Aims.We have estimated the age of the young moving group TW Hydrae Association, a cohort of a few dozen stars and brown dwarfs located near the Sun which share the same kinematic properties and, ...presumably, the same origin and age. Methods.The chronology has been determined by analyzing different properties (magnitudes, colors, activity, lithium) of its members and comparing them with several well-known star forming regions and open clusters, as well as theoretical models. In addition, by using medium-resolution optical spectra of two M8 members of the association (2M1139 and 2M1207 – an accreting brown dwarf with a planetary mass companion), we have derived spectral types and measured Hα and lithium equivalent widths. We have also estimated their effective temperature and gravity, which were used to produce an independent age estimation for these two brown dwarfs. We have also collected spectra of 2M1315, a candidate member with a L5 spectral type and measured its Hα equivalent width. Results.Our age estimate for the association, 10$^{+10}_{-7}$ Myr, agrees with previous values cited in the literature. In the case of the two brown dwarfs, we have derived an age of 15$^{+15}_{-10}$ Myr, which also agree with our estimate for the whole group. Conclusions.We compared our results with recent articles published on the same subject using other techniques, and discuss the limits of the age-dating techniques.
Gaia Data Release 1 Brown, A G A; Vallenari, A; Prusti, T ...
Astronomy and astrophysics (Berlin),
11/2016, Letnik:
595
Journal Article, Web Resource
Recenzirano
Odprti dostop
Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7. ...Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release. Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue. Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the Hipparcos and Tycho-2 catalogues - a realisation of the Tycho-Gaia Astrometric Solution (TGAS) - and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ~3000 Cepheid and RR Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr super(-1) for the proper motions. A systematic component of ~0.3 mas should be added to the parallax uncertainties. For the subset of ~94000 Hipparcos stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr super(-1). For the secondary astrometric data set, the typical uncertainty of the positions is ~10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ~0.03 mag over the magnitude range 5 to 20.7. Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data.
ABSTRACT We describe a large-scale far-infrared line and continuum survey of protoplanetary disk through to young debris disk systems carried out using the ACS instrument on the Herschel Space ...Observatory. This Open Time Key program, known as GASPS (Gas Survey of Protoplanetary Systems), targeted ∼250 young stars in narrow wavelength regions covering the OI fine structure line at 63 μm the brightest far-infrared line in such objects. A subset of the brightest targets were also surveyed in OI145 μm, CII at 157 μm, as well as several transitions of H2O and high-excitation CO lines at selected wavelengths between 78 and 180 μm. Additionally, GASPS included continuum photometry at 70, 100 and 160 μm, around the peak of the dust emission. The targets were SED Class II-III T Tauri stars and debris disks from seven nearby young associations, along with a comparable sample of isolated Herbig AeBe stars. The aim was to study the global gas and dust content in a wide sample of circumstellar disks, combining the results with models in a systematic way. In this overview paper we review the scientific aims, target selection and observing strategy of the program. We summarise some of the initial results, showing line identifications, listing the detections, and giving a first statistical study of line detectability. The OI line at 63 μm was the brightest line seen in almost all objects, by a factor of ∼10. Overall OI63 μm detection rates were 49%, with 100% of HAeBe stars and 43% of T Tauri stars detected. A comparison with published disk dust masses (derived mainly from sub-mm continuum, assuming standard values of the mm mass opacity) shows a dust mass threshold for OI63 μm detection of ∼10-5 M⊙. Normalising to a distance of 140 pc, 84% of objects with dust masses ≥10-5 M⊙ can be detected in this line in the present survey; 32% of those of mass 10-6-10-5 M⊙, and only a very small number of unusual objects with lower masses can be detected. This is consistent with models with a moderate UV excess and disk flaring. For a given disk mass, OI detectability is lower for M stars compared with earlier spectral types. Both the continuum and line emission was, in most systems, spatially and spectrally unresolved and centred on the star, suggesting that emission in most cases was from the disk. Approximately 10 objects showed resolved emission, most likely from outflows. In the GASPS sample, OI detection rates in T Tauri associations in the 0.3-4 Myr age range were ∼50%. For each association in the 5-20 Myr age range, ∼2 stars remain detectable in OI63 μm, and no systems were detected in associations with age >20 Myr. Comparing with the total number of young stars in each association, and assuming a ISM-like gas/dust ratio, this indicates that ∼18% of stars retain a gas-rich disk of total mass ∼1 MJupiter for 1-4 Myr, 1-7% keep such disks for 5-10 Myr, but none are detected beyond 10-20 Myr. The brightest OI objects from GASPS were also observed in OI145 μm, CII157 μm and CO J = 18 - 17, with detection rates of 20-40%. Detection of the CII line was not correlated with disk mass, suggesting it arises more commonly from a compact remnant envelope.
