We use moderate-resolution optical spectrophotometry and the new MARCS stellar atmosphere models to determine the effective temperatures of 74 Galactic red supergiants (RSGs). The stars are mostly members of OB associations or clusters with known distances, allowing a critical comparison with modern stellar evolutionary tracks. We find we can achieve excellent matches between the observations and the reddened model fluxes and molecular transitions, although the atomic lines Ca I ?4226 and Ca II H and K are found to be unrealistically strong in the models. Our new effective temperature scale is significantly warmer than those in the literature, with the differences amounting to 400 K for the latest type M supergiants (i.e., M5 I). We show that the newly derived temperatures and bolometric corrections give much better agreement with stellar evolutionary tracks. This agreement provides a completely independent verification of our new temperature scale. The combination of effective temperature and bolometric luminosities allows us to calculate stellar radii; the coolest and most luminous stars (KW Sgr, Case 75, KY Cyg, HD 206936=? Cep) have radii of roughly 1500 Rsolar (7 AU), in excellent accordance with the largest stellar radii predicted from current evolutionary theory, although smaller than that found by others for the binary VV Cep and for the peculiar star VY CMa. We find that similar results are obtained for the effective temperatures and bolometric luminosities using only the dereddened V-K colors, providing a powerful demonstration of the self-consistency of the MARCS models.

https://iopscience.iop.org/article/10.1086/430901/pdf

The effective temperature scale of galactic red supergiants : Cool, but not as cool as we thought

In this paper we present a new look at the distribution of emission responsible for the soft X-ray diffuse background (SXRB) using ROSAT all-sky survey data. This is prompted by the demonstration of the existence of extensive 1/4 keV emission in the Galactic halo that was not considered in the most successful previous model, in which the bulk of the observed 1/4 keV X-rays originated in a Local Hot Bubble (LHB) that surrounds the Sun. The basic distribution can be represented by two angularly varying Galactic components (the LHB and an X-ray halo) and an isotropic extragalactic component. In addition, there are the distinct enhancements of supernova remnants, superbubbles, and clusters of galaxies. Using the negative correlation between the SXRB and DIRBE-corrected IRAS 100 μm intensity, we find a LHB that is similar to previous models, although it is possibly more limited in extent. The emission of the LHB varies by a factor of ~3.3 with direction and produces ROSAT 1/4 keV intensities between ~250 and ~820 × 10-6 counts s-1 arcmin-2 (typically brighter at higher Galactic latitudes), with inferred emission measures of ~0.0018 to 0.0058 cm-6 pc (assuming thermal ionization equilibrium). The distribution of the emission temperature of the LHB is peaked at 106.07 K with a range of ±12% at FWHM. While this variation is small and is perhaps due to systematic uncertainties in the analysis, there is a suggestion of a cooler region in the anticenter direction consistent with previous studies. We derive halo plus extragalactic intensities that vary from ~400 to 3000 × 10-6 counts s-1 arcmin-2 outside the absorbing gas of the Galactic disk. As the low end of the range is comparable to what is expected for the extragalactic background, this implies that the halo emission varies considerably over the sky, with inferred emission measures ranging from near zero to >0.02 cm-6 pc. The distribution of emission temperatures in the halo, again derived from thermal equilibrium emission models, peaks near 106.02 K and varies over ±20% at FWHM.

https://iopscience.iop.org/article/10.1086/305135/pdf

Progress on Establishing the Spatial Distribution of Material Responsible for the 1/4 keV Soft X-Ray Diffuse Background Local and Halo Components

Recent advances in ground-based gamma ray astronomy have led to the discovery of more than 70 sources of very high energy (E? ? 100?GeV) gamma rays, falling into a number of source populations including pulsar wind nebulae, shell type supernova remnants, Wolf-Rayet stars, giant molecular clouds, binary systems, the Galactic Center, active galactic nuclei and 'dark' (yet unidentified) galactic objects. We summarize the history of TeV gamma ray astronomy up to the current status of the field including a description of experimental techniques and highlight recent astrophysical results. We also discuss the potential of ground-based gamma ray astronomy for future discoveries and describe possible directions for future instrumental developments.

