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Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| Masses and luminosities of O- and B-type stars and red supergiants
Massive stars are of interest as progenitors of supernovae, i.e.neutron stars and black holes, which can be sources of gravitationalwaves. Recent population synthesis models can predict neutron star andgravitational wave observations but deal with a fixed supernova rate oran assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernovaprogenitors, i.e. with O- and early B-type stars, and also allsupergiants within 3 kpc. We restrict our sample to those massive starsdetected both in 2MASS and observed by Hipparcos, i.e. only those starswith parallax and precise photometry. To determine the luminositieswe calculated the extinctions from published multi-colour photometry,spectral types, luminosity class, all corrected for multiplicity andrecently revised Hipparcos distances. We use luminosities andtemperatures to estimate the masses and ages of these stars usingdifferent models from different authors. Having estimated theluminosities of all our stars within 3 kpc, in particular for all O- andearly B-type stars, we have determined the median and mean luminositiesfor all spectral types for luminosity classes I, III, and V. Ourluminosity values for supergiants deviate from earlier results: Previouswork generally overestimates distances and luminosities compared to ourdata, this is likely due to Hipparcos parallaxes (generally moreaccurate and larger than previous ground-based data) and the fact thatmany massive stars have recently been resolved into multiples of lowermasses and luminosities. From luminosities and effective temperatureswe derived masses and ages using mass tracks and isochrones fromdifferent authors. From masses and ages we estimated lifetimes andderived a lower limit for the supernova rate of ?20 events/Myraveraged over the next 10 Myr within 600 pc from the sun. These data arethen used to search for areas in the sky with higher likelihood for asupernova or gravitational wave event (like OB associations).
| An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328Galactic B and late-O stars. We use a new technique which employsstellar atmosphere models in lieu of unreddened "standard" stars. Thistechnique is capable of virtually eliminating spectral mismatch errorsin the curves. It also allows a quantitative assessment of the errorsand enables a rigorous testing of the significance of relationshipsbetween various curve parameters, regardless of whether theiruncertainties are correlated. Analysis of the curves gives the followingresults: (1) In accord with our previous findings, the central positionof the 2175 A extinction bump is mildly variable, its width is highlyvariable, and the two variations are unrelated. (2) Strong correlationsare found among some extinction properties within the UV region, andwithin the IR region. (3) With the exception of a few curves withextreme (i.e., large) values of R(V), the UV and IR portions of Galacticextinction curves are not correlated with each other. (4) The largesightline-to-sightline variation seen in our sample implies that anyaverage Galactic extinction curve will always reflect the biases of itsparent sample. (5) The use of an average curve to deredden a spectralenergy distribution (SED) will result in significant errors, and arealistic error budget for the dereddened SED must include the observedvariance of Galactic curves. While the observed largesightline-to-sightline variations, and the lack of correlation among thevarious features of the curves, make it difficult to meaningfullycharacterize average extinction properties, they demonstrate thatextinction curves respond sensitively to local conditions. Thus, eachcurve contains potentially unique information about the grains along itssightline.
| Small Magellanic Cloud-Type Interstellar Dust in the Milky Way
It is well known that the sight line toward HD 204827 in the clusterTrumpler 37 shows a UV extinction curve that does not follow the averageGalactic extinction relation. However, when a dust component, foregroundto the cluster, is removed, the residual extinction curve is identicalto that found in the SMC within the uncertainties. The curve is verysteep and has little or no 2175 Å bump. The position of HD 204827in the sky is projected onto the edge of the Cepheus IRAS bubble. Inaddition, HD 204827 has an IRAS bow shock, indicating that it may beembedded in dust swept up by the supernova that created the IRAS bubble.Shocks due to the supernova may have led to substantial processing ofthis dust. The HD 204827 cloud is dense and rich in carbon molecules.The 3.4 μm feature indicating a C-H grain mantle is present in thedust toward HD 204827. The environment of the HD 204827 cloud dust maybe similar to the dust associated with HD 62542, which lies on the edgeof a stellar wind bubble and is also dense and rich in molecules. Thissight line may be a Rosetta Stone if its environment can be related tothose in the SMC having similar dust.
| Dust properties in the direction of Trumpler 37.