We present the discovery by optical and near-infrared imaging of an extremely red, low-luminosity population of isolated objects in the young, nearby stellar cluster around the multiple, massive star ...σ Orionis. The proximity (352 parsecs), youth (1 million to 5 million years), and low internal extinction make this cluster an ideal site to explore the substellar domain from the hydrogen mass limit down to a few Jupiter masses. Optical and near-infrared low-resolution spectroscopy of three of these objects confirms the very cool spectral energy distribution (atmospheric effective temperatures of 1700 to 2200 kelvin) expected for cluster members with masses in the range 5 to 15 times that of Jupiter. Like the planets of the solar system, these objects are unable to sustain stable nuclear burning in their interiors, but in contrast they are not bound to stars. This new kind of isolated giant planet, which apparently forms on time scales of less than a few million years, offers a challenge to our understanding of the formation processes of planetary mass objects.
We investigate the long‐term motion of Saturn's north pole hexagon and the structure of its associated eastward jet, using Cassini imaging science system and ground‐based images from 2008 to 2014. We ...show that both are persistent features that have survived the long polar night, the jet profile remaining essentially unchanged. During those years, the hexagon vertices showed a steady rotation period of 10 h 39 min 23.01 ± 0.01 s. The analysis of Voyager 1 and 2 (1980–1981) and Hubble Space Telescope and ground‐based (1990–1991) images shows a period shorter by 3.5 s due to the presence at the time of a large anticyclone. We interpret the hexagon as a manifestation of a vertically trapped Rossby wave on the polar jet and, because of their survival and unchanged properties under the strong seasonal variations in insolation, we propose that both hexagon and jet are deep‐rooted atmospheric features that could reveal the true rotation of the planet Saturn.
Key Points
Hexagon wave steady motion
Jet stream unchanged to seasonal changes
Saturn's rotation
We present new high spatial resolution (less than or similar to 0 ''.1) 1-5 mu m adaptive optics images, interferometric 1.3 mm continuum and (CO)-C-12 2-1 maps, and 350 mu m, 2.8 and 3.3 mm fluxes ...measurements of the HV Tau system. Our adaptive optics images unambiguously demonstrate that HV Tau AB-C is a common proper motion pair. They further reveal an unusually slow orbital motion within the tight HV Tau AB pair that suggests a highly eccentric orbit and/or a large deprojected physical separation. Scattered light images of the HV Tau C edge-on protoplanetary disk suggest that the anisotropy of the dust scattering phase function is almost independent of wavelength from 0.8 to 5 mu m, whereas the dust opacity decreases significantly over the same range. The images further reveal a marked lateral asymmetry in the disk that does not vary over a timescale of two years. We further detect a radial velocity gradient in the disk in our (CO)-C-12 map that lies along the same position angle as the elongation of the continuum emission, which is consistent with Keplerian rotation around a 0.5-1 M-circle dot central star, suggesting that it could be the most massive component in the triple system. To obtain a global representation of the HV Tau C disk, we search for a model that self-consistently reproduces observations of the disk from the visible regime up to millimeter wavelengths. We use a powerful radiative transfer model to compute synthetic disk observations and use a Bayesian inference method to extract constraints on the disk properties. Each individual image, as well as the spectral energy distribution, of HV Tau C can be well reproduced by our models with fully mixed dust provided grain growth has already produced larger-than-interstellar dust grains. However, no single model can satisfactorily simultaneously account for all observations. We suggest that future attempts to model this source include more complex dust properties and possibly vertical stratification. While both grain growth and stratification have already been suggested in many disks, only a panchromatic analysis, such as presented here, can provide a complete picture of the structure of a disk, a necessary step toward quantitatively testing the predictions of numerical models of disk evolution.
Mid-Infrared Variability of Protostars in IC 1396A Morales-Calderón, M; Stauffer, J. R; Rebull, L ...
