The lifetime of isolated protoplanetary disks is thought to be set by the combination of viscous accretion and photoevaporation driven by stellar high-energy photons. Observational evidence for ...magnetospheric accretion in young Sun-like stars is robust. Here we report the first observational evidence for disk photoevaporation driven by the central star. We acquired high-resolution (R~ 30,000) spectra of the Ne II 12.81 Delta *mm line from seven circumstellar disks using VISIR on Melipal/VLT. We show that the three transition disks in the sample all have Ne II line profiles consistent with those predicted by a photoevaporative flow driven by stellar extreme-ultraviolet (EUV) photons. The ~6 km s-1 blueshift of the line from the almost face-on disk of TW Hya is clearly inconsistent with emission from a static disk atmosphere and convincingly points to the presence of a photoevaporative wind. We do not detect any Ne II line close to the stellar velocity from the sample of classical optically thick (nontransition) disks. We conclude that most of the spectrally unresolved Ne II emission in these less-evolved systems arises from jets/outflows rather than from the disk. The pattern of the Ne II detections and nondetections suggests that EUV-driven photoevaporation starts only at a later stage in the disk evolution.
We have surveyed a sample of 165 solar-type spectroscopic binaries (SB) with periods from 1 to 30 days for higher-order multiplicity. A subsample of 62 targets were observed with the NACO adaptive ...optics system and 13 new physical tertiary companions were detected. An additional 12 new wide companions (5 still tentative) were found using the 2MASS all-sky survey. The binaries belong to 161 stellar systems; of these 64 are triple, 11 quadruple and 7 quintuple. After correction for incompleteness, the fraction of SBs with additional companions is found to be 63% ± 5%. We find that this fraction is a strong function of the SB period P, reaching 96% for $P<3^{\rm d}$ and dropping to 34% for $P>12^{\rm d}$. Period distributions of SBs with and without tertiaries are significantly different, but their mass ratio distributions are identical. The statistical data on the multiplicity of close SBs presented in this paper indicates that the periods and mass ratios of SBs were established very early, but the periods of SB systems with triples were further shortened by angular momentum exchange with companions.
Abstract
Spectroastrometry is used to investigate the low-velocity component (LVC) of the optical forbidden emission from the T Tauri stars RU Lupi and AS 205 N. Both stars also have high-velocity ...forbidden emission, which is tracing a jet. For AS 205 N, analysis reveals a complicated outflow system. For RU Lupi, the O
i
λ
6300 and S
ii
λλ
6716,6731 LV narrow component (NC) is offset along the same position angle (PA) as the high-velocity component but with a different velocity gradient than the jet, in that displacement from the stellar position along the rotation axis is decreasing with increasing velocity. From the LVC, NC, PA, and velocity gradient, it is inferred that the NC is tracing a wide-angled magnetohydrodynamic (MHD) disk wind. A photoevaporative wind is ruled out. This is supported by a comparison with a previous spectroastrometric study of the CO fundamental line. The decrease in offset with increasing velocity is interpreted as tracing an increase in the height of the wind with increasing disk radius. This is one of the first measurements of the spatial extent of the forbidden emission line LVC NC (∼40 au, 8 au for RU Lupi in the S
ii
λ
6731 and O
i
λ
6300 lines) and the first direct confirmation that the LVC narrow component can trace an MHD disk wind.
