thick dust at the dust sublimation radius can account for the near-infrared characteristics of the SEDs. It can also naturally explain the H and K band interferometric observations of AB Aur (Millan-Gabet et al. CITE), which reveal a ring of emission of radius ~0.3 AU. Finally, irradiated disk models can easily explain the observed intensity of the 10 μm silicate features and their variation from star to star.
The
Gaia
-ESO Survey is an European Southern Observatory (ESO) public spectroscopic survey that targeted 10
5
stars in the Milky Way covering the major populations of the disk, bulge and halo. The ...observations were made using FLAMES on the VLT obtaining both UVES high (
R
~ 47 000) and GIRAFFE medium (
R
~ 20 000) resolution spectra. The analysis of the
Gaia
-ESO spectra was the work of multiple analysis teams (nodes) within five working groups (WG). The homogenisation of the stellar parameters within WG11 (high resolution observations of FGK stars) and the homogenisation of the stellar parameters within WG10 (medium resolution observations of FGK stars) is described here. In both cases, the homogenisation was carried out using a Bayesian Inference method developed specifically for the
Gaia
-ESO Survey by WG11. The method was also used for the chemical abundance homogenisation within WG11, however, the WG10 chemical abundance data set was too sparsely populated so basic corrections for each node analysis were employed for the homogenisation instead. The WG10 homogenisation primarily used the cross-match of stars with WG11 as the reference set in both the stellar parameter and chemical abundance homogenisation. In this way the WG10 homogenised results have been placed directly onto the WG11 stellar parameter and chemical abundance scales. The reference set for the metal-poor end was sparse which limited the effectiveness of the homogenisation in that regime. For WG11, the total number of stars for which stellar parameters were derived was 6 231 with typical uncertainties for
T
eff
, log
g
and Fe/H of 32 K, 0.05 and 0.05 respectively. One or more chemical abundances out of a possible 39 elements were derived for 6 188 of the stars. For WG10, the total number of stars for which stellar parameters were derived was 76 675 with typical uncertainties for
T
eff
, log
g
and Fe/H of 64 K, 0.15 and 0.07 respectively. One or more chemical abundances out of a possible 30 elements were derived for 64177 of the stars.
Investigating the physical mechanisms driving the dynamical evolution of young star clusters is fundamental to our understanding of the star formation process and the properties of the Galactic field ...stars. The young (~2 Myr) and partially embedded cluster Chamaeleon I is one of the closest laboratories for the study of the early stages of star cluster dynamics in a low-density environment. The aim of this work is to study the structural and kinematical properties of this cluster combining parameters from the high-resolution spectroscopic observations of the Gaia-ESO Survey with data from the literature. Our main result is the evidence of a large discrepancy between the velocity dispersion (σstars = 1.14 ± 0.35 km s-1) of the stellar population and the dispersion of the pre-stellar cores (~0.3 km s-1) derived from submillimeter observations. The origin of this discrepancy, which has been observed in other young star clusters, is not clear. It has been suggested that it may be due to either the effect of the magnetic field on the protostars and the filaments or to the dynamical evolution of stars driven by two-body interactions. Furthermore, the analysis of the kinematic properties of the stellar population puts in evidence a significant velocity shift (~1 km s-1) between the two subclusters located around the north and south main clouds of the cluster. This result further supports a scenario where clusters form from the evolution of multiple substructures rather than from a monolithic collapse. Using three independent spectroscopic indicators (the gravity indicator γ, the equivalent width of the Li line at 6708 Å, and the Hα 10% width), we performed a new membership selection. We found six new cluster members all located in the outer region of the cluster, proving that Chamaeleon I is probably more extended than previously thought. Starting from the positions and masses of the cluster members, we derived the level of substructure Q, the surface density Σ, and the level of mass segregation ΛMSR of the cluster. The comparison between these structural properties and the results of N-body simulations suggests that the cluster formed in a low-density environment, in virial equilibrium or a supervirial state, and highly substructured.
