Calcium-rich gap transients: Tidal detonations of white dwarfs?

We hypothesize that at least some of the recently discovered class of calcium-rich gap transients are tidal detonation events of white dwarfs (WDs) by black holes (BHs) or possibly neutron stars. We show that the properties of the calcium-rich gap transients agree well with the predictions of the tidal detonation model. Under the predictions of this model, we use a follow-up X-ray observation of one of these transients, SN 2012hn, to place weak upper limits on the detonator mass of this system that include all intermediate-mass BHs (IMBHs). As these transients are preferentially in the stellar haloes of galaxies, we discuss the possibility that these transients are tidal detonations of WDs caused by random flyby encounters with IMBHs in dwarf galaxies or globular clusters. This possibility has been already suggested in the literature but without connection to the calcium-rich gap transients. In order for the random flyby cross-section to be high enough, these events would have to be occurring inside these dense stellar associations. However, there is a lack of evidence for IMBHs in these systems, and recent observations have ruled out all but the very faintest dwarf galaxies and globular clusters for a few of these transients. Another possibility is that these are tidal detonations caused by three-body interactions, where a WD is perturbed towards the detonator in isolated multiple star systems. We highlight a number of ways this could occur, even in lower mass systems with stellar-mass BHs or neutron stars. Finally, we outline several new observational tests of this scenario, which are feasible with current instrumentation.

The VLT-FLAMES Tarantula Survey XXI. Stellar spin rates of O-type spectroscopic binaries

Context: The initial distribution of spin rates of massive stars is a fingerprint of their elusive formation process. It also sets a key initial condition for stellar evolution and is thus an important ingredient in stellar population synthesis. So far, most studies have focused on single stars. Most O stars are, however, found in multiple systems. 

Aims: By establishing the spin-rate distribution of a sizeable sample of O-type spectroscopic binaries and by comparing the distributions of binary subpopulations with one another and with that of presumed-single stars in the same region, we aim to constrain the initial spin distribution of O stars in binaries, and to identify signatures of the physical mechanisms that affect the evolution of the spin rates of massive stars. 

Methods: We use ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial rotational velocities (ν_esini) for components of 114 spectroscopic binaries in 30 Doradus. The ν_esini values are derived from the full width at half maximum (FWHM) of a set of spectral lines, using a FWHM vs. ν_esini calibration that we derive based on previous line analysis methods applied to single O-type stars in the VFTS sample. 

Results: The overall ν_esini distribution of the primary stars resembles that of single O-type stars in the VFTS, featuring a low-velocity peak (at ν_esini<200 kms^-1) and a shoulder at intermediate velocities (200 <ν_esini<300 kms^-1). The distributions of binaries and single stars, however, differ in two ways. First, the main peak at ν_esini ~ 100kms^-1 is broader and slightly shifted towards higher spin rates in the binary distribution than that of the presumed-single stars. This shift is mostly due to short-period binaries (P_orb~<10 d). Second, the ν_esini distribution of primaries lacks a significant population of stars spinning faster than 300 kms^-1, while such a population is clearly present in the single-star sample. The ν_esini distribution of binaries with amplitudes of radial velocity variation in the range of 20 to 200 kms^-1 (mostly binaries with P_orb ~ 10-1000 d and/or with q<0.5) is similar to that of single O stars below ν_esini_~<170kms^-1. 

Conclusions: Our results are compatible with the assumption that binary components formed with the same spin distribution as single stars, and that this distribution contains few or no fast-spinning stars. The higher average spin rate of stars in short-period binaries may either be explained by spin-up through tides in such tight binary systems, or by spin-down of a fraction of the presumed-single stars and long-period binaries through magnetic braking (or by a combination of both mechanisms). Most primaries and secondaries of SB2 systems with P_orb~<10 d appear to have similar rotational velocities. This is in agreement with tidal locking in close binaries where the components have similar radii. The lack of very rapidly spinning stars among binary systems supports the idea that most stars with ν_esini_~> 300kms^-1 in the single-star sample are actually spun-up post-binary interaction products. Finally, the overall similarities (low-velocity peak and intermediate-velocity shoulder) of the spin distribution of binary and single stars argue for a massive star formation process in which the initial spin is set independently of whether stars are formed as single stars or as components of a binary system.

The VLT-FLAMES tarantula survey:XII. Rotational velocities of the single O-type stars

Context. The 30 Doradus (30 Dor) region of the Large Magellanic Cloud, also known as the Tarantula nebula, is the nearest starburst region. It contains the richest population of massive stars in the Local Group, and it is thus the best possible laboratory to investigate open questions on the formation and evolution of massive stars. Aims. Using ground-based multi-object optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to establish the (projected) rotational velocity distribution for a sample of 216 presumably single O-type stars in 30 Dor. The sample is large enough to obtain statistically significant information and to search for variations among subpopulations - in terms of spectral type, luminosity class, and spatial location - in the field of view. Methods. We measured projected rotational velocities, 3e sin i, by means of a Fourier transform method and a profile fitting method applied to a set of isolated spectral lines. We also used an iterative deconvolution procedure to infer the probability density, P(3e), of the equatorial rotational velocity, 3e. Results. The distribution of 3e sin i shows a two-component structure: a peak around 80 km s1 and a high-velocity tail extending up to 600 km s^-1 This structure is also present in the inferred distribution P(3e) with around 80% of the sample having 0 <3e ≤ 300 km s^-1 and the other 20% distributed in the high-velocity region. The presence of the low-velocity peak is consistent with what has been found in other studies for late O- and early B-type stars. Conclusions. Most of the stars in our sample rotate with a rate less than 20% of their break-up velocity. For the bulk of the sample, mass loss in a stellar wind and/or envelope expansion is not efficient enough to significantly spin down these stars within the first few Myr of evolution. If massive-star formation results in stars rotating at birth with a large portion of their break-up velocities, an alternative braking mechanism, possibly magnetic fields, is thus required to explain the present-day rotational properties of the O-type stars in 30 Dor. The presence of a sizeable population of fast rotators is compatible with recent population synthesis computations that investigate the influence of binary evolution on the rotation rate of massive stars. Even though we have excluded stars that show significant radial velocity variations, our sample may have remained contaminated by post-interaction binary products. That the highvelocity tail may be populated primarily (and perhaps exclusively) by post-binary interaction products has important implications for the evolutionary origin of systems that produce gamma-ray bursts. © 2013 Author(s).

