Mapping young stellar populations towards Orion with Gaia DR1

We use the first data release of the Gaia mission to explore the three dimensional arrangement and the age ordering of the many stellar groups towards the Orion OB association, aiming at a new classification and characterization of the stellar population. We make use of the parallaxes and proper motions provided in the Tycho Gaia Astrometric Solution (TGAS) sub-set of the Gaia catalogue, and of the combination of Gaia and 2MASS photometry. In TGAS we find evidence for the presence of a young population, at a parallax $\varpi \sim 2.65 \, \mathrm{mas}$, loosely distributed around some known clusters: 25 Ori, $\epsilon$ Ori and $\sigma$ Ori, and NGC 1980 ($\iota$ Ori). The low mass counterpart of this population is visible in the color-magnitude diagrams constructed by combining Gaia and 2MASS photometry. We study the density distribution of the young sources in the sky. We find the same groups as in TGAS, and also some other density enhancements that might be related to the recently discovered Orion X group, the Orion dust ring, and to the $\lambda$ Ori complex. We estimate the ages of this population and we infer the presence of an age gradient going from 25 Ori (13-15 Myr) to the ONC (1-2 Myr). We confirm this age ordering by repeating the Bayesian fit using the Pan-STARRS1 data. The estimated ages towards the NGC 1980 cluster span a broad range of values. This can either be due to the presence of two populations coming from two different episodes of star formation or to a large spread along the line of sight of the same population. Our results form the first step towards using the Gaia data to unravel the complex star formation history of the Orion region in terms of the different star formation episodes, their duration, and their effects on the surrounding interstellar medium.Comment: 17 pages, 17 figure

Runaway and walkaway stars from the ONC with Gaia DR2

Theory predicts that we should find fast, ejected (runaway) stars of all masses around dense, young star-forming regions. N-body simulations show that the number and distribution of these ejected stars could be used to constrain the initial spatial and kinematic substructure of the regions. We search for runaway and slower walkaway stars within 100 pc of the Orion Nebula Cluster (ONC) using Gaia DR2 astrometry and photometry. We compare our findings to predictions for the number and velocity distributions of runaway stars from simulations that we run for 4 Myr with initial conditions tailored to the ONC. In Gaia DR2, we find 31 runaway and 54 walkaway candidates based on proper motion, but not all of these are viable candidates in three dimensions. About 40 per cent are missing radial velocities, but we can trace back 9 3D-runaways and 24 3D-walkaways to the ONC, all of which are low/intermediate-mass (<8 M⊙). Our simulations show that the number of runaways within 100 pc decreases the older a region is (as they quickly travel beyond this boundary), whereas the number of walkaways increases up to 3 Myr. We find fewer walkaways in Gaia DR2 than the maximum suggested from our simulations, which may be due to observational incompleteness. However, the number of Gaia DR2 runaways agrees with the number from our simulations during an age of ∼1.3-2.4 Myr, allowing us to confirm existing age estimates for the ONC (and potentially other star-forming regions) using runaway stars

How accurately can we age-date solar-type dwarfs using activity/rotation diagnostics?

It is well established that activity and rotation diminishes during the life of sun-like main sequence (~F7-K2V) stars. Indeed, the evolution of rotation and activity among these stars appears to be so deterministic that their rotation/activity diagnostics are often utilized as estimators of stellar age. A primary motivation for the recent interest in improving the ages of solar-type field dwarfs is in understanding the evolution of debris disks and planetary systems. Reliable isochronal age-dating for field, solar-type main sequence stars is very difficult given the observational uncertainties and multi-Gyr timescales for significant structural evolution. Observationally, significant databases of activity/rotation diagnostics exist for field solar-type field dwarfs (mainly from chromospheric and X-ray activity surveys). But how well can we empirically age-date solar-type field stars using activity/rotation diagnostics? Here I summarize some recent results for F7-K2 dwarfs from an analysis by Mamajek & Hillenbrand (2008), including an improved "gyrochronology" [Period(color, age)] calibration, improved chromospheric (R'_HK and X-ray (log Lx/Lbol) activity vs. rotation (via Rossby number) relations, and a chromospheric vs. X-ray activity relation that spans four orders of magnitude in log Lx/Lbol. Combining these relations, one can produce predicted chromospheric and X-ray activity isochrones as a function of color and age for solar type dwarfs.Comment: 8 pgs, to appear in proc. of 258th IAU Symposium "The Ages of Stars", eds. E.E. Mamajek, D.R. Soderblom, & R.F.G. Wyse (in press