Astrophysical journal/The Astrophysical journal,
09/2009, Letnik:
702, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We have used Spitzer/Infrared Array Camera (IRAC) to conduct a photometric monitoring program of the IC1396A dark globule in order to study the mid-IR (3.6-8 Delta *mm) variability of the heavily ...embedded young stellar objects (YSOs) present in that area. We obtained light curves covering a 14 day timespan with a twice daily cadence for 69 YSOs, and continuous light curves with approximately 12 s cadence over 7 hr for 38 YSOs. Typical accuracies for our relative photometry were 1%-2% for the long timespan data and a few millimagnitude, corresponding to less than 0.5%, for the 7 hr continuous 'staring-mode' data. More than half of the YSOs showed detectable variability, with amplitudes from ~0.05 mag to ~0.2 mag. About 30% of the YSOs showed quasi-sinusoidal light-curve shapes with apparent periods from 5 to 12 days and light-curve amplitudes approximately independent of wavelength over the IRAC bandpasses. We have constructed models which simulate the time-dependent spectral energy distributions of Class I and II YSOs in order to attempt to explain these light curves. Based on these models, the apparently periodic light curves are best explained by YSO models where one or two high-latitude photospheric spots heat the inner wall of the circumstellar disk, and where we view the disk at fairly large inclination angle. Disk inhomogeneities, such as increasing the height where the accretion funnel flows to the stellar hot spot, enhances the light-curve modulations. The other YSOs in our sample show a range of light-curve shapes, some of which are probably due to varying accretion rate or disk shadowing events. One star, IC1396A-47, shows a 3.5 hr periodic light curve; this object may be a PMS Delta Scuti star.
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, Letnik:
395, Številka:
3
Journal Article
Recenzirano
Odprti dostop
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.
Convective storms occur regularly in Saturn's atmosphere. Huge storms known as Great White Spots, which are ten times larger than the regular storms, are rarer and occur about once per Saturnian year ...(29.5 Earth years). Current models propose that the outbreak of a Great White Spot is due to moist convection induced by water. However, the generation of the global disturbance and its effect on Saturn's permanent winds have hitherto been unconstrained by data, because there was insufficient spatial resolution and temporal sampling to infer the dynamics of Saturn's weather layer (the layer in the troposphere where the cloud forms). Theoretically, it has been suggested that this phenomenon is seasonally controlled. Here we report observations of a storm at northern latitudes in the peak of a weak westward jet during the beginning of northern springtime, in accord with the seasonal cycle but earlier than expected. The storm head moved faster than the jet, was active during the two-month observation period, and triggered a planetary-scale disturbance that circled Saturn but did not significantly alter the ambient zonal winds. Numerical simulations of the phenomenon show that, as on Jupiter, Saturn's winds extend without decay deep down into the weather layer, at least to the water-cloud base at pressures of 10-12 bar, which is much deeper than solar radiation penetrates.
Context.
Gas giants orbiting close to hot and massive early-type stars can reach dayside temperatures that are comparable to those of the coldest stars. These ‘ultra-hot Jupiters’ have atmospheres ...made of ions and atomic species from molecular dissociation and feature strong day-to-night temperature gradients. Photometric observations at different orbital phases provide insights on the planet’s atmospheric properties.
Aims.
We aim to analyse the photometric observations of WASP-189 acquired with the Characterising Exoplanet Satellite (CHEOPS) to derive constraints on the system architecture and the planetary atmosphere.
Methods.
We implemented a light-curve model suited for an asymmetric transit shape caused by the gravity-darkened photosphere of the fast-rotating host star. We also modelled the reflective and thermal components of the planetary flux, the effect of stellar oblateness and light-travel time on transit-eclipse timings, the stellar activity, and CHEOPS systematics.
Results.
From the asymmetric transit, we measure the size of the ultra-hot Jupiter WASP-189 b, R
p
= 1.600
−0.016
+0.017
R
J
, with a precision of 1%, and the true orbital obliquity of the planetary system, Ψ
p
= 89.6 ± 1.2deg (polar orbit). We detect no significant hotspot offset from the phase curve and obtain an eclipse depth of δ
ecl
= 96.5
−5.0
+4.5
ppm, from which we derive an upper limit on the geometric albedo:
A
g
< 0.48. We also find that the eclipse depth can only be explained by thermal emission alone in the case of extremely inefficient energy redistribution. Finally, we attribute the photometric variability to the stellar rotation, either through superficial inhomogeneities or resonance couplings between the convective core and the radiative envelope.
Conclusions.
Based on the derived system architecture, we predict the eclipse depth in the upcoming Transiting Exoplanet Survey Satellite (TESS) observations to be up to ~165 ppm. High-precision detection of the eclipse in both CHEOPS and TESS passbands might help disentangle reflective and thermal contributions. We also expect the right ascension of the ascending node of the orbit to precess due to the perturbations induced by the stellar quadrupole moment
J
2
(oblateness).
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