Archive for black holes

A new route for the formation of black hole binaries and a prediction for their mass distribution

Posted in astronomy with tags , , on February 15, 2016 by Tim Kendall

hs-2007-04-a-full-jpgIn a new Oxford University press release astronomers have revealed an evolutionary route to the formation of stellar-mass black hole binaries such as the one recently seen by Advanced LIGO. While the observed mass deficit of three solar masses exactly fitted Einstein’s predictions, the actual masses, around 30 MSun each, are unexpectedly large. However, tidally induced internal mixing often occurs in massive close binary stars, and careful modelling of this process has revealed new insights into exactly how this affects the masses of the final black holes:

One complication to theories around the formation of tight pairs of compact stars stems from the fact that stars are generally thought to expand when they age. This new theory avoids such an expansion by invoking a mechanism that keeps the interior of very close and sufficiently massive stars completely chemically mixed. The reason for this is that tidal forces keep the stars in bound rotation – that is, they continue to show the same side to each other, similar to how the same side of the Moon always faces the Earth. The fast orbital motion involved in this process leads to extremely rapid rotation of the stars, which triggers internal chemical mixing inside the star. Marchant and colleagues show, through a large number of detailed evolutionary calculations, that these stars never expand and that the binaries remain compact up to the collapse phase, when the stars turn into relatively massive black holes.

An online preprint is available and the abstract (which I reproduce in part) yields the bigger picture:

With recent advances in gravitational-wave astronomy, the direct detection of gravitational waves from the merger of two stellar-mass compact objects has become a realistic prospect. Evolutionary scenarios towards mergers of various double compact objects generally invoke so-called common-envelope evolution, which is poorly understood and leads to large uncertainties in the predicted merger rates. Here we explore, as an alternative, the scenario of massive overcontact binary (MOB) evolution, which involves two very massive stars in a very tight binary that remain fully mixed as a result of their tidally induced high spin. While many of these systems merge early on, we find many MOBs that swap mass several times, but survive as a close binary until the stars collapse. The simplicity of the MOB scenario allows us to use the effcient public stellar-evolution code MESA to explore it systematically by means of detailed numerical calculations. We find that, at low metallicity, MOBs produce double-black-hole (BH+BH) systems that will merge within a Hubble time with mass-ratios close to one, in two mass ranges, about 25 to 60 MSun and > 130 M, with pair- instability supernovae (PISNe) [no remnant at all] being produced at intermediate masses.

Image: Massive stars in NGC 602 (Hubble 25th Anniversary). Updates: News from the Fermi gamma-ray observatories in orbit: LAT finds no counterpart but GBM data contains a transient event on the same date, and the discoverers themselves write on the implications of GW150914 for a large stochastic gravitational wave background from merging black hole binaries. Now there is a further paper dealing with the weak GBM transient which arrived 0.4 sec after the GW event.

VLT/MUSE spectroscopy suggests a central intermediate mass black hole in globular cluster NGC 6397

Posted in astronomy with tags , on February 5, 2016 by Tim Kendall

NGC6397-NRGBhiTwo new papers appearing today on the astro-ph preprint server have highlighted the capabilities of a new instrument at ESO/VLT, the Multi Unit Spectroscopic Explorer, MUSE. Studying the velocity dispersion of stars in the globular cluster NGC 6397, astronomers infer a central black hole of  some six hundred solar masses. They have also been able to construct the first complete spectroscopic HR diagram for a globular cluster, using nearly 19000 stellar spectra. Image: Antilhue/Chile; astrosurf.com, 14.5″ mirror, prime focus f/9. From the abstracts: (Paper I/Paper II)

We demonstrate the high multiplex advantage of crowded field 3D spectroscopy using the new integral field spectrograph MUSE by means of a spectroscopic analysis of more than 12,000 individual stars in the globular cluster NGC 6397. The stars are deblended with a PSF (point spread function) fitting technique, using a photometric reference catalogue from HST as prior, including relative positions and brightnesses. This catalogue is also used for a first analysis of the extracted spectra, followed by an automatic in-depth analysis using a full-spectrum fitting method based on a large grid of PHOENIX [theoretical model] spectra. With 18,932 spectra from 12,307 stars in NGC 6397 we have analysed the largest sample so far available for a single globular cluster. We derived a mean radial velocity of 17.84 ± 0.07 km/s and a mean metallicity of [Fe/H]= −2.120 ± 0.002, with the latter seemingly varying with temperature for stars on the RGB. We determine effective temperature and [Fe/H] from the spectra, and surface gravity from HST photometry. This is the first very comprehensive HRD for a globular cluster based on the analysis of several thousands of stellar spectra. Furthermore, two interesting objects were identified with one being a post-AGB star and the other a possible millisecond-pulsar companion.