We use Gaia-TGAS data to compare the transverse velocities in Galactic longitude (coming from proper motions and parallaxes) in the Milky Way disk for negative and positive longitudes as a function ...of distance. The transverse velocities are strongly asymmetric and deviate significantly from the expectations for an axisymmetric galaxy. The value and sign of the asymmetry changes at spatial scales of several tens of degrees in Galactic longitude and about 0.5 kpc in distance. The asymmetry is statistically significant at 95% confidence level for 57% of the region probed, which extends up to ~ 1.2 kpc. A percentage of 24% of the region shows absolute differences at this confidence level larger than 5 km s-1 and 7% larger than 10 km s-1. The asymmetry pattern shows mild variations in the vertical direction and with stellar type. A first qualitative comparison with spiral arm models indicates that the arms are probably not the main source of the asymmetry. We briefly discuss alternative origins. This is the first time that global all-sky asymmetries are detected in the Milky Way kinematics beyond the local neighbourhood and with a purely astrometric sample.
As part of the science demonstration phase of the Herschel mission of the Gould Belt key program, the Aquila rift molecular complex has been observed. The complete ~ 3.3° × 3.3° imaging with SPIRE ...250/350/500 μm and PACS 70/160 μm allows a deep investigation of embedded protostellar phases, probing of the dust emission from warm inner regions at 70 and 160 μm to the bulk of the cold envelopes between 250 and 500 μm. We used a systematic detection technique operating simultaneously on all Herschel bands to build a sample of protostars. Spectral energy distributions are derived to measure luminosities and envelope masses, and to place the protostars in an Menv – Lbol evolutionary diagram. The spatial distribution of protostars indicates three star-forming sites in Aquila, with W40/Sh2-64 HII region by far the richest. Most of the detected protostars are newly discovered. For a reduced area around the Serpens South cluster, we could compare the Herschel census of protostars with Spitzer results. The Herschel protostars are younger than in Spitzer with 7 Class 0 YSOs newly revealed by Herschel. For the entire Aquila field, we find a total of ~45–60 Class 0 YSOs discovered by Herschel. This confirms the global statistics of several hundred Class 0 YSOs that should be found in the whole Gould Belt Survey.
The dependence of the mass accretion rate on the stellar properties is a key constraint for star formation and disk evolution studies. Here we present a study of a sample of stars in the Chamaeleon I ...star-forming region carried out using spectra taken with the ESO VLT/X-shooter spectrograph. The sample is nearly complete down to stellar masses (M⋆) ~0.1 M⊙ for the young stars still harboring a disk in this region. We derive the stellar and accretion parameters using a self-consistent method to fit the broadband flux-calibrated medium resolution spectrum. The correlation between accretion luminosity to stellar luminosity, and of mass accretion rate to stellar mass in the logarithmic plane yields slopes of 1.9 ± 0.1 and 2.3 ± 0.3, respectively. These slopes and the accretion rates are consistent with previous results in various star-forming regions and with different theoretical frameworks. However, we find that a broken power-law fit, with a steeper slope for stellar luminosity lower than ~0.45 L⊙ and for stellar masses lower than ~0.3 M⊙ is slightly preferred according to different statistical tests, but the single power-law model is not excluded. The steeper relation for lower mass stars can be interpreted as a faster evolution in the past for accretion in disks around these objects, or as different accretion regimes in different stellar mass ranges. Finally, we find two regions on the mass accretion versus stellar mass plane that are empty of objects: one region at high mass accretion rates and low stellar masses, which is related to the steeper dependence of the two parameters we derived. The second region is located just above the observational limits imposed by chromospheric emission, at M⋆ ~ 0.3 − 0.4 M⊙. These are typical masses where photoevaporation is known to be effective. The mass accretion rates of this region are ~10-10M⊙/yr, which is compatible with the value expected for photoevaporation to rapidly dissipate the inner disk.
Context. Carte du Ciel was a global international project at the end of the nineteenth and beginning of the twentieth century to map the sky to about magnitude 14 on photographic plates. The full ...project was never observationally completed and a large fraction of the observations made remain unanalyzed. Aims. We want to study whether the astrometric and photometric accuracies obtained for the Carte du Ciel plates digitized with a commercial digital camera are high enough for scientific exploitation of the plates. Methods. We use a digital camera Canon EOS 5Ds, with a 100 mm macrolens for digitizing. We analyze six single-exposure plates and four triple-exposure plates from the Helsinki zone of Carte du Ciel (+39∘ ≤ δ ≤ +47∘). Each plate is digitized using four images, with a significant central area being covered twice for quality control purposes. The astrometric calibration of the digitized images is done with the data from the Tycho-Gaia Astrometric Solution (Gaia TGAS) of the first Gaia data release (Gaia DR1), Tycho-2, Hot Stuff for One Year (HSOY), USNO CCD Astrograph Catalog (UCAC5), and PMA catalogs. Results. The best astrometric accuracy is obtained with the UCAC5 reference stars. The astrometric accuracy for single-exposure plates is σ(α cos(δ)) = 0.16″ and σ(δ)=0.15″, expressed as a Gaussian deviation of the astrometric residuals. For triple-exposure plates the astrometric accuracy is σ(α cos(δ)) = 0.12″ and σ(δ)=0.13″. The 1 − σ uncertainty of photometric calibration is about 0.28 mag and 0.24 mag for single- and triple-exposure plates, respectively. We detect the photographic adjacency (Kostinsky) effect in the triple-exposure plates. Conclusions. We show that accuracies at least of the level of scanning machines can be achieved with a digital camera, without any corrections for possible distortions caused by our instrumental setup. This method can be used to rapidly and inexpensively digitize and calibrate old photographic plates enabling their scientific exploitation.