https://iopscience.iop.org/article/10.1088/0034-4885/71/9/096901/pdf

High energy astrophysics with ground-based gamma ray detectors

The diffuse gamma radiation arising from the interaction of cosmic-ray particles with matter and radiation in the Galaxy is one of the few probes available to study the origin of the cosmic rays. Data from the Milagro gamma-ray observatory—a water Cerenkov detector that continuously views ~2 sr of the overhead sky—shows that the brightest extended region in the entire northern sky is the Cygnus region of the Galactic plane. The TeV image of the Cygnus region contains at least one new source, MGRO J2019+37, which is 10.9 σ above the isotropic background, as well as correlations with the matter density in the region. However, the gamma-ray flux from the Cygnus region (after excluding MGRO J2019+37) as measured at ~12 TeV exceeds that predicted from a model of cosmic-ray production and propagation. This observation indicates the existence of either hard-spectrum cosmic-ray sources and/or unresolved sources of TeV gamma rays in the region.

/pdf/discovery-of-tev-gamma-ray-emission-from-the-cygnus-region-5aglndo6pe.pdf

Discovery of TeV Gamma-Ray Emission from the Cygnus Region of the Galaxy

To probe the distribution and physical characteristics of interstellar gas at temperatures -->T ? 3 ? 105 K in the disk of the Milky Way, we have used the Far Ultraviolet Spectroscopic Explorer (FUSE) to observe absorption lines of O VI ?1032 toward 148 early-type stars situated at distances >1 kpc. After subtracting off a mild excess of O VI arising from the Local Bubble, combining our new results with earlier surveys of O VI, and eliminating stars that show conspicuous localized X-ray emission, we find an average O VI midplane density -->n0 = 1.3 ? 10?8 cm?3. The density decreases away from the plane of the Galaxy in a way that is consistent with an exponential scale height of 3.2 kpc at negative latitudes or 4.6 kpc at positive latitudes. Average volume densities of O VI along different sight lines exhibit a dispersion of about 0.26 dex, irrespective of the distances to the target stars. This indicates that O VI does not arise in randomly situated clouds of a fixed size and density, but instead is distributed in regions that have a very broad range of column densities, with the more strongly absorbing clouds having a lower space density. Line widths and centroid velocities are much larger than those expected from differential Galactic rotation, but they are nevertheless correlated with distance and N(O VI), which reinforces our picture of a diverse population of hot plasma regions that are ubiquitous over the entire Galactic disk. The velocity extremes of the O VI profiles show a loose correlation with those of very strong lines of less ionized species, supporting a picture of a turbulent, multiphase medium churned by shock-heated gas from multiple supernova explosions.

The Far Ultraviolet Spectroscopic Explorer Survey of O VI Absorption in the Disk of the Milky Way

This paper summarizes and analyzes the results of radio optical, infrared, and X-ray observations of a large sector of the sky in the constellation Cygnus (α≈19h20m-22h, σ=30–50°;lII=65–90°, |bII|≲10°). This region is associated with an extended X-ray source referred to as the Cygnus superbubble. About a quarter of the superbubble region is occupied by the extensively investigated multicomponent thermal radio source Cyg X. The region contains eight OB-associations which, when projected on the sky, duplicate the outline of the X-ray superbubble. These associations contains 110 stars of high luminosity (about 40 Wolf-Rayet and Of stars). The observations suggest that the X-ray superbubble is not a single object. Between 50 and 75% of its X-ray emission can be ascribed to discrete sources, the rest being probably due to regions of coronal gas about 100 pc in diameter, created by stellar winds and, possibly, supernova explosions in individual associations. The objects that produce the X-ray and optical radiation of the presumed superbubble are located at distances from 0.5 to 2.5 kpc from the Sun in the Carina-Cygnus spiral arm. The eastern portion of the region presumed superbubble contains the associations Cyg OB7 and Cyg OB4 and is generally less than 1 kpc distant, while the western portion contains the associations Cyg OB1, 2, 3, 8, and 9 and is 1 to 2 kpc distant.

Tatiana G. Sitnik

Papers

Structure and origin of the cygnus superbubble