Showing papers by "Geert Barentsen published in 2015"

The Gaia-ESO Survey: Chromospheric emission, accretion properties, and rotation in γ Velorum and Chamaeleon I

Abstract: One of the goals of the Gaia-ESO Survey (GES), which is conducted with FLAMES at the VLT, is the census and the characterization of the low-mass members of very young clusters and associations. We conduct a comparative study of the main properties of the sources belonging to gamma Velorum (gamma Vel) and Chamaeleon I (Cha I) young associations, focusing on their rotation, chromospheric radiative losses, and accretion. Methods. We used the fundamental parameters (effective temperature, surface gravity, lithium abundance, and radial velocity) delivered by the GES consortium in the first internal data release to select the members of gamma Vel and Cha I among the UVES and GIRAFFE spectroscopic observations. A total of 140 gamma Vel members and 74 Cha I members were studied. The procedure adopted by the GES to derive stellar fundamental parameters also provided measures of the projected rotational velocity (v sin i). We calculated stellar luminosities through spectral energy distributions, while stellar masses were derived by comparison with evolutionary tracks. The spectral subtraction of low-activity and slowly rotating templates, which are rotationally broadened to match the v sin i of the targets, enabled us to measure the equivalent widths (EWs) and the fluxes in the H alpha and H beta lines. The Ha line was also used for identifying accreting objects, on the basis of its EW and the width at the 10% of the line peak (10% W), and for evaluating the mass accretion rate ((M)over dot(acc)). Results. The distribution of v sin i for the members of. Vel displays a peak at about 10 km s(-1) with a tail toward faster rotators. There is also some indication of a different v sin i distribution for the members of its two kinematical populations. Most of these stars have Ha fluxes corresponding to a saturated activity regime. We find a similar distribution, but with a narrower peak, for Cha I. Only a handful of stars in gamma Vel display signatures of accretion, while many more accretors were detected in the younger Cha I, where the highest Ha fluxes are mostly due to accretion, rather than to chromospheric activity. Accreting and active stars occupy two different regions in a T-eff-flux diagram and we propose a criterion for distinguishing them. We derive (M)over dot(acc) in the ranges 10(-11)-10(-9) M-circle dot yr(-1) and 10(-10)-10(-7) M-circle dot yr(-1) for gamma Vel and Cha I accretors, respectively. We find less scatter in the (M)over dot(acc)-M-star relation derived through the H alpha EWs, when compared to the Ha 10% W diagnostics, in agreement with other authors.

Campaign 9 of the $K2$ Mission: Observational Parameters, Scientific Drivers, and Community Involvement for a Simultaneous Space- and Ground-based Microlensing Survey

Abstract: $K2$'s Campaign 9 ($K2$C9) will conduct a $\sim$3.7 deg$^{2}$ survey toward the Galactic bulge from 7/April through 1/July of 2016 that will leverage the spatial separation between $K2$ and the Earth to facilitate measurement of the microlens parallax $\pi_{\rm E}$ for $\gtrsim$127 microlensing events. These will include several that are planetary in nature as well as many short-timescale microlensing events, which are potentially indicative of free-floating planets (FFPs). These satellite parallax measurements will in turn allow for the direct measurement of the masses of and distances to the lensing systems. In this white paper we provide an overview of the $K2$C9 space- and ground-based microlensing survey. Specifically, we detail the demographic questions that can be addressed by this program, including the frequency of FFPs and the Galactic distribution of exoplanets, the observational parameters of $K2$C9, and the array of resources dedicated to concurrent observations. Finally, we outline the avenues through which the larger community can become involved, and generally encourage participation in $K2$C9, which constitutes an important pathfinding mission and community exercise in anticipation of $WFIRST$.

The K2 Ecliptic Plane Input Catalog (EPIC) and Stellar Classifications of 138,600 Targets in Campaigns 1-8

