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Register a new userThe deep OB star population in Carina from the VST Photometric H$alpha$ Survey (VPHAS+)
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Massive OB stars are critical to the ecology of galaxies, and yet our knowledge of OB stars in the Milky Way, fainter than $V sim 12$, remains patchy. Data from the VST Photometric H$alpha$ Survey (VPHAS+) permit the construction of the first deep catalogues of blue excess-selected OB stars, without neglecting the stellar field. A total of 14900 candidates with 2MASS cross-matches are blue-selected from a 42 square-degree region in the Galactic Plane, capturing the Carina Arm over the Galactic longitude range $282^{circ} lesssim ell lesssim 293^{circ}$. Spectral energy distribution fitting is performed on these candidates combined VPHAS+ $u,g,r,i$ and 2MASS $J,H,K$ magnitudes. This delivers: effective temperature constraints, statistically separating O from early-B stars; high-quality extinction parameters, $A_0$ and $R_V$ (random errors typically $< 0.1$). The high-confidence O-B2 candidates number 5915 and a further 5170 fit to later B spectral type. Spectroscopy of 276 of the former confirms 97% of them. The fraction of emission line stars among all candidate B stars is 7--8% . Greyer ($R_V > 3.5$) extinction laws are ubiquitous in the region, over the distance range 2.5--3 kpc to $sim$10~kpc. Near prominent massive clusters, $R_V$ tends to rise, with particularly large and chaotic excursions to $R_V sim 5$ seen in the Carina Nebula. The data reveal a hitherto unnoticed association of 108 O-B2 stars around the O5If$+$ star LSS 2063 ($ell = 289.77^{circ}$, $b = -1.22^{circ}$). Treating the OB star scale-height as a constant within the thin disk, we find an orderly mean relation between extinction ($A_0$) and distance in the Galactic longitude range, $287.6^{circ} < ell < 293.5^{circ}$, and infer the subtle onset of thin-disk warping. A halo around NGC 3603, roughly a degree in diameter, of $sim$500 O-B2 stars with $4 < A_0 (rm{mag}) < 7$ is noted.
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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 identified. We present the results of a pilot study to select and parametrise OB star candidates in the Southern Galactic plane, down to a limiting magnitude of $g=20$. A 2 square-degree field capturing the Carina Arm around the young massive star cluster, Westerlund 2, is examined. The confirmed OB stars in this cluster are used to validate our identification method, based on selection from the $(u-g, g-r)$ diagram for the region. Our Markov Chain Monte Carlo fitting method combines VPHAS+ $u, g, r, i$ with published $J, H, K$ photometry in order to derive posterior probability distributions of the stellar parameters $log(rm T_{rm eff})$ and distance modulus, together with the reddening parameters $A_0$ and $R_V$. The stellar parameters are sufficient to confirm OB status while the reddening parameters are determined to a precision of $sigma(A_0)sim0.09$ and $sigma(R_V)sim0.08$. There are 489 objects that fit well as new OB candidates, earlier than $sim$B2. This total includes 74 probable massive O stars, 5 likely blue supergiants and 32 reddened subdwarfs. This increases the number of previously known and candidate OB stars in the region by nearly a factor of 10. Most of the new objects are likely to be at distances between 3 and 6 kpc. We have confirmed the results of previous studies that, at these longer distances, these sight lines require non-standard reddening laws with $3.5<R_V<4$.
The VST Photometric Halpha Survey of the Southern Galactic Plane and Bulge (VPHAS+) is surveying the southern Milky Way in u, g, r, i and Halpha at 1 arcsec angular resolution. Its footprint spans the Galactic latitude range -5 < b < +5 at all longitudes south of the celestial equator. Extensions around the Galactic Centre to Galactic latitudes +/-10 bring in much of the Galactic Bulge. This ESO public survey, begun on 28th December 2011, reaches down to 20th magnitude (10-sigma) and will provide single-epoch digital optical photometry for around 300 million stars. The observing strategy and data pipelining is described, and an appraisal of the segmented narrowband Halpha filter in use is presented. Using model atmospheres and library spectra, we compute main-sequence (u - g), (g - r), (r - i) and (r - Halpha) stellar colours in the Vega system. We report on a preliminary validation of the photometry using test data obtained from two pointings overlapping the Sloan Digital Sky Survey. An example of the (u - g, g - r) and (r - Halpha, r - i) diagrams for a full VPHAS+ survey field is given. Attention is drawn to the opportunities for studies of compact nebulae and nebular morphologies that arise from the image quality being achieved. The value of the u band as the means to identify planetary-nebula central stars is demonstrated by the discovery of the central star of NGC 2899 in survey data. Thanks to its excellent imaging performance, the VST/OmegaCam combination used by this survey is a perfect vehicle for automated searches for reddened early-type stars, and will allow the discovery and analysis of compact binaries, white dwarfs and transient sources.
