VOSA: virtual observatory SED analyzer Bayo, A.; Rodrigo, C.; Barrado y Navascués, D. ...
Astronomy and astrophysics (Berlin),
12/2008, Volume:
492, Issue:
1
Journal Article
Peer reviewed
Context. The physical properties of almost any kind of astronomical object can be derived by fitting synthetic spectra or photometry extracted from theoretical models to observational data. Aims. We ...want to develop an automatic procedure to perform this kind of fitting to a relatively large sample of members of a stellar association and apply this methodology to the case of Collinder 69. Methods. We combine the multiwavelength data of our sources and follow a work-flow to derive the physical parameters of the sources. The key step of the work-flow is performed by a new VO-tool, VOSA. All the steps in this process are done in a VO environment. Results. We present this new tool, and provide physical parameters such as T_{eff}, gravity, luminosity, etc. for similar to 170 candidate members of Collinder 69, and an upper-limit for the age of this stellar association. Conclusions. This kind of study of star forming regions, clusters, etc. produces a huge amount of data, very tedious to analyse using the traditional methodology. Thus, they are excellent examples of where to apply the VO techniques.
We analyze Spitzer images of 30 long-duration gamma-ray burst (GRB) host galaxies. We estimate their total stellar masses (M{sub *}) based on the rest-frame K-band luminosities (L{sub K{sub r{sub ...e{sub s{sub t}}}) and constrain their star formation rates (SFRs; not corrected for dust extinction) based on the rest-frame UV continua. Further, we compute a mean M{sub *}/L{sub K{sub r{sub e{sub s{sub t}}} = 0.45 M{sub sun}/L{sub sun}. We find that the hosts are low M{sub *}, star-forming systems. The median M{sub *} in our sample ((M{sub *}) = 10{sup 9.7} M{sub sun}) is lower than that of 'field' galaxies (e.g., Gemini Deep Deep Survey). The range spanned by M{sub *} is 10{sup 7} M{sub sun} < M{sub *} < 10{sup 11} M{sub sun}, while the range spanned by the dust-uncorrected UV SFR is 10{sup -2} M{sub sun} yr{sup -1} < SFR < 10 M{sub sun} yr{sup -1}. There is no evidence for intrinsic evolution in the distribution of M{sub *} with redshift. We show that extinction by dust must be present in at least 25% of the GRB hosts in our sample and suggest that this is a way to reconcile our finding of a relatively lower UV-based, specific SFR ({phi} {identical_to} SFR/M{sub *}) with previous claims that GRBs have some of the highest {phi} values. We also examine the effect that the inability to resolve the star-forming regions in the hosts has on {phi}.
The discovery of an M4+T8.5 binary system Burningham, Ben; Pinfield, D. J.; Leggett, S. K. ...
Monthly Notices of the Royal Astronomical Society,
05/2009, Volume:
395, Issue:
3
Journal Article
Peer reviewed
Open access
We report the discovery of a T8.5 dwarf, which is a companion to the M4 dwarf Wolf 940. At a distance of 12.50+0.75−0.67 pc, the angular separation of 32 arcsec corresponds to a projected separation ...of 400 au. The M4 primary displays no Hα emission, and we apply the age–activity relations of West et al. to place a lower limit on the age of the system of 3.5 Gyr. Weak Hα absorption suggests some residual activity, and we estimate an upper age limit of 6 Gyr. We apply the relations of Bonfils et al. for V−Ks and to determine the metallicity, Fe/H=−0.06 ± 0.20 for Wolf 940A, and by extension the T8.5 secondary, Wolf 940B. We have obtained JHK NIRI spectroscopy and JHKL′ photometry of Wolf 940B, and use these data, in combination with theoretical extensions, to determine its bolometric flux, Fbol= 1.75 ± 0.18 × 10−16 W m−2, and thus its luminosity log (L*/L⊙) =−6.07 ± 0.04. Using the age constraints for the system and evolutionary structural models of Baraffe et al., we determine Teff= 570 ± 25 K and log g= 4.75 − 5.00 for Wolf 940B, based on its bolometric luminosity. This represents the first determination of these properties for a T8+ dwarf that does not rely on the fitting of T dwarf spectral models. This object represents the first system containing a T8+ dwarf for which fiducial constraints on its properties are available, and we compare its spectra with those of the latest very cool BT–Settl models. This clearly demonstrates that the use of the (WJ, K/J) spectral ratios (used previously to constrain Teff and log g) would have overestimated Teff by ∼100 K.
Context.
Stellar ages are key to improving our understanding of different astrophysical phenomena. However, many techniques to estimate stellar ages are highly model-dependent. The lithium depletion ...boundary (LDB), based on the presence or absence of lithium in low-mass stars, can be used to derive ages in stellar associations of between 20 and 500 Ma.
Aims.
The purpose of this work is to revise former LDB ages in stellar associations in a consistent way, taking advantage of the homogeneous
Gaia
parallaxes as well as bolometric luminosity estimations that do not rely on monochromatic bolometric corrections.