Abstract: The K2 Mission uses the Kepler spacecraft to obtain high-precision photometry over ~80 day campaigns in the ecliptic plane. The Ecliptic Plane Input Catalog (EPIC) provides coordinates, photometry and kinematics based on a federation of all-sky catalogs to support target selection and target management for the K2 mission. We describe the construction of the EPIC, as well as modifications and shortcomings of the catalog. Kepler magnitudes (Kp) are shown to be accurate to ~0.1 mag for the Kepler field, and the EPIC is typically complete to Kp~17 (Kp~19 for campaigns covered by SDSS). We furthermore classify 138,600 targets in Campaigns 1-8 (~88% of the full target sample) using colors, proper motions, spectroscopy, parallaxes, and galactic population synthesis models, with typical uncertainties for G-type stars of ~3% in Teff, ~0.3 dex in log(g), ~40% in radius, ~10% in mass, and ~40% in distance. Our results show that stars targeted by K2 are dominated by K-M dwarfs (~41% of all selected targets), F-G dwarfs (~36%) and K giants (~21%), consistent with key K2 science programs to search for transiting exoplanets and galactic archeology studies using oscillating red giants. However, we find a significant variation of the fraction of cool dwarfs with galactic latitude, indicating a target selection bias due to interstellar reddening and the increased contamination by giant stars near the galactic plane. We discuss possible systematic errors in the derived stellar properties, and differences to published classifications for K2 exoplanet host stars. The EPIC is hosted at the Mikulski Archive for Space Telescopes (MAST): this http URL.

Classical T Tauri stars with VPHAS+ – I. H α and u-band accretion rates in the Lagoon Nebula M8

Abstract: We estimate the accretion rates of 235 Classical T Tauri star (CTTS) candidates in the Lagoon Nebula using $ugri$H$\alpha$ photometry from the VPHAS+ survey. Our sample consists of stars displaying H$\alpha$-excess, the intensity of which is used to derive accretion rates. For a subset of 87 stars, the intensity of the $u$-band excess is also used to estimate accretion rates. We find the mean variation in accretion rates measured using H$\alpha$ and $u$-band intensities to be $\sim$ 0.17 dex, agreeing with previous estimates (0.04-0.4 dex) but for a much larger sample. The spatial distribution of CTTS align with the location of protostars and molecular gas suggesting that they retain an imprint of the natal gas fragmentation process. Strong accretors are concentrated spatially, while weak accretors are more distributed. Our results do not support the sequential star forming processes suggested in the literature.

A deep catalogue of classical Be stars in the direction of the Perseus Arm: spectral types and interstellar reddenings

Abstract: We present a catalogue of 247 photometrically and spectroscopically confirmed fainter classical Be stars (13 lsim r lsim 16) in the direction of the Perseus Arm of the Milky Way (-1ẹg lt b lt +4ẹg, 120ẹg lt \ell lt 140ẹg). The catalogue consists of 181 IPHAS-selected new classical Be stars, in addition to 66 objects that we studied in our previous work more closely, and three stars identified as classical Be stars in earlier work. This study more than doubles the number known in the region. Photometry spanning 0.6-5 μm, spectral types, and interstellar reddenings are given for each object. The spectral types were determined from low-resolution spectra (λ/Δλ \ap 800-2000), to a precision of 1-3 subtypes. The interstellar reddenings are derived from the (r - i) colour, using a method that corrects for circumstellar disc emission. The colour excesses obtained range from E(B - V) = 0.3 up to 1.6 - a distribution that modestly extends the range reported in the literature for Perseus-Arm open clusters. For around half the sample, the reddenings obtained are compatible with measures of the total sightline Galactic extinction. Many of these are likely to lie well beyond the Perseus Arm.

The OmegaWhite Survey for Short-Period Variable Stars - I : Overview and First Results

Abstract: We present the goals, strategy and first results of the OmegaWhite survey: a wide-field high-cadence $g$-band synoptic survey which aims to unveil the Galactic population of short-period variable stars (with periods $<$ 80 min), including ultracompact binary star systems and stellar pulsators. The ultimate goal of OmegaWhite is to cover 400 square degrees along the Galactic Plane reaching a depth of $g = $ 21.5 mag (10$\sigma$), using OmegaCam on the VLT Survey Telescope (VST). The fields are selected to overlap with surveys such as the Galactic Bulge Survey (GBS) and the VST Photometric H$\alpha$ Survey of the Southern Galactic Plane (VPHAS+) for multi-band colour information. Each field is observed using 38 exposures of 39 s each, with a median cadence of $\sim$2.7 min for a total duration of two hours. Within an initial 26 square degrees, we have extracted the light curves of 1.6 million stars, and have identified 613 variable candidates which satisfy our selection criteria. Furthermore, we present the light curves and statistical properties of 20 sources which have the highest-likelihood of being variable stars. One of these candidates exhibits the colours and light curve properties typically associated with ultracompact AM CVn binaries, although its spectrum exhibits weak Balmer absorption lines and is thus not likely to be such a binary system. We also present follow-up spectroscopy of five other variable candidates, which identifies them as likely low-amplitude $\delta$ Sct pulsating stars.

New OB star candidates in the Carina Arm around Westerlund 2 from VPHAS

Abstract: O and early B stars are at the apex of galactic ecology, but in the Milky Way, only a minority of them may yet have been identied.