* Astronomy

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M33

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M33 X-7

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RE: Messier33

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Title: Spitzer Observations of M33 and the Hot Star, H II Region Connection
Authors: Robert H. Rubin, Janet P. Simpson, Sean W.J. Colgan, Reginald J. Dufour, Gregory Brunner, Ian A. McNabb, Adalbert W.A. Pauldrach, Edwin F. Erickson, Michael R. Haas, Robert I. Citron

We have observed emission lines of [S IV] 10.51, H(7-6) 12.37, [Ne II] 12.81, [Ne III] 15.56, and [S III] 18.71 um in a number of extragalactic H II regions with the Spitzer Space Telescope. A previous paper presented our data and analysis for the substantially face-on spiral galaxy M83. Here we report our results for the local group spiral galaxy M33. The nebulae selected cover a wide range of galactocentric radii (R_G). The observations were made with the Infrared Spectrograph with the short wavelength, high resolution module. The above set of five lines is observed cospatially, thus permitting a reliable comparison of the fluxes. From the measured fluxes, we determine the ionic abundance ratios including Ne++/Ne+, S3+/S++, and S++/Ne+ and find that there is a correlation of increasingly higher ionisation with larger R_G. By sampling the dominant ionisation states of Ne (Ne+, Ne++) and S (S++, S3+) for H II regions, we can estimate the Ne/H, S/H, and Ne/S ratios. We find from linear least-squares fits that there is a decrease in metallicity with increasing R_G: d log (Ne/H)/dR_G = -0.058 ±0.014 and d log (S/H)/dR_G = -0.052 ±0.021 dex kpc-1. There is no apparent variation in the Ne/S ratio with R_G. Unlike our previous similar study of M83, where we conjectured that this ratio was an upper limit, for M33 the derived ratios are likely a robust indication of Ne/S. This occurs because the H II regions have lower metallicity and higher ionisation than those in M83. Both Ne and S are primary elements produced in alpha-chain reactions, following C and O burning in stars, making their yields depend very little on the stellar metallicity. Thus, it is expected that Ne/S remains relatively constant throughout a galaxy. The median (average) Ne/S ratio derived for H II regions in M33 is 16.3 (16.9), just slightly higher than the Orion Nebula value of 14.3. The same methodology is applied to Spitzer observations recently published for three massive H II regions: NGC 3603 (Milky Way), 30 Dor (LMC), and N 66 (SMC) as well as for a group of blue compact dwarf galaxies.

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Newborn stars shine like celestial sparklers in a new portrait of the nearby Triangulum Galaxy the most detailed ultraviolet image of a galaxy ever taken. Astronomers will use the image, taken by NASA's Swift telescope, to create an "age map" of the galaxy's components to understand how galaxies evolve over time

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Credit: NASA/Swift/Stefan Immler

A Swift satellite UVOT UV mosaic of 13 observations (red: uvw1, green: uvm2, blue: uvw2) of M33. The 13 individual pointings (with 1 ks exposure time per UVOT UV filter) cover the entire disk of the galaxy. The resulting image is one of the best ultraviolet observations of any galaxy obtained to date.

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Title: Spitzer photometry of discrete sources in M33
Authors: S. Verley, L. K. Hunt, E. Corbelli, C. Giovanardi

Combining the relative vicinity of the Local Group spiral galaxy M33 with the Spitzer images, we investigate the properties of infrared (IR) emission sites and assess the reliability of the IR emission as a star formation tracer. We compared the photometric results for several samples of three known types of discrete sources (HII regions, supernovae remnants and planetary nebulae) with theoretical diagnostic diagrams, and derived the spectral energy distribution (from 3.6 to 24 microns) of each type of object. Moreover, we generated a catalogue of 24 microns sources and inferred their nature from the observed and theoretical colours of the known type sources. We estimated the star formation rate in M33 both globally and locally, from the IR emission and from the Halpha emission line.

