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OB Stars (ob + star)

Distribution by Scientific Domains
Physics and Astronomy100%

Selected Abstracts

A census of the Carina Nebula , II.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
Energy budget, global properties of the nebulosity
ABSTRACT The first paper in this series took a direct census of energy input from the known OB stars in the Carina Nebula, and in this paper we study the global properties of the surrounding nebulosity. This detailed comparison may prove useful for interpreting observations of extragalactic giant H ii regions and ultraluminous infrared (IR) galaxies. We find that the total IR luminosity of Carina is about 1.2 × 107 L,, accounting for only about 50,60 per cent of the known stellar luminosity from Paper I. Similarly, the ionizing photon luminosity derived from the integrated radio continuum is about 7 × 1050 s,1, accounting for ,75 per cent of the expected Lyman continuum from known OB stars. The total kinetic energy of the nebula is about 8 × 1051 erg, or ,30 per cent of the mechanical energy from stellar winds over the lifetime of the nebula, so there is no need to invoke a supernova (SN) explosion based on energetics. Warm dust grains residing in the H ii region interior dominate emission at 10,30 ,m, but cooler grains at 30,40 K dominate the IR luminosity and indicate a likely gas mass of ,106 M,. We find an excellent correlation between the radio continuum and 20,25 ,m emission, consistent with the idea that the ,80-K grain population is heated by trapped Ly, photons. Similarly, we find a near perfect correlation between the far-IR optical depth map of cool grains and 8.6-,m hydrocarbon emission, indicating that most of the nebular mass resides as atomic gas in photodissociation regions and not in dense molecular clouds. Synchronized star formation around the periphery of Carina provides a strong case that star formation here was indeed triggered by stellar winds and ultraviolet radiation. This second generation appears to involve a cascade toward preferentially intermediate- and low-mass stars, but this may soon change when , Carinae and its siblings explode. If the current reservoir of atomic and molecular gas can be tapped at that time, massive star formation may be rejuvenated around the periphery of Carina much as if it were a young version of Gould's Belt. Furthermore, when these multiple SNe occur, the triggered second generation will be pelted repeatedly with SN ejecta bearing short-lived radioactive nuclides. Carina may therefore represent the most observable analogue to the cradle of our own Solar system. [source]

Displacement of the Sun from the Galactic plane

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007
Y. C. Joshi
ABSTRACT We have carried out a comparative statistical study for the displacement of the Sun from the Galactic plane (z,) following three different methods. The study has been carried out using a sample of 537 young open clusters (YOCs) with log (Age) < 8.5, lying within a heliocentric distance of 4 kpc, and 2030 OB stars observed up to a distance of 1200 pc, all of which have distance information. We statistically separated the members of the Gould Belt before investigating the variation in the z, estimation with different upper cut-off limits in the heliocentric distance and distance perpendicular to the Galactic plane. We have found that z, varies in the range ,13,20 pc from the analysis of YOCs and in the range ,6,28 pc from the analysis of OB stars. A significant scatter in z,, because of different cut-off values, is noticed for the OB stars, although no such deviation is seen for the YOCs. We have also determined scaleheights of 56.9+3.8,3.4 and 61.4+2.7,2.4 pc for the distribution of YOCs and OB stars, respectively. [source]

MSX mid-infrared imaging of massive star birth environments , II.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2004
Giant H ii regions
ABSTRACT We conduct a Galactic census of giant H ii (GH ii) regions, based on the all-sky 6-cm data set of Kuchar & Clark, plus the kinematic distances obtained by Russeil. From an inspection of mid-infrared (MIR) Mid-course Space Experiment (MSX) and far-IR IRAS Sky Survey Atlas images, we identify a total of 56 GH ii regions in the Milky Way, of which 15 per cent (65 per cent) can be seen at optical (near-IR) wavelengths. The mid to far-IR fluxes from each GH ii region are measured, and sample the thermal emission from the ubiquitous dust present within the exciting clusters of OB stars, arising from the integrated luminosity of the hot stars heating the cluster dust, for which we obtain log L(IR) = 5.5,7.3 L,. The MIR 21-,m spatial morphology is presented for each GH ii region, and often indicates multiple emission sources, suggesting complicated cluster formation. IR colour,colour diagrams are presented, providing information concerning the temperature distribution and the optical depth of the dust. For the clusters of our study, the dust is not optically thick to all stellar radiation, thus the measured infrared luminosity is lower than Lbol. As the dust environment of a cluster begins to dissipate, the thermal emission and its optical depth ought to decrease even before the stars evolve appreciably. We see evidence of this in our empirical relationship between the integrated IR and Lyman continuum luminosities. [source]

The total-to-selective extinction ratio determined from near IR photometry of OB stars

ASTRONOMISCHE NACHRICHTEN, Issue 3 2003
W. WegnerArticle first published online: 8 APR 200
Abstract The paper presents an extensive list of the total to selective extinction ratios R calculated from the infrared magnitudes of 597 O and B stars using the extrapolation method. The IR magnitudes of these stars were taken from the literature. The IR colour excesses are determined with the aid of "artificial standards" , Wegner (1994). The individual and mean values of total to selective extinction ratios R differ in most cases from the average value R= 3.10 ±0.05 , Wegner (1993) in different OB associations. The relation between total to selective extinction ratios R determined in this paper and those calculated using the "method of variable extinction" and the Cardelli et al. (1989) formulae is discussed. The R values presented in this paper can be used to determine individual absolute magnitudes of reddened OB stars with known trigonometric parallaxes. [source]