Methods.
We studied nine open clusters and three moving groups characterised by a previous determination of the LDB age. We gathered all the available information from our data and the literature: membership, distances, photometric data, reddening, metallicity, and surface gravity. We re-assigned membership and calculated bolometric luminosities and effective temperatures using distances derived from
Gaia
DR2 and multi-wavelength photometry for individual objects around the former LDB. We located the LDB using a homogeneous method for all the stellar associations. Finally, we estimated the age by comparing it with different evolutionary models.
Results.
We located the LDB for the twelve stellar associations and derived their ages using several theoretical evolutionary models. We compared the LDB ages among them, along with data obtained with other techniques, such as isochrone fitting, ultimately finding some discrepancies among the various approaches. Finally, we remark that the 32 Ori MG is likely to be composed of at least two populations of different ages.
Although mid-to-late type M dwarfs are the most common stars in our stellar neighbourhood, our knowledge of these objects is still limited. Open questions include the evolution of their angular ...momentum, internal structures, dust settling in their atmospheres and age dispersion within populations. In addition, at young ages, late-type Ms have masses below the hydrogen burning limit and therefore are key objects in the debate on the brown dwarf mechanism of formation. In this work, we determine and study in detail the physical parameters of two samples of young, late M-type sources belonging to either the Chamaeleon I dark cloud or the TW Hydrae Association and compare them with the results obtained in the literature for other young clusters and also for older, field, dwarfs. We used multiwavelength photometry to construct and analyse SEDs to determine general properties of the photosphere and disc presence. We also used low-resolution optical and near-infrared spectroscopy to study activity, accretion, gravity and effective temperature sensitive indicators. We propose a Virtual Observatory-based spectral index that is both temperature and age sensitive. We derived physical parameters using independent techniques confirming the already common feature/problem of the age/luminosity spread. In particular, we highlight two brown dwarfs showing very similar temperatures but clearly different surface gravity (explained invoking extreme early accretion). We also show how, despite large improvement in the dust treatment in theoretical models, there is still room for further progress in the simultaneous reproduction of the optical and near-infrared features of these cold young objects.
Experimental results show that hosing of a long particle bunch in plasma can be induced by wakefields driven by a short, misaligned preceding bunch. Hosing develops in the plane of misalignment, ...self-modulation in the perpendicular plane, at frequencies close to the plasma electron frequency, and are reproducible. Development of hosing depends on misalignment direction, its growth on misalignment extent and on proton bunch charge. Results have the main characteristics of a theoretical model, are relevant to other plasma-based accelerators and represent the first characterization of hosing.
A long, narrow, relativistic charged particle bunch propagating in plasma is subject to the self -modulation (SM) instability. We show that SM of a proton bunch can be seeded by the wakefields driven ...by a preceding electron bunch. SM timing reproducibility and control are at the level of a small fraction of the modulation period. With this seeding method, we independently control the amplitude of the seed wakefields with the charge of the electron bunch and the growth rate of SM with the charge of the proton bunch. Seeding leads to larger growth of the wakefields than in the instability case.
Context. The classical T Tauri star (CTTS) AA Tau has presented photometric variability that was attributed to an inner disk warp, caused by the interaction between the inner disk and an inclined ...magnetosphere. Previous studies of the young cluster NGC 2264 have shown that similar photometric behavior is common among CTTS. Aims. The goal of this work is to investigate the main causes of the observed photometric variability of CTTS in NGC 2264 that present AA Tau-like light curves, and verify if an inner disk warp could be responsible for their observed variability. Methods. In order to understand the mechanism causing these stars’ photometric behavior, we investigate veiling variability in their spectra and u − r color variations and estimate parameters of the inner disk warp using an occultation model proposed for AA Tau. We also compare infrared Spitzer IRAC and optical CoRoT light curves to analyze the dust responsible for the occultations. Results. AA Tau-like variability proved to be transient on a timescale of a few years. We ascribe this variability to stable accretion regimes and aperiodic variability to unstable accretion regimes and show that a transition, and even coexistence, between the two is common. We find evidence of hot spots associated with occultations, indicating that the occulting structures could be located at the base of accretion columns. We find average values of warp maximum height of 0.23 times its radial location, consistent with AA Tau, with variations of on average 11% between rotation cycles. We also show that extinction laws in the inner disk indicate the presence of grains larger than interstellar grains. Conclusions. The inner disk warp scenario is consistent with observations for all but one star with AA Tau-like variability in our sample. AA Tau-like systems are fairly common, comprising 14% of CTTS observed in NGC 2264, though this number increases to 35% among systems of mass 0.7 M⊙ ≲ M ≲ 2.0 M⊙. Assuming random inclinations, we estimate that nearly all systems in this mass range likely possess an inner disk warp. We attribute this to a possible change in magnetic field configurations among stars of lower mass.
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