Archive for massive stars

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.

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Cosmography of nearby OB associations with HIPPARCOS

Posted in astronomy with tags , , on November 29, 2015 by Tim Kendall

ESO’s VLT reveals the Carina Nebula's hidden secrets(ESA press release:) Astronomers have used modern techniques to visualise data from ESA’s Hipparcos space astrometry mission in three dimensions. The treatment of the data has offered insights into the distribution of nearby stars and uncovered new groupings of stars in the solar neighbourhood, shedding light on the origins of the stars in Orion and calling into question the existence of the Gould Belt – an iconic ring-shaped structure of stars in the Milky Way. The results show the potential of 3D visualisation of the solar neighbourhood, an approach which is of particular relevance to ESA’s Gaia mission which will map the Milky Way and Local Group in 3D with unprecedented sensitivity and accuracy. The above image is the Carina Nebula, imaged using VLT/HAWK-I and roughly centred on the extremely young (~0.3 – 0.5 Myr) cluster Trumpler 14 (right), part of the Carina OB1 association. The paper is “Cosmography of OB stars in the solar neighbourhood” H. Bouy & J. Alves, 2015 A&A, 584, 13 and I reproduce in part the abstract. The must-view visualisation is here (screenshot below).ESO-Trumpler14-cluster

We construct a 3D map of the spatial density of OB stars within 500 pc from the Sun using the Hipparcos  catalogue and find three large-scale stream-like structures that allow a new view on the solar neighbourhood. The spatial coherence of these blue streams and the monotonic age sequence over hundreds of parsecs suggest that they are made of young stars, similar to the young streams that are conspicuous in nearby spiral galaxies. The three streams are 1) the Scorpius to Canis Majoris stream, covering 350 pc and 65 Myr of star formation history; 2) the Vela stream, encompassing at least 150 pc and 25 Myr of star formation history; and 3) the Orion stream, including not only the well-known Orion OB1abcd associations, but also a large previously unreported foreground stellar group lying only 200 pc from the Sun. The map also reveals a remarkable and previously unknown nearby OB association, between the Orion stream and the Taurus molecular clouds, which might be responsible for the observed structure and star formation activity in this cloud complex. This new association also appears to be the birthplace of Betelgeuse, as indicated by the proximity and velocity of the red giant. If this is confirmed, it would solve the long-standing puzzle of the origin of Betelgeuse. The well-known nearby star-forming low-mass clouds, including the nearby T and R associations Lupus, Cha, Oph, CrA, Taurus, Vela R1, and various low-mass cometary clouds in Vela and Orion, appear in this new view of the local neighbourhood to be secondary star formation episodes that most likely were triggered by the feedback from the massive stars in the streams. We also recover well-known star clusters of various ages that are currently cruising through the solar neighbourhood. Finally, we find no evidence of an elliptical structure such as the Gould belt, a structure we suggest is a 2D projection effect, and not a physical ring.Screen Shot 2015-11-28 at 10.16.01

Massive overcontact binary VFTS 352 in the Tarantula Nebula

Posted in astronomy with tags , , on October 21, 2015 by Tim Kendall

Location of VFTS 352 in the Large Magellanic Cloud

This image shows the location of  VFTS 352 — the hottest and most massive double star system to date where the two components are in contact and sharing material. The two stars in this extreme system lie about 160 000 light-years from Earth in the Large Magellanic Cloud. This intriguing system could be heading for a dramatic end, either merging to form a single giant star or forming a binary black hole. This view of the Tarantula star-forming region includes visible-light images from the Wide Field Imager at the MPG/ESO 2.2-metre telescope at La Silla and infrared images from the 4.1-metre infrared VISTA telescope at Paranal. Image and text courtesy ESO Science Release eso1540. The discovery at the ESO/VLT of this remarkable binary is reported in “Discovery of the massive overcontact binary VFTS 352: Evidence for enhanced internal mixing”, L.M. Almeida et al., in Astrophysical Journal, vol 812, 2, 102 (2015) (DOI: 10.1088/0004-637X/812/2/102) and I reproduce here the abstract:

The contact phase expected to precede the coalescence of two massive stars is poorly characterized due to the paucity of observational constraints. Here we report on the discovery of VFTS 352, an O-type binary in the 30 Doradus region, as the most massive and earliest spectral type overcontact system known to date. We derived the 3D geometry of the system, its orbital period  Porb = 1.1241452(4) day, components’ effective temperatures — T1 = 42 540 ± 280 K and T2 = 41 120 ± 290 K — and dynamical masses M1 = 28.63 ± 0.30 M⊙ and M2 = 28.85 ± 0.30 M⊙. Compared to single-star evolutionary models, the VFTS 352 components are too hot for their dynamical masses by about 2700 and 1100 K, respectively. These results can be explained naturally as a result of enhanced mixing, theoretically predicted to occur in very short-period tidally locked systems. The VFTS 352 components are two of the best candidates identified so far to undergo this so-called chemically homogeneous evolution. The future of VFTS 352 is uncertain. If the two stars merge, a very rapidly rotating star will be produced. Instead, if the stars continue to evolve homogeneously and keep shrinking within their Roche Lobes, coalescence can be avoided. In this case, tides may counteract the spin down by winds such that the VFTS 352 components may, at the end of their life, fulfill the requirements for long gamma-ray burst (GRB) progenitors in the collapsar scenario. Independently of whether the VFTS 352 components become GRB progenitors, this scenario makes VFTS 352 interesting as a progenitor of a black hole binary, hence as a potential gravitational wave source through black hole–black hole merger.