Origin Of The Far Off-Axis GRB171205A

We show that observed properties of the low luminosity GRB171205A and its afterglow, like those of most other low-luminosity (LL) gamma ray bursts (GRBs) associate with a supernova (SN), indicate that it is an ordinary SN-GRB, which was produced by inverse Compton scattering of glory light by a highly relativistic narrowly collimated jet ejected in a supernova explosion and viewed from a far off-axis angle. As such, VLA/VLBI follow-up radio observations of a superluminal displacement of its bright radio afterglow from its parent supernova, will be able to test clearly whether it is an ordinary SN-GRB viewed from far off-axis or it belongs to a distinct class of GRBs, which are different from ordinary GRBs, and cannot be explained by standard fireball models of GRBs as ordinary GRBsComment: 5 pages, 6 figures, updated data in Fig. 3, Corrected GRB angular distance used in Fig.

Solar-Type Post-T Tauri Stars in the Nearest OB Subgroups

I discuss results from the recent spectroscopic survey for solar-type pre-MS stars in the Lower Centaurus-Crux (LCC) and Upper Centaurus-Lupus (UCL) OB subgroups by Mamajek, Meyer, & Liebert (2002, AJ, 124, 1670). LCC and UCL are subgroups of the Sco-Cen OB association, and the two nearest OB subgroups to the Sun. In the entire survey of 110 pre-main sequence stars, there exists only one Classical T Tauri star (PDS 66), implying that only ~1% of ~1 Msun stars are still accreting at age 13$\pm$7 (1$\sigma$) Myr. Accounting for observational errors, the HRD placement of the pre-MS stars is consistent with the bulk of star-formation taking place within 5-10 Myr. In this contribution, I estimate conservative upper limits to the intrinsic velocity dispersions of the post-T Tauri stars in the LCC and UCL subgroups (<1.6 km/s and <2.2 km/s, respectively; 95% CL) using Monte-Carlo simulations of Tycho-2 proper motions for candidate subgroup members. I also demonstrate that a new OB subgroup recently proposed to exist in Chamaeleon probably does not.Comment: 8 pages, 2 figures, to appear in proceedings for "Open Issues in Local Star Formation and Early Stellar Evolution", eds. J. Gregorio-Hetem & J. Lepine. Minor edits (5/30/03

First results of a cryogenic optical photon counting imaging spectrometer using a DROID array

Context. In this paper we present the first system test in which we demonstrate the concept of using an array of Distributed Read Out Imaging Devices (DROIDs) for optical photon detection. Aims. After the successful S-Cam 3 detector the next step in the development of a cryogenic optical photon counting imaging spectrometer under the S-Cam project is to increase the field of view using DROIDs. With this modification the field of view of the camera has been increased by a factor of 5 in area, while keeping the number of readout channels the same. Methods. The test has been performed using the flexible S-Cam 3 system and exchanging the 10x12 Superconducting Tunnel Junction array for a 3x20 DROID array. The extra data reduction needed with DROIDs is performed offline. Results. We show that, although the responsivity (number of tunnelled quasiparticles per unit of absorbed photon energy, e- /eV) of the current array is too low for direct astronomical applications, the imaging quality is already good enough for pattern detection, and will improve further with increasing responsivity. Conclusions. The obtained knowledge can be used to optimise the system for the use of DROIDs.Comment: 7 pages, 9 figures, accepted for publicaiton in A&

