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تسجيل مستخدم جديدA Brief History of the Study of Nearby Young Moving Groups and Their Members
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Joel H. Kastner
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Beginning with the enigmatic (and now emblematic) TW Hya, the scutiny of individual stars and star-disk systems has both motivated and benefitted from the identification of nearby young moving groups (NYMGs). I briefly outline the emergence of this relatively new subfield of astronomy over the past two decades, and offer a few examples illustrating how the study of NYMGs and their members enables unique investigations of pre-main sequence stellar evolution, evolved protoplanetary disks, and young exoplanets.
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We study a target sample of 68 low-mass objects (with spectral types in the range M4.5-L1) previously selected via photometric and astrometric criteria, as possible members of five young moving groups: the Local Association (Pleiades moving group, ag
e=20 - 150 Myr), the Ursa Mayor group (Sirius supercluster, age=300 Myr), the Hyades supercluster (age=600 Myr), IC 2391 supercluster (age=35 - 55 Myr) and the Castor moving group (age=200 Myr). In this paper we assess their membership by using different kinematic and spectroscopic criteria. We use high resolution echelle spectroscopic observations of the sample to measure accurate radial velocities (RVs). Distances are calculated and compared to those of the moving group from the literature, we also calculate the kinematic Galactic components (U,V,W) of the candidate members and apply kinematic criterion of membership to each group. In addition we measure rotational velocities (v sin i) to place further constraints on membership of kinematic members. We find that 49 targets have young disk kinematics and that 36 of them possibly belong to one of our five moving groups. From the young disk target ob jects, 31 have rotational velocities in agreement with them belonging to the young disk population. We also find that one of our moving group candidates, 2MASS0123- 3610, is a low-mass double lined spectroscopic binary, with probable spectral types around M7.
We present a search for new planetary-mass members of nearby young moving groups (YMGs) using astrometry for 694 T and Y dwarfs, including 447 objects with parallaxes, mostly produced by recent large parallax programs from UKIRT and Spitzer. Using th
e BANYAN $Sigma$ and LACEwING algorithms, we identify 30 new candidate YMG members, with spectral types of T0$-$T9 and distances of $10-43$ pc. Some candidates have unusually red colors and/or faint absolute magnitudes compared to field dwarfs with similar spectral types, providing supporting evidence for their youth, including 4 early-T dwarfs. We establish one of these, the variable T1.5 dwarf 2MASS J21392676$+$0220226, as a new planetary-mass member ($14.6^{+3.2}_{-1.6}$ M$_{rm Jup}$) of the Carina-Near group ($200pm50$ Myr) based on its full six-dimensional kinematics, including a new parallax measurement from CFHT. The high-amplitude variability of this object is suggestive of a young age, given the coexistence of variability and youth seen in previously known YMG T dwarfs. Our four latest-type (T8$-$T9) YMG candidates, WISE J031624.35$+$430709.1, ULAS J130217.21$+$130851.2, WISEPC J225540.74$-$311841.8, and WISE J233226.49$-$432510.6, if confirmed, will be the first free-floating planets ($approx2-6$ M$_{rm Jup}$) whose ages and luminosities are compatible with both hot-start and cold-start evolutionary models, and thus overlap the properties of the directly-imaged planet 51 Eri b. Several of our early/mid-T candidates have peculiar near-infrared spectra, indicative of heterogenous photospheres or unresolved binarity. Radial velocity measurements needed for final membership assessment for most of our candidates await upcoming 20$-$30 meter class telescopes. In addition, we compile all 15 known T7$-$Y1 benchmarks and derive a homogeneous set of their effective temperatures, surface gravities, radii, and masses.
A significant fraction of nearby young moving group members harbor circumstellar debris dust disks. Due to their proximity and youth, these disks are attractive targets for studying the early evolution of debris dust and planetesimal belts. Here we p
resent 70 and 160$mu$m observations of 31 systems in the $beta$ Pic moving group, and in the Tucana-Horologium, Columba, Carina and Argus associations, using the Herschel Space Observatory. None of these stars were observed at far-infrared wavelengths before. Our Herschel measurements were complemented by photometry from the WISE satellite for the whole sample, and by submillimeter/millimeter continuum data for one source, HD 48370. We identified six stars with infrared excess, four of them are new discoveries. By combining our new findings with results from the literature, we examined the incidence and general characteristics of debris disks around Sun-like members of the selected groups. With their dust temperatures of <45 K the newly identified disks around HD 38397, HD 48370, HD 160305, and BD-20 951 represent the coldest population within this sample. For HD 38397 and HD 48370, the emission is resolved in the 70$mu$m PACS images, the estimated radius of these disks is ~90 au. Together with the well-known disk around HD 61005, these three systems represent the highest mass end of the known debris disk population around young G-type members of the selected groups. In terms of dust content, they resemble the hypothesized debris disk of the ancient Solar System.
We image 104 newly identified low-mass (mostly M-dwarf) pre-main sequence members of nearby young moving groups with Magellan Adaptive Optics (MagAO) and identify 27 binaries with instantaneous projected separation as small as 40 mas. 15 were previou
sly unknown. The total number of multiple systems in this sample including spectroscopic and visual binaries from the literature is 36, giving a raw multiplicity rate of at least $35^{+5}_{-4}%$ for this population. In the separation range of roughly 1 - 300 AU in which infrared AO imaging is most sensitive, the raw multiplicity rate is at least $24^{+5}_{-4}%$ for binaries resolved by the MagAO infrared camera (Clio). The M-star sub-sample of 87 stars yields a raw multiplicity of at least $30^{+5}_{-4}%$ over all separations, $21^{+5}_{-4}%$ for secondary companions resolved by Clio from 1 to 300 AU ($23^{+5}_{-4}%$ for all known binaries in this separation range). A combined analysis with binaries discovered by the Search for Associations Containing Young stars shows that multiplicity fraction as a function of mass and age over the range of 0.2 to 1.2 $M_odot$ and 10 - 200 Myr appears to be linearly flat in both parameters and across YMGs. This suggests that multiplicity rates are largely set by 100 Myr without appreciable evolution thereafter. After bias corrections are applied, the multiplicity fraction of low-mass YMG members ($< 0.6 M_odot$) is in excess of the field.
Optical spectroscopic observations are reported for 24 and 23, nearby, proper-motion-selected M-dwarf candidate members of the Beta Pictoris and AB Doradus moving groups (BPMG and ABDMG). Using kinematic criteria, the presence of both Halpha emission
and high X-ray-to-bolometric luminosity, and position in absolute colour-magnitude diagrams, 10 and 6 of these candidates are confirmed as likely members of the BPMG and ABDMG respectively. Equivalent widths or upper limits for the Li I 6708A line are reported and the lithium depletion boundary (LDB) age of the BPMG is revisited. Whilst non-magnetic evolutionary models still yield an estimated age of 21 +/- 4 Myr, models that incorporate magnetic inhibition of convection imply an older age of 24 +/- 4 Myr. A similar systematic increase would be inferred if the stars were 25 per cent covered by dark magnetic starspots. Since young, convective M-dwarfs are magnetically active and do have starspots, we suggest that the original LDB age estimate is a lower limit. The LDB age of the ABDMG is still poorly constrained -- non-magnetic evolutionary models suggest an age in the range 35-150 Myr, which could be significantly tightened by new measurements for existing candidate members.
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