We present a detailed analysis of the kinematics of the galactic globular cluster NGC 6397 based on more than ~18,000 spectra obtained with the novel integral field spectrograph MUSE. While NGC 6397 is often considered a core collapse cluster, our analysis suggests a flattening of the surface brightness profile at the smallest radii. Although it is among the nearest globular clusters, the low velocity dispersion of NGC 6397 of <5 km/s imposes heavy demands on the quality of the kinematical data. We show that despite its limited spectral resolution, MUSE reaches an accuracy of 1 km/s in the analysis of stellar spectra. We find slight evidence for a rotational component in the cluster and the velocity dispersion profile that we obtain shows a mild central cusp. To investigate the nature of this feature, we calculate spherical Jeans models and compare these models to our kinematical data. This comparison shows that if a constant mass-to-light ratio is assumed, the addition of an intermediate-mass black hole with a mass of 600 M_sun brings the model predictions into agreement with our data, and therefore could be at the origin of the velocity dispersion profile. We further investigate cases with varying mass-to-light ratios and find that a compact dark stellar component can also explain our observations. However, such a component would closely resemble the black hole from the constant mass-to-light ratio models as this component must be confined to the central ~5 arcsec of the cluster and must have a similar mass. Independent constraints on the distribution of stellar remnants in the cluster or kinematic measurements at the highest possible spatial resolution should be able to distinguish the two alternatives.

IDL TIFF fileHubble image of the central regions of NGC 6397 (Wikipedia). In 2006, a study using such data was published that showed a clear lower limit in the intrinsic brightness of the cluster population of faint stars at around visual magnitude 26. The authors therefore were able to deduce observationally the lower limit for the mass necessary for stars to develop a core capable of fusion: roughly 0.083 times the mass of the Sun.

Emission nebula NGC 6357 and rich open cluster Pismis 24

Posted in astronomy with tags , , , on October 24, 2013 by Tim Kendall

EmissionNebula_NGC6357
Image credit: APOD The most prominent star of the open cluster Pismis 24 (top) is another object thought at one time to have several hundred solar masses, but now known to be at least a triple system – one where the components are each a hundred Suns. Wikipedia has a list of the most massive stars. Top of the list are the stars in R136 in the Large Magellanic Cloud, and at the bottom is the extreme red supergiant VY Canis Majoris (right), which can be seen undergoing copious mass loss.

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I have drawn attention to the black hole firewall paradox previously, and Scientific American has an article, highlighting new work which argues for the reality of the paradox:

Polchinski and his colleagues conclude that not only is space not smooth at a black hole horizon—at that point the laws of physics completely break down. Instead of an unobtrusive boundary, the scientists argue that there must actually be a sharp division they call a firewall. “The firewall is kind of a wall of energy—it could be the end of spacetime itself,” Polchinski says. “Anything hitting it would break up into its fundamental bits and effectively dissolve.”

A full treatment of the subject is here, and dissenting arguments can be found here. If the new paradigm proves true, the event horizon will be seen as the place where the laws of physics break down, not the singularity itself.

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In addition, there are a pair of new brown dwarf discoveries, not yet peer-reviewed but by leading authors in the field. In the first, overseen by Prof. S.J. Warren at Imperial College, the largest homogeneous sample of ultracool dwarfs to date has been assembled, and appears to have increased the number of known L dwarfs enormously. This has been accomplished by mining existing surveys (SDSS, UKIDSS and WISE) down to apparent magnitude J = 17.5. In the second paper, a bizarre underluminous M dwarf – that is, an extreme M subdwarf – appears to have been found with a large proper motion of 1.8 arcseconds per year, in the WISE dataset. The distance is, however, uncertain.

Update: The 25 year Lick planet search, using the Hamilton spectrograph, has been completed and a homogeneous dataset released:

The Lick Observatory planet search program began in 1987 when the first spectrum of τ Ceti was taken with an iodine cell and the Hamilton Spectrograph. Upgrades to the instrument improved the Doppler precision from about 10 m/s in 1992 to about 3 m/s in 1995. The project detected dozens of exoplanets with orbital periods ranging from a few days to several years. The Lick survey identified the first planet in an eccentric orbit (70 Virginis) and the first multi-planet system around a normal main sequence star (Upsilon Andromedae).

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Lick will continue at the forefront of exoplanet research with the new robotic Automated Planet Finder (APF). The practice of using the spectrum of an arc lamp as a wavelength comparison has a long history. Above is the high resolution visible spectrum of the Sun, showing numerous absorption lines, strong and weak. Image courtesy W.M. Keck observatories.