Constraining the Mass Profiles of Stellar Systems: Schwarzschild Modeling of Discrete Velocity Datasets

(ABRIDGED) We present a new Schwarzschild orbit-superposition code designed to model discrete datasets composed of velocities of individual kinematic tracers in a dynamical system. This constitutes an extension of previous implementations that can only address continuous data in the form of (the moments of) velocity distributions, thus avoiding potentially important losses of information due to data binning. Furthermore, the code can handle any combination of available velocity components, i.e., only line-of-sight velocities, only proper motions, or a combination of both. It can also handle a combination of discrete and continuous data. The code finds the distribution function (DF, a function of the three integrals of motion E, Lz, and I3) that best reproduces the available kinematic and photometric observations in a given axisymmetric gravitational potential. The fully numerical approach ensures considerable freedom on the form of the DF f(E,Lz,I3). This allows a very general modeling of the orbital structure, thus avoiding restrictive assumptions about the degree of (an)isotropy of the orbits. We describe the implementation of the discrete code and present a series of tests of its performance based on the modeling of simulated datasets generated from a known DF. We find that the discrete Schwarzschild code recovers the original orbital structure, M/L ratios, and inclination of the input datasets to satisfactory accuracy, as quantified by various statistics. The code will be valuable, e.g., for modeling stellar motions in Galactic globular clusters, and those of individual stars, planetary nebulae, or globular clusters in nearby galaxies. This can shed new light on the total mass distributions of these systems, with central black holes and dark matter halos being of particular interest.Comment: ApJ, in press; 51 pages, 11 figures; manuscript revised following comments by refere

The origin of runaway stars

Milli-arcsecond astrometry provided by Hipparcos and by radio observations makes it possible to retrace the orbits of some of the nearest runaway stars and pulsars to determine their site of origin. The orbits of the runaways AE Aurigae and mu Columbae and of the eccentric binary iota Orionis intersect each other about 2.5 Myr ago in the nascent Trapezium cluster, confirming that these runaways were formed in a binary-binary encounter. The path of the runaway star zeta Ophiuchi intersects that of the nearby pulsar PSR J1932+1059, about 1 Myr ago, in the young stellar group Upper Scorpius. We propose that this neutron star is the remnant of a supernova that occurred in a binary system which also contained zeta Oph, and deduce that the pulsar received a kick velocity of about 350 km/s in the explosion. These two cases provide the first specific kinematic evidence that both mechanisms proposed for the production of runaway stars, the dynamical ejection scenario and the binary-supernova scenario, operate in nature.Comment: 5 pages, including 2 eps-figures and 1 table, submitted to the ApJ Letters. The manuscript was typeset using aaste

Axisymmetric Three-Integral Models for Galaxies

We describe an improved, practical method for constructing galaxy models that match an arbitrary set of observational constraints, without prior assumptions about the phase-space distribution function (DF). Our method is an extension of Schwarzschild's orbit superposition technique. As in Schwarzschild's original implementation, we compute a representative library of orbits in a given potential. We then project each orbit onto the space of observables, consisting of position on the sky and line-of-sight velocity, while properly taking into account seeing convolution and pixel binning. We find the combination of orbits that produces a dynamical model that best fits the observed photometry and kinematics of the galaxy. A key new element of this work is the ability to predict and match to the data the full line-of-sight velocity profile shapes. A dark component (such as a black hole and/or a dark halo) can easily be included in the models. We have tested our method, by using it to reconstruct the properties of a two-integral model built with independent software. The test model is reproduced satisfactorily, either with the regular orbits, or with the two-integral components. This paper mainly deals with the technical aspects of the method, while applications to the galaxies M32 and NGC 4342 are described elsewhere (van der Marel et al., Cretton & van den Bosch). (abridged)Comment: minor changes, accepted for publication in the Astrophysical Journal Supplement

A Gaia study of the Hyades open cluster

Galaxie