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Register a new userYoung Stars Near Earth: The Octans-Near Association and Castor Moving Group
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All cataloged stellar moving groups and associations with ages <100 Myr and within 100 pc of Earth have Galactic space motions (UVW) situated in a good box with dimensions ~20 km/s on a side. Torres et al. defined the Octans Association as a group of 15 stars with age 20 Myr? and located ~140 pc from Earth, but with average V space velocity -3.6 km/s that is well outside of the good box. We present a list of 14 Hipparcos star systems within 100 pc of Earth that we call Octans-Near; these systems have UVW similar to those of the much more distant Octans Association. The Octans-Near stars have apparent ages between about 30 and 100 Myr and their relationship to the Octans Association stars is unclear. Six additional star systems have UVW similar to those of Octans-Near stars and likely ages <200 Myr. These six systems include the late-type binary star EQ Peg -- 6.2 pc from Earth with likely age <100 Myr and thus likely to be the nearest known pre-main sequence star system. The UVW of stars in a previously proposed ~200 Myr old Castor moving group are not too dissimilar from the UVW of Octans-Near stars. However, stars in the Castor group -- if it exists at all -- are mostly substantially older than 200 Myr and thus generally can readily be distinguished from the much younger Octans-Near stars.
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Context. Precise determination of stellar masses is necessary to test the validity of pre-main-sequence (PMS) stellar evolutionary models, whose predictions are in disagreement with measurements for masses below 1.2 Msun. To improve such a test, and based on our previous studies, we selected the AB Doradus moving group (AB Dor-MG) as the best-suited association on which to apply radio-based high-precision astrometric techniques to study binary systems. Aims. We seek to determine precise estimates of the masses of a set of stars belonging to the AB Dor-MG using radio and infrared observations. Methods. We observed in phase-reference mode with the Very Large Array (VLA) at 5 GHz and with the European VLBI Network (EVN) at 8.4 GHz the stars HD 160934, EK Dra, PW And, and LO Peg. We also observed some of these stars with the near-infrared CCD AstraLux camera at the Calar Alto observatory to complement the radio observations. Results. We determine model-independent dynamical masses of both components of the star HD 160934, A and c, which are 0.70+/-0.07 Msun and 0.45+/-0.04 Msun , respectively. We revised the orbital parameters of EK Dra and we determine a sum of the masses of the system of 1.38+/-0.08 Msun. We also explored the binarity of the stars LO Peg and PW And. Conclusions. We found observational evidence that PMS evolutionary models underpredict the mass of PMS stars by 10%-40%, as previously reported by other authors. We also inferred that the origin of the radio emission must be similar in all observed stars, that is, extreme magnetic activity of the stellar corona that triggers gyrosynchrotron emission from non-thermal, accelerated electrons.
We present the first results from a 124 night J, H, K near-infrared monitoring campaign of the dark cloud L 1003 in Cygnus OB7, an active star-forming region. Using 3 seasons of UKIRT observations spanning 1.5 years, we obtained high-quality photometry on 9,200 stars down to J=17 mag, with photometric uncertainty better than 0.04 mag. On the basis of near-infrared excesses from disks, we identify 30 pre-main sequence stars, including 24 which are newly discovered. We analyze those stars and find the NIR excesses are significantly variable. All 9,200 stars were monitored for photometric variability; among the field star population, about 160 exhibited near-infrared variability (1.7% of the sample). Of the 30 YSOs (young stellar objects), 28 of them (93%) are variable at a significant level. 25 of the 30 YSOs have near-infrared excess consistent with simple disk-plus-star classical T Tauri models. Nine of these (36%) drift in color space over the course of these observations and/or since 2MASS observations such that they cross the boundary defining the NIR excess criteria; effectively, they have a transient near-infrared excess. About half of the YSOs have color-space variations parallel to either the classical T Tauri star locus or a hybrid track which includes the dust reddening trajectory. This indicates that the NIR variability in YSOs that possess accretion disks arises from a combination of variable extinction and changes in the inner accretion disk: either in accretion rate, central hole size and/or the inclination of the inner disk. While some variability may be due to stellar rotation, the level of variability on the individual stars can exceed a magnitude. This is a strong empirical suggestion that protoplanetary disks are quite dynamic and exhibit more complex activity on short timescales than is attributable to rotation alone or captured in static disk models.