Gaia Data Release 2 Luri, X.; Brown, A. G. A.; Sarro, L. M. ...
Astronomy and astrophysics (Berlin),
08/2018, Letnik:
616
Journal Article
Recenzirano
Context. The second Gaia data release (Gaia DR2) provides precise five-parameter astrometric data (positions, proper motions, and parallaxes) for an unprecedented number of sources (more than 1.3 ...billion, mostly stars). This new wealth of data will enable the undertaking of statistical analysis of many astrophysical problems that were previously infeasible for lack of reliable astrometry, and in particular because of the lack of parallaxes. However, the use of this wealth of astrometric data comes with a specific challenge: how can the astrophysical parameters of interest be properly inferred from these data? Aims. The main focus of this paper, but not the only focus, is the issue of the estimation of distances from parallaxes, possibly combined with other information. We start with a critical review of the methods traditionally used to obtain distances from parallaxes and their shortcomings. Then we provide guidelines on how to use parallaxes more efficiently to estimate distances by using Bayesian methods. In particular we also show that negative parallaxes, or parallaxes with relatively large uncertainties still contain valuable information. Finally, we provide examples that show more generally how to use astrometric data for parameter estimation, including the combination of proper motions and parallaxes and the handling of covariances in the uncertainties. Methods. The paper contains examples based on simulated Gaia data to illustrate the problems and the solutions proposed. Furthermore, the developments and methods proposed in the paper are linked to a set of tutorials included in the Gaia archive documentation that provide practical examples and a good starting point for the application of the recommendations to actual problems. In all cases the source code for the analysis methods is provided. Results. Our main recommendation is to always treat the derivation of (astro-)physical parameters from astrometric data, in particular when parallaxes are involved, as an inference problem which should preferably be handled with a full Bayesian approach. Conclusions. Gaia will provide fundamental data for many fields of astronomy. Further data releases will provide more data, and more precise data. Nevertheless, to fully use the potential it will always be necessary to pay careful attention to the statistical treatment of parallaxes and proper motions. The purpose of this paper is to help astronomers find the correct approach.
Gaia is an ESA mission to build a census of our Milky Way Galaxy. This is done with a suite of three instruments combining two fields of view into a single focal plane. On-board detection of all ...sources will ensure a bias free survey down to the 20th magnitude. In addition to astrometry also photometry is done down to the 20th magnitude. The third, spectroscopic instrument will provide radial velocities down to the 17th magnitude of the Galactic stellar population. Currently the scientific requirements on Gaia are largely met except for bright stars for which in astrometry additional efforts are needed to squeeze the remaining μarcsecs out of the positional information error budgets. In the mid-magnitude range (15 mag) the end of mission parallax errors are from 10 to 25 μarcsec and photometry errors are between 5 and 16 mmag. Radial velocities are better than 1 km s-1 for the bright stars with errors increasing to 15 km s-1 for the faintest ones. At the moment there are no unsolvable technical challenges on the table and Gaia is progressing toward the launch in 2012.
We report on the Herschel Gould Belt survey (HGBS) of the Chamaeleon II (Cha II) star-forming region, focusing on the detection of Class I to III young stellar objects (YSOs). We aim at ...characterizing the circumstellar material around these YSOs and at understanding which disk parameters are most likely constrained by the new HGBS data, which are expected to be crucial for studying the transition from optically thick disks to evolved debris-type disks. The estimated flaring angles, although very uncertain, point toward very flat disks, as found for low-mass M-type YSO samples in other star-forming regions. Thus, our results support the idea that disk properties show a dependence on stellar properties.
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