Supernova 2013fc in a circumnuclear ring of a luminous infrared galaxy: The big brother of SN 1998S

We present photometric and spectroscopic observations of SN 2013fc, a bright type II supernova (SN) in a circumnuclear star-forming ring in the luminous infrared galaxy ESO 154-G010, observed as part of the Public ESO Spectroscopic Survey of Transient Objects. SN 2013fc is both photometrically and spectroscopically similar to the well-studied type IIn SN 1998S and to the bright type II-L SN 1979C. It exhibits an initial linear decline, followed by a short plateau phase and a tail phase with a decline too fast for ⁵⁶Co decay with full γ -ray trapping. Initially, the spectrum was blue and featureless. Later on, a strong broad (~8000 km s^-1) H α emission profile became prominent. We apply a STARLIGHT stellar population model fit to the SN location (observed when the SN had faded) to estimate a high extinction of A_V = 2.9 ± 0.2 mag and an age of 10⁺³ _-2 Myr for the underlying cluster.We compare the SN to SNe 1998S and 1979C and discuss its possible progenitor star considering the similarities to these events. With a peak brightness of B = -20.46 ± 0.21 mag, SN 2013fc is 0.9 mag brighter than SN 1998S and of comparable brightness to SN 1979C.We suggest that SN 2013fc was consistent with a massive red supergiant (RSG) progenitor. Recent mass loss probably due to a strong RSG wind created the circumstellar matter illuminated through its interaction with the SN ejecta. We also observe a near-infrared excess, possibly due to newly condensed dust.

Commentary on Star Chamber Decree 1586

A decree prohibiting the publication of any book contrary to statute, injunction, ordinance and letters patents, as well as any ordinance set down by the Company of Stationers. The formal protection of the Star Chamber was extended not only to books protected under royal printing privileges (see: uk_1566) but to books printed in contravention of the internal regulations of the Stationers' Company itself, further enhancing the significance of ‘stationers' copyright'.
The commentary describes the background to the decree, in particular the religious controversies of the 1570s and 1580s, as well as the dissatisfaction within the general printing trade during this period at the manner in which a number of the printing privileges granted by Elizabeth resulted in the monopolistic control of commercially lucrative works within the hands of a few stationers only. The commentary also details the efforts of the dominant members of the Stationers' Company to influence the substance of the decree and further augment their control over the internal operation of the book trade.

The fastest unbound star in our Galaxy ejected by a thermonuclear supernova

Hypervelocity stars (HVSs) travel with velocities so high that they exceed the escape velocity of the Galaxy. Several acceleration mechanisms have been discussed. Only one HVS (US 708, HVS 2) is a compact helium star. Here we present a spectroscopic and kinematic analysis of US 708. Traveling with a velocity of ∼1200 kilometers per second, it is the fastest unbound star in our Galaxy. In reconstructing its trajectory, the Galactic center becomes very unlikely as an origin, which is hardly consistent with the most favored ejection mechanism for the other HVSs. Furthermore, we detected that US 708 is a fast rotator. According to our binary evolution model, it was spun-up by tidal interaction in a close binary and is likely to be the ejected donor remnant of a thermonuclear supernova.

The VLT-FLAMES Tarantula survey: XX. The nature of the X-ray bright emission-line star VFTS 399

Context: The stellar population of the 30 Doradus star-forming region in the Large Magellanic Cloud contains a subset of apparently single, rapidly rotating O-type stars. The physical processes leading to the formation of this cohort are currently uncertain. 

Aims: One member of this group, the late O-type star VFTS 399, is found to be unexpectedly X-ray bright for its bolometric luminosity-in this study we aim to determine its physical nature and the cause of this behaviour. 

Methods: To accomplish this we performed a time-resolved analysis of optical, infrared and X-ray observations. 

Results: We found VFTS 399 to be an aperiodic photometric variable with an apparent near-IR excess. Its optical spectrum demonstrates complex emission profiles in the lower Balmer series and select He i lines-taken together these suggest an OeBe classification. The highly variable X-ray luminosity is too great to be produced by a single star, while the hard, non-thermal nature suggests the presence of an accreting relativistic companion. Finally, the detection of periodic modulation of the X-ray lightcurve is most naturally explained under the assumption that the accretor is a neutron star. 

Conclusions: VFTS 399 appears to be the first high-mass X-ray binary identified within 30 Dor, sharing many observational characteristics with classical Be X-ray binaries. Comparison of the current properties of VFTS 399 to binary-evolution models suggests a progenitor mass 25 M_⊙ for the putative neutron star, which may host a magnetic field comparable in strength to those of magnetars. VFTS 399 is now the second member of the cohort of rapidly rotating "single" O-type stars in 30 Dor to show evidence of binary interaction resulting in spin-up, suggesting that this may be a viable evolutionary pathway for the formation of a subset of this stellar population.