Early stages of massive star formation: “Yellow balls” in W33

Posted in astronomy with tags , , on February 7, 2015 by Tim Kendall

Volunteers using the web-based Milky Way Project brought star-forming features nicknamed "yellowballs" to the attention of researchers, who later showed that they are a phase of massive star formation.Image Credit: NASA/JPL-Caltech

Citizen scientists have helped identify a particular stage in the still poorly understood process of massive star formation:

Infrared wavelengths of 3.6, 8.0, and 24.0 microns observed by the Spitzer Space Telescope are mapped into visible colors red, green, and blue in this striking image. The cosmic cloud of gas and dust is W33, a massive starforming complex some 13,000 light-years distant, near the plane of our Milky Way Galaxy. So what are all those yellow balls? Citizen scientists of the web-based Milky Way Project found the features they called yellow balls as they scanned many Spitzer images and persistently asked that question of researchers. Now there is an answer. The yellow balls in Spitzer images are identified as an early stage of massive star formation. They appear yellow because they are overlapping regions of red and green, the assigned colors that correspond to dust and organic molecules known as PAHs at Spitzer wavelengths. Yellow balls represent the stage before newborn massive stars clear out cavities in their surrounding gas and dust and appear as green-rimmed bubbles with red centers in the Spitzer image. Of course, the astronomical crowdsourcing success story is only part of the Zooniverse. The Spitzer image spans 0.5 degrees or about 100 light-years at the estimated distance of W33.

Text and image: APOD. Update: Here’s a great article from Scientific American about the future of exoplanet research and in particular the difficulties of studying different types of exoplanet with the same instrument. It is well worth a read.

Cloud-sculpting star cluster NGC 6823

Posted in astronomy with tags , on October 15, 2014 by Tim Kendall

ngc6823_waid_1092
Image credit and copyright: Donald P. Waid (Waid Observatory) courtesy Astronomy Picture of the Day

Star cluster NGC 6823 is slowly turning gas clouds into stars. The center of the open cluster, visible on the upper left, formed only about two million years ago and is dominated in brightness by a host of bright young blue stars. Some outer parts of the cluster, visible in the featured image’s center as the stars and pillars of emission nebula NGC 6820, contain even younger stars. The huge pillars of gas and dust likely get their elongated shape by erosion from hot radiation emitted from the brightest cluster stars. Striking dark globules of gas and dust are also visible across the lower left of the featured image. Open star cluster NGC 6823 spans about 50 light years and lies about 6000 light years away toward the constellation of the Fox (Vulpecula).

Ou4: a huge, extended and faint bipolar outflow only ~ 700 parsecs distant

Posted in astronomy with tags , , on July 18, 2014 by Tim Kendall

ou4_colourreduced_corradi
Image credit: Romano Corradi/(Instituto de Astrofísica de Canarias). Text by the incomparable Astronomy Picture of the Day (APOD) where I recommend you go for links to further web material concerning the discovery.

This nebula is very faint, but also very large in planet Earth’s sky. In the mosaic image, composed with narrowband data from the 2.5 meter Isaac Newton Telescope, it spans some 2.5 full moons toward the constellation Cepheus. Recently discovered by French astro-imager Nicolas Outters, the remarkable nebula’s bipolar shape and emission are consistent with it being a planetary nebula (PN), the gaseous shroud of a dying sun-like star, but its actual distance and origin are unknown. A new investigation suggests Ou4 really lies within the emission region SH2-129 some 2,300 light-years away. Consistent with that scenario, the cosmic squid would represent a spectacular outflow of material driven by a triple system of hot, massive stars, cataloged as HR 8119, seen near the center of the nebula. If so, this truly giant squid nebula would physically be nearly 50 light-years across.