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Title: A 15.65 solar mass black hole in an eclipsing binary in the nearby spiral galaxy Messier 33
Authors: Jerome A. Orosz (San Diego State), Jeffrey E. McClintock, Ramesh Narayan (CfA), Charles D. Bailyn (Yale), Joel D. Hartman (CfA), Lucas Mracri (NOAO), Jiefeng Liu (CfA), Wolfgang Pietsch (Max Planck, Garching), Ronald A. Remillard (MIT), Avi Shporer, Tsevi Mazeh (Wise Observatory)

Stellar-mass black holes are discovered in X-ray emitting binary systems, where their mass can be determined from the dynamics of their companion stars. Models of stellar evolution have difficulty producing black holes in close binaries with masses >10 solar masses, which is consistent with the fact that the most massive stellar black holes known so all have masses within 1 sigma of 10 solar masses. Here we report a mass of 15.65 ± 1.45 solar masses for the black hole in the recently discovered system M33 X-7, which is located in the nearby galaxy Messier 33 (M33) and is the only known black hole that is in an eclipsing binary. In order to produce such a massive black hole, the progenitor star must have retained much of its outer envelope until after helium fusion in the core was completed. On the other hand, in order for the black hole to be in its present 3.45 day orbit about its 70.0 ± 6.9 solar mass companion, there must have been a "common envelope"' phase of evolution in which a significant amount of mass was lost from the system. We find the common envelope phase could not have occurred in M33 X-7 unless the amount of mass lost from the progenitor during its evolution was an order of magnitude less than what is usually assumed in evolutionary models of massive stars.

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M33 X-7

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Astronomers have located an exceptionally massive black hole in orbit around a huge companion star. This result has intriguing implications for the evolution and ultimate fate of massive stars.
The black hole is part of a binary system in M33, a nearby galaxy about 3 million light years from Earth. By combining data from NASAs Chandra X-ray Observatory and the Gemini telescope on Mauna Kea, Hawaii, the mass of the black hole, known as M33 X-7, was determined to be 15.7 times that of the Sun. This makes M33 X-7 the most massive stellar black hole known. A stellar black hole is formed from the collapse of the core of a massive star at the end of its life.


Credit: Illustration: NASA/CXC/M.Weiss; X-ray: NASA/CXC/CfA/P.Plucinsky et al.; Optical: NASA/STScI/SDSU/J.Orosz et al.
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"This discovery raises all sorts of questions about how such a big black hole could have been formed - Jerome Orosz of San Diego State University, lead author of the paper appearing in the October 18th issue of the journal Nature.

M33 X-7 orbits a companion star that eclipses the black hole every three and a half days. The companion star also has an unusually large mass, 70 times that of the Sun. This makes it the most massive companion star in a binary system containing a black hole.

"This is a huge star that is partnered with a huge black hole. Eventually, the companion will also go supernova and then well have a pair of black holes" - coauthor Jeffrey McClintock of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Mass.

The properties of the M33 X-7 binary system a massive black hole in a close orbit around a massive companion star are difficult to explain using conventional models for the evolution of massive stars. The parent star for the black hole must have had a mass greater than the existing companion in order to have formed a black hole before the companion star.
Such a massive star would have had a radius larger than the present separation between the stars, so the stars must have been brought closer while sharing a common outer atmosphere. This process typically results in a large amount of mass being lost from the system, so much that the parent star should not have been able to form a 15.7 solar-mass black hole.
The black hole's progenitor must have shed gas at a rate about 10 times less than predicted by models before it exploded. If even more massive stars also lose very little material, it could explain the incredibly luminous supernova seen recently as SN 2006gy. The progenitor for SN 2006gy is thought to have been about 150 times the mass of the Sun when it exploded.

"Massive stars can be much less extravagant than people think by hanging onto a lot more of their mass toward the end of their lives. This can have a big effect on the black holes that these stellar time-bombs make - Jerome Orosz.

Coauthor Wolfgang Pietsch was also the lead author of an article in the Astrophysical Journal that used Chandra observations to report that M33 X-7 is the first black hole in a binary system observed to undergo eclipses. The eclipsing nature enables unusually accurate estimates for the mass of the black hole and its companion.

"Because it's eclipsing and because it has such extreme properties, this black hole is an incredible test-bed for studying astrophysics" - Wolfgang Pietsch .

The length of the eclipse seen by Chandra gives information about the size of the companion. The scale of the companion's motion, as inferred from the Gemini observations, gives information about the mass of the black hole and its companion. Other observed properties of the binary were used to constrain the mass estimates.
NASA's Marshall Space Flight Centre, Huntsville, Ala., manages the Chandra program for the agency's Science Mission Directorate. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Centre in Cambridge, Mass. Gemini is an international partnership managed by the Association of Universities for Research in Astronomy under a cooperative agreement with the National Science Foundation.

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