The ESO public survey VISTA Variables in the Via Lactea (VVV) has contributed with deep multi-epoch photometry of the Galactic bulge and the adjacent part of the disk over 526 square degrees. More than a hundred cluster candidates have been reported thanks to this survey. We present the fifth article in a series of papers focused on young and massive clusters discovered in the VVV survey. In this paper, we present the physical characterization of five clusters with a spectroscopically confirmed OB-type stellar population. To characterize the clusters, we used near-infrared photometry ($J$, $H,$ and $K_S$) from the VVV survey and near-infrared $K$-band spectroscopy from ISAAC at VLT, following the methodology presented in the previous articles of the series. All clusters in our sample are very young (ages between 1-20 Myr), and their total mass are between $(1.07^{+0.40}_{-0.30})cdot10^2$ $M_{odot}$ and $(4.17^{+4.15}_{-2.08})cdot10^3$ $M_{odot}$. We observed a relation between the clusters total mass $M_{ecl}$ and the mass of their most massive stellar member $m_{max}$, even for clusters with an age $<$ 10 Myr.
We performed a deep wide-field (6.76 deg^2) near-infrared survey with the VISTA telescope that covers the entire extent of the Carina nebula complex (CNC). The point-source catalog created from these data contains around four million individual objects down to masses of 0.1 M_sun. We present a statistical study of the large-scale spatial distribution and an investigation of the clustering properties of infrared-excesses objects, which are used to trace disk-bearing young stellar objects (YSOs). We find that a (J - H) versus (Ks - [4.5]) color-color diagram is well suited to tracing the population of YSO-candidates (cYSOs) by their infrared excess. We identify 8781 sources with strong infrared excess, which we consider as cYSOs. This sample is used to investigate the spatial distribution of the cYSOs with a nearest-neighbor analysis. The surface density distribution of cYSOs agrees well with the shape of the clouds as seen in our Herschel far-infrared survey. The strong decline in the surface density of excess sources outside the area of the clouds supports the hypothesis that our excess-selected sample consists predominantly of cYSOs with a low level of background contamination. This analysis allows us to identify 14 groups of cYSOs outside the central area. Our results suggest that the total population of cYSOs in the CNC comprises about 164000 objects, with a substantial fraction (~35%) located in the northern, still not well studied parts. Our cluster analysis suggests that roughly half of the cYSOs constitute a non-clustered, dispersed population.
We present the main steps that will be taken to extract H$alpha$ emission flux from Javalambre Photometric Local Universe Survey (J-PLUS) photometric data. For galaxies with $zlesssim0.015$, the H$alpha$+[NII] emission is covered by the J-PLUS narrow-band filter $F660$. We explore three different methods to extract the H$alpha$ + [NII] flux from J-PLUS photometric data: a combination of a broad-band and a narrow-band filter ($r$ and $F660$), two broad-band and a narrow-band one ($r$, $i$ and $F660$), and a SED-fitting based method using 8 photometric points. To test these methodologies, we simulated J-PLUS data from a sample of 7511 SDSS spectra with measured H$alpha$ flux. Based on the same sample, we derive two empirical relations to correct the derived H$alpha$+[NII] flux from dust extinction and [NII] contamination. We find that the only unbiased method is the SED fitting based one. The combination of two filters underestimates the measurements of the H$alpha$ + [NII] flux by a 28%, while the three filters method by a 9%. We study the error budget of the SED-fitting based method and find that, in addition to the photometric error, our measurements have a systematic uncertainty of a 4.3%. Several sources contribute to this uncertainty: differences between our measurement procedure and the one used to derive the spectroscopic values, the use of simple stellar populations as templates, and the intrinsic errors of the spectra, which were not taken into account. Apart from that, the empirical corrections for dust extinction and [NII] contamination add an extra uncertainty of 14%. Given the J-PLUS photometric system, the best methodology to extract H$alpha$ + [NII] flux is the SED-fitting based one. Using this method, we are able to recover reliable H$alpha$ fluxes for thousands of nearby galaxies in a robust and homogeneous way.
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