Context. As a building block for amino acids, formamide (NH$_2$CHO) is an important molecule in astrobiology and astrochemistry, but its formation path in the interstellar medium is not understood well. Aims. We aim to find empirical evidence to support the chemical relationships of formamide to HNCO and H$_2$CO. Methods. We examine high angular resolution (~0.2) Atacama Large Millimeter/submillimeter Array (ALMA) maps of six sources in three high-mass star-forming regions and compare the spatial extent, integrated emission peak position, and velocity structure of HNCO and H$_2$CO line emission with that of NH$_2$CHO by using moment maps. Through spectral modeling, we compare the abundances of these three species. Results. In these sources, the emission peak separation and velocity dispersion of formamide emission is most often similar to HNCO emission, while the velocity structure is generally just as similar to H$_2$CO and HNCO (within errors). From the spectral modeling, we see that the abundances between all three of our focus species are correlated, and the relationship between NH$_2$CHO and HNCO reproduces the previously demonstrated abundance relationship. Conclusions. In this first interferometric study, which compares two potential parent species to NH$_2$CHO, we find that all moment maps for HNCO are more similar to NH$_2$CHO than H$_2$CO in one of our six sources (G24 A1). For the other five sources, the relationship between NH$_2$CHO, HNCO, and H$_2$CO is unclear as the different moment maps for each source are not consistently more similar to one species as opposed to the other.
We investigate the young (proto)stellar population in NGC 2023 and the L 1630 molecular cloud bordering the HII region IC 434, using Spitzer IRAC and MIPS archive data, JCMT SCUBA imaging and spectroscopy as well as targeted BIMA observations of one of the Class 0 protostars, NGC 2023 MM1. We have performed photometry of all IRAC and MIPS images, and used color-color diagrams to identify and classify all young stars seen within a 22x26 field along the boundary between IC 434 and L 1630. For some stars, which have sufficient optical, IR, and/or sub-millimeter data we have also used the online SED fitting tool for a large 2D archive of axisymmetric radiative transfer models to perform more detailed modeling of the observed SEDs. We identify 5 sub-millimeter cores in our 850 and 450 micron SCUBA images, two of which have embedded class 0 or I protostars. Observations with BIMA are used to refine the position and characteristics of the Class 0 source NGC 2023 MM 1. These observations show that it is embedded in a very cold cloud core, which is strongly enhanced in NH2D. We find that HD 37903 is the most massive member of a cluster with 20 -- 30 PMS stars. We also find smaller groups of PMS stars formed from the Horsehead nebula and another elephant trunk structure to the north of the Horsehead. We refine the spectral classification of HD 37903 to B2 Ve. Our study shows that the expansion of the IC 434 HII region has triggered star formation in some of the dense elephant trunk structures and compressed gas inside the L 1630 molecular cloud. This pre-shock region is seen as a sub-millimeter ridge in which stars have already formed. The cluster associated with NGC 2023 is very young, and has a large fraction of Class I sources.
To analyze the SACY (Search for Associations Containing Young stars) survey we developed a method to find young associations and to define their high probability members. These bona fide members enable to obtain the kinematical and the physical properties of each association in a proper way. Recently we noted a concentration in the UV plane and we found a new association we are calling ASYA (All Sky Young Association) for its overall distribution in the sky with a total of 38 bonafide members and an estimated age of 110 Myr, the oldest young association found in the SACY survey. We present here its kinematical, space and Li distributions and its HR diagram.
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