Aims.This paper describes a systematic search for high-order multiplicity among wide visual Pre-Main Sequence (PMS) binaries. Methods.We conducted an Adaptive Optics survey of a sample of 58 PMS wide ...binaries from various star-forming regions, which include 52 T Tauri systems with mostly K- and M-type primaries, with the NIR instrument NACO at the VLT. Results.Of these 52 systems, 7 are found to be triple (2 new) and 7 quadruple (1 new). The new close companions are most likely physically bound based on their probability of chance projection and, for some of them, on their position on a color–color diagram. The corresponding degree of multiplicity among wide binaries (number of triples and quadruples divided by the number of systems) is 26.9 ± 7.2% in the projected separation range ~0$\farcs$07–12″, with the largest contribution from the Taurus-Auriga cloud. We also found that this degree of multiplicity is twice in Taurus compared to Ophiuchus and Chamaeleon for which the same number of sources are present in our sample. Considering a restricted sample composed of systems at distance 140–190 pc, the degree of multiplicity is 26.8 ± 8.1%, in the separation range 10/14 AU–1700/2300 AU (30 binaries, 5 triples, 6 quadruples). The observed frequency agrees with results from previous multiplicity surveys within the uncertainties, although a significant overabundance of quadruple systems compared to triple systems is apparent. Tentatively including the spectroscopic pairs in our restricted sample and comparing the multiplicity fractions to those measured for solar-type main-sequence stars in the solar neighborhood leads to the conclusion that both the ratio of triples to binaries and the ratio of quadruples to triples seems to be in excess among young stars. Most of the current numerical simulations of multiple star formation, and especially smoothed particles hydrodynamics simulations, over-predict the fraction of high-order multiplicity when compared to our results. The circumstellar properties around the individual components of our high-order multiple systems tend to favor mixed systems (i.e. systems including components of wTTS and cTTS type), which is in general agreement with previous studies of disks in binaries, with the exception of Taurus, where we find a preponderance of similar type of components among the multiples studied.
Context.
Young nearby stars are good candidates in the search for planets with both radial velocity (RV) and direct imaging techniques. This, in turn, allows for the computation of the giant planet ...occurrence rates at all separations. The RV search around young stars is a challenge as they are generally faster rotators than older stars of similar spectral types and they exhibit signatures of magnetic activity (spots) or pulsation in their RV time series. Specific analyses are necessary to characterize, and possibly correct for, this activity.
Aims.
Our aim is to search for planets around young nearby stars and to estimate the giant planet (GP) occurrence rates for periods up to 1000 days.
Methods.
We used the HARPS spectrograph on the 3.6 m telescope at La Silla Observatory to observe 89
A
−
M
young (<600 Myr) stars. We used our SAFIR (Spectroscopic data via Analysis of the Fourier Interspectrum Radial velocities) software to compute the RV and other spectroscopic observables. Then, we computed the companion occurrence rates on this sample.
Results.
We confirm the binary nature of HD 177171, HD 181321 and HD 186704. We report the detection of a close low mass stellar companion for HIP 36985. No planetary companion was detected. We obtain upper limits on the GP (<13
M
Jup
) and BD (∈ 13;80
M
Jup
) occurrence rates based on 83 young stars for periods less than 1000 days, which are set, 2
−2
+3
and 1
−1
+3
%.
We explore the possible connection between the open cluster IC 2391 and the unbound Argus association identified by the search for associations containing young stars survey. In addition to common ...kinematics and ages between these two systems, here we explore their chemical abundance patterns to confirm if the two substructures shared a common origin. We carry out a homogenous high-resolution elemental abundance study of eight confirmed members of IC 2391 as well as six members of the Argus association using UVES spectra. We derive spectroscopic stellar parameters and abundances for Fe, Na, Mg, Al, Si, Ca, Ti, Cr, Ni and Ba.
All stars in the open cluster and Argus association were found to share similar abundances with the scatter well within the uncertainties, where Fe/H = −0.04 ± 0.03 for cluster stars and Fe/H = −0.06 ± 0.05 for Argus stars. Effects of overionization/excitation were seen for stars cooler than roughly 5200 K as previously noted in the literature. Also, enhanced Ba abundances of around 0.6 dex were observed in both systems. The common ages, kinematics and chemical abundances strongly support the fact that the Argus association stars originated from the open cluster IC 2391. Simple modelling of this system finds this dissolution to be consistent with two-body interactions.