The fourteen brightest OB stars of the Trumpler 37 Cluster have beenobserved in the V,R,I,J,H,K photometric bands. These have been used toderive accurate measurements of the quantity R_V_=A_V_/E(B-V). Differentfrom previous studies it is found that in Trumpler 37 isclose to the mean interstellar value of 3.1. Most of the reddeningtowards Trumpler 37 is found to be of foreground origin. This isconsistent with the lack of variations, observed by us, of R_V_ with theprojected distance from the center of the cluster. In two highlyreddened stars of the cluster, whose reddening is certainly mostlylocal, the observed R_V_ is lower than the mean interstellar value of3.1. This is to be compared with the behaviour of the Trapezium stars inOrion, whose R_V_ is quite above such value whereas the age of thecluster is similar to the one of Trumpler 37.
| The structure of the IC1396 region.
We have made an extensive study of the molecular clouds associated withthe Hii region IC1396 in the rotational transitions J=1-0 and J=2-1 of^12^CO and ^13^CO and J=3-2 of ^12^CO with an average spatial resolutionof 2.5' and an average sampling of about 2.0', in order to getinformation on its structure and evolution. On the basis of ourobservations, which cover an area of more than 6deg^2^, we can classifythe molecular clouds into those directly associated with the ionizingO6.5V star HD 206267, producing the bright-rimmed clouds, and the coldgas along the line of sight, which is mainly foreground material. Thebright-rimmed clouds show the presence of warmer molecular gas throughhigher ^12^CO (2-1)/(1-0), ^13^CO (2-1)/(1-0) and ^12^CO (3-2)/(2-1)line ratios than the cold foreground gas. The warm clouds form roughly ashell-like arrangement with a diameter of 25 to 40pc around HD206267(though most are slightly closer to the Sun than the O star), and theyseem to be the remainder of the now dispersing molecular cloud whichgave birth to the O6.5 star and the star cluster Tr37 associated withit. All bright-rimmed clouds show internal structure on all size scales,including bipolar outflows. Optical, FIR (IRAS 12 to 100μm) and COmaps are in close agreement over the whole region, especially for thebright-rimmed clouds: Exceptions are some optically bright (ionized)regions, which show FIR, but no CO emission, and the cold foregroundclouds, which are very weak at FIR wavelengths. The entire mass of themapped IC1396 region is estimated to be 12000Msun_, which iscomposed of molecular (4000Msun_), atomic(5000Msun_), and ionized material (3000Msun_) innearly equal amounts. The masses of the bright-rimmed clouds range froma few to several 100Msun_.
| Tracing the Roots of Interstellar Mid Infrared Emission
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...275..549J&db_key=AST
| Anomalous dust in the open cluster Trumpler 37
New IUE data were obtained towards stars in the open cluster Trumpler37. Extinction curves derived from these data show that the anomalousdust detected towards HD 204827 and HD 210072 is not unique to thoselines of sight but occurs generally in that region of the sky. The dustis apparently associated with the Trumpler 37 cluster, which forms thecenter of Cepheus OB2. This is a complex region of star formation wherethe evolution of dust grains through shock waves could be consideredlikely. A supernova in a slightly older star-forming region of Cep OB2located next to Trumpler 37 is a possible source of such a shock wave.
| Some characteristics of the young open cluster Trumpler 37.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976PASP...88..865G&db_key=AST
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Observation and Astrometry data
| Constellation: | Cepheus |
| Right ascension: | 21h46m11.64s |
| Declination: | +57°47'53.0" |
| Apparent magnitude: | 10.199 |
| Proper motion RA: | 0.6 |
| Proper motion Dec: | 7.3 |
| B-T magnitude: | 10.798 |
| V-T magnitude: | 10.249 |
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