Ou4 might be one of the nearest planetary nebulae although the discoverers caution its nature is not yet fully established. The morphology does seem quite clear from the beautiful image. If the scenario given by Corradi et al. (2014) is correct though, Ou4 is not a PN, as the central object(s) are not sufficiently evolved and in no way like the central stars of true planetary nebulae. [my italics]

Global collapse of a dark molecular cloud sees hundred solar mass star caught at formation

Posted in astronomy with tags , on July 14, 2013 by Tim Kendall

eso1331c

Images: (top) This wide-field optical view shows a region of sky in the southern constellation of Norma. At the centre lies the massive star-forming region SDC 335.579-0.292, but this is too obscured by dust to be visible. The very hot blue star HD 147937 and its surrounding ejected clouds can be seen at upper right. Credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin (below) Observations of the dark cloud SDC 335.579-0.292 using the Atacama Large Millimeter/submillimeter array (ALMA) have given astronomers the best view yet of a monster star in the process of forming. A stellar womb with over 500 times the mass than the Sun has been found and appears as the yellow blob near the centre of this picture. This is the largest ever seen in the Milky Way — and it is still growing. Credit: ALMA (ESO/NRAJ/NRAO)/NASA/Spitzer/JPL-Caltech/GLIMPSE

eso1331a There are two theories on the formation of the most massive stars. One suggests that the parental dark cloud fragments, creating several small cores that collapse on their own and eventually form stars. The other is more dramatic: the entire cloud begins to collapse inwards, with material racing towards the cloud’s centre to form one or more massive behemoths there. A team led by Nicolas Peretto of CEA/AIM Paris-Saclay, France, and Cardiff University, UK, realised that ALMA was the perfect tool to help find out what was really happening. SDC335.579-0.292 was first revealed as a dramatic environment of dark, dense filaments of gas and dust through observations with NASA’s Spitzer Space Telescope and ESA’s Herschel Space Observatory. Now the team has used the unique sensitivity of ALMA to look in detail at both the amount of dust and the motion of the gas moving around within the dark cloud — and they have found a true monster. This core — the womb of the embryonic star — has over 500 times the mass of our Sun swirling around within it. The ALMA observations show that much more material is still flowing inwards and increasing the mass still further. This material will eventually collapse to form a young star up to 100 times as massive as our home star — a very rare beast.

The paper is N. Peretto et al., “Global collapse of molecular clouds as a formation mechanism for the most massive stars”, A&A, 555, A112 (2013). The data imply a huge mass infall rate of around 0.0025 solar masses per year.

Stars forming in the Rosette Nebula

Posted in astronomy with tags , , on May 18, 2013 by Tim Kendall

hobys_rosette_05_A3_3d
Original image download(11 Mb jpeg). I think this is probably the best image from Herschel where the forming stars are truly pinpointed:

The Rosette Nebula resides some 5000 light-years from Earth and is associated with a larger cloud that contains enough dust and gas to make the equivalent of 10 000 Sun-like stars. The Herschel image shows half of the nebula and most of the Rosette cloud. The massive stars powering the nebula lie to the right of the image but are invisible at these wavelengths. Each colour represents a different temperature of dust, from –263ºC (only 10ºC above absolute zero) in the red emission to –233ºC in the blue. The bright smudges are dusty cocoons hiding massive protostars. These will eventually become stars containing around ten times the mass of the Sun. The small spots near the centre and in the redder regions of the image are lower mass protostars, similar in mass to the Sun.

A new paper today by Gahm et al. (arXiv.org pdf) addresses the question of brown dwarf formation in the region. From the abstract:

We conclude that the entire complex of shells, elephant trunks, and globulettes in the northern part of the nebula is expanding with nearly the same velocity of ~22 km/s, and with a very small spread in velocity among the globulettes. Some globulettes are in the process of detaching from elephant trunks and shells, while other more isolated objects must have detached long ago and are lagging behind in the general expansion of the molecular shell. The suggestion that some globulettes might collapse to form planetary-mass objects or brown dwarfs is strengthened by our finding of dense cores in several objects.

“Baby stars in the Rosette Cloud” (ESA)

Update: The results from the Herschel mission have shown us a pre-picture of star formation, as material flows under the influence of magnetic fields into and along streams, or filaments, which lead to condensation under self-gravitation and all the accretion phenomena of star formation – disks – which have been observed in the near-infrared. Now, in a new paper by P. Hennebelle, “On the origin of non-self-gravitating filaments in the ISM”, Astronomy & Astrophysics 556, A153, the physical origins of the filaments themselves, as they exist in the interstellar medium before any onset of self-gravitation and star formation, have been investigated thoroughly by supercomputer simulations of the underlying magnetohydrodynamic processes within the material. The formation of the filaments themselves is thought to be simply driven by energy dissipation.

Filaments are ubiquitous in the interstellar medium, as recently emphasized by Herschel observations, but their physical origin remains elusive. In this paper, the author uses ideal MHD simulations to study the formation of non-gravitating clumps in various conditions, including different setups, magnetization, and Mach numbers. On average, clumps in MHD simulations are more filamentary than clumps in hydrodynamic simulations. Detailed analyses reveal that the filaments are in general preferentially aligned with the strain, indicating that they simply result from the stretch induced by turbulence. Moreover, filaments tend to be confined by the Lorentz forces, which therefore lead them to survive longer in magnetized flows. The author concludes that filaments are ubiquitous because they are the results of the very generic turbulent strain, and because the magnetic field helps to keep them coherent. Energy dissipation appears to play a fundamental role in filament formation.