Nighttime polarimetric measurements are often obtained very close to the limits of the instrumental capabilities. It is important to be aware of the possible sources of spurious polarization, and to ...adopt data reduction techniques that best compensate for the instrumental effects intrinsic to the design of the most common polarimeters adopted for nighttime observations. We define a self-consistent framework starting from the basic definitions of the Stokes parameters, and we present an analytical description of the data reduction techniques commonly used with a polarimeter (consisting of a retarder wave plate and a Wollaston prism) to explore their advantages and limitations. We first consider an ideal polarimeter in which all optical components are perfectly defined by their nominal characteristics. We then introduce deviations from the nominal behavior of the polarimetric optics, and develop an analytical model to describe the polarization of the outgoing radiation. We study and compare the results of two different data reduction methods, one based on the differences of the signals, and one based on their ratios, to evaluate the residual amount of spurious polarization. We show that data reduction techniques may fully compensate for small deviations of the polarimetric optics from their nominal values, although some important (first-order) corrections have to be adopted for linear polarization data. We include a detailed discussion of quality checking by means of null parameters. We present an application to data obtained with the FORS1 instrument of the ESO VLT, in which we have detected a significant amount of cross talk between circular and linear polarization. We show that this cross-talk effect is not due to the polarimetric optics themselves, but is most likely caused by spurious birefringence due to the instrument’s collimator lens.
Context. The young associations offer us one of the best opportunities to study the properties of young stellar and substellar objects and to directly image planets thanks to their proximity (<200pc) ...and age (approximate5-150Myr). However, many previous works have been limited to identifying the brighter, more active members (approximate1M sub(middot in circle)) owing to photometric survey sensitivities limiting the detections of lower mass objects. Aims. We search the field of view of 542 previously identified members of the young associations to identify wide or extremely wide (1000-100000au in physical separation) companions. Methods. We combined 2MASS near-infrared photometry (J, H, K) with proper motion values (from UCAC4, PPMXL, NOMAD) to identify companions in the field of view of known members. We collated further photometry and spectroscopy from the literature and conducted our own high-resolution spectroscopic observations for a subsample of candidate members. This complementary information allowed us to assess the efficiency of our method. Results. We identified 84 targets (45:0.2-1.3M sub(middot in circle), 17:0.08-0.2M sub(middot in circle), 22:<0.08M sub(middot in circle)) in our analysis, ten of which have been identified from spectroscopic analysis in previous young association works. For 33 of these 84, we were able to further assess their membership using a variety of properties (X-ray emission, UV excess, Halpha, lithium and KI equivalent widths, radial velocities, and CaH indices). We derive a success rate of 76-88% for this technique based on the consistency of these properties. Conclusions. Once confirmed, the targets identified in this work would significantly improve our knowledge of the lower mass end of the young associations. Additionally, these targets would make an ideal new sample for the identification and study of planets around nearby young stars. Given the predicted substellar mass of the majority of these new candidate members and their proximity, high-contrast imaging techniques would facilitate the search for new low-mass planets.
We report results from a high-resolution optical spectroscopic survey aimed to search for nearby young associations and young stars among optical counterparts of ROSAT All-Sky Survey X-ray sources in ...the Southern Hemisphere. We selected 1953 late-type ( B-V similar to \geq similar to 0.6), potentially young, optical counterparts out of a total of 9574 1RXS sources for follow-up observations. At least one high-resolution spectrum was obtained for each of 1511 targets. This paper is the first in a series presenting the results of the SACY survey. Here we describe our sample and our observations. We describe a convergence method in the ( UVW) velocity space to find associations. As an example, we discuss the validity of this method in the framework of the \beta Pic Association.
Giant exoplanets on wide orbits have been directly imaged around young stars. If the thermal background in the mid-infrared can be mitigated, then exoplanets with lower masses can also be imaged. ...Here we present a ground-based mid-infrared observing approach that enables imaging low-mass temperate exoplanets around nearby stars, and in particular within the closest stellar system, α Centauri. Based on 75-80% of the best quality images from 100 h of cumulative observations, we demonstrate sensitivity to warm sub-Neptune-sized planets throughout much of the habitable zone of α Centauri A. This is an order of magnitude more sensitive than state-of-the-art exoplanet imaging mass detection limits. We also discuss a possible exoplanet or exozodiacal disk detection around α Centauri A. However, an instrumental artifact of unknown origin cannot be ruled out. These results demonstrate the feasibility of imaging rocky habitable-zone exoplanets with current and upcoming telescopes.
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