Francesca Valsecchi

ON THE MAXIMUM MASS OF STELLAR BLACK HOLES

We present the spectrum of compact object masses: neutron stars and black holes (BHs) that originate from single stars in different environments. In particular, we calculate the dependence of maximum BH mass on metallicity and on some specific wind mass loss rates (e.g., Hurley et al. and Vink et al.). Our calculations show that the highest mass BHs observed in the Galaxy M bh ~ 15 M ☉ in the high metallicity environment (Z = Z ☉ = 0.02) can be explained with stellar models and the wind mass loss rates adopted here. To reach this result we had to set luminous blue variable mass loss rates at the level of ~10–4 M ☉ yr–1 and to employ metallicity-dependent Wolf-Rayet winds. With such winds, calibrated on Galactic BH mass measurements, the maximum BH mass obtained for moderate metallicity (Z = 0.3 Z ☉ = 0.006) is M bh,max = 30 M ☉. This is a rather striking finding as the mass of the most massive known stellar BH is M bh = 23-34 M ☉ and, in fact, it is located in a small star-forming galaxy with moderate metallicity. We find that in the very low (globular cluster-like) metallicity environment the maximum BH mass can be as high as M bh,max = 80 M ☉ (Z = 0.01 Z ☉ = 0.0002). It is interesting to note that X-ray luminosity from Eddington-limited accretion onto an 80 M ☉ BH is of the order of ~1040 erg s–1 and is comparable to luminosities of some known ultra-luminous X-ray sources. We emphasize that our results were obtained for single stars only and that binary interactions may alter these maximum BH masses (e.g., accretion from a close companion). This is strictly a proof-of-principle study which demonstrates that stellar models can naturally explain even the most massive known stellar BHs.

Formation of the black-hole binary M33 X-7 through mass exchange in a tight massive system

The X-ray source M33 X-7 in the nearby galaxy Messier 33 is among the most massive X-ray binary stellar systems known, hosting a rapidly spinning, 15.65M⊙ black hole orbiting an underluminous, 70M⊙ main-sequence companion in a slightly eccentric 3.45-day orbit (M⊙, solar mass). Although post-main-sequence mass transfer explains the masses and tight orbit, it leaves unexplained the observed X-ray luminosity, the star’s underluminosity, the black hole’s spin and the orbital eccentricity. A common envelope phase, or rotational mixing, could explain the orbit, but the former would lead to a merger and the latter to an overluminous companion. A merger would also ensue if mass transfer to the black hole were invoked for its spin-up. Here we report simulations of evolutionary tracks which reveal that if M33 X-7 started as a primary body of 85M⊙–99M⊙ and a secondary body of 28M⊙–32M⊙, in a 2.8–3.1-d orbit, its observed properties can be consistently explained. In this model, the main-sequence primary transfers part of its envelope to the secondary and loses the rest in a wind; it ends its life as a ∼16M⊙ helium star with an iron–nickel core that collapses to a black hole (with or without an accompanying supernova). The release of binding energy, and possibly collapse asymmetries, ‘kick’ the nascent black hole into an eccentric orbit. Wind accretion explains the X-ray luminosity, and the black-hole spin can be natal.

PLANETS ON THE EDGE

Hot Jupiters formed through circularization of high-eccentricity orbits should be found at orbital separations a exceeding twice that of their Roche limit a R. Nevertheless, about a dozen giant planets have now been found well within this limit (a R < a < 2 a R), with one coming as close as 1.2 a R. In this Letter, we show that orbital decay (starting beyond 2 a R) driven by tidal dissipation in the star can naturally explain these objects. For a few systems (WASP-4 and 19), this explanation requires the linear reduction in convective tidal dissipation proposed originally by Zahn and verified by recent numerical simulations, but rules out the quadratic prescription proposed by Goldreich & Nicholson. Additionally, we find that WASP-19-like systems could potentially provide direct empirical constraints on tidal dissipation, as we could soon be able to measure their orbital decay through high precision transit timing measurements.

ON THE RARITY OF X-RAY BINARIES WITH NAKED HELIUM DONORS

The paucity of known high-mass X-ray binaries (HMXBs) with naked He donor stars (hereafter He star) in the Galaxy has been noted over the years as a surprising fact, given the significant number of Galactic HMXBs containing H-rich donors, which are expected to be their progenitors. This contrast has further sharpened in light of recent observations uncovering a preponderance of HMXBs hosting loosely bound Be donors orbiting neutron stars (NSs), which would be expected to naturally evolve into He-HMXBs through dynamical mass transfer onto the NS and a common-envelope (CE) phase. Hence, reconciling the large population of Be-HMXBs with the observation of only one He-HMXB can help constrain the dynamics of CE physics. Here, we use detailed stellar structure and evolution models and show that binary mergers of HMXBs during CE events must be common in order to resolve the tension between these observed populations. We find that, quantitatively, this scenario remains consistent with the typically adopted energy parameterization of CE evolution, yielding expected populations which are not at odds with current observations. However, future observations which better constrain the underlying population of loosely bound O/B-NS binaries are likely to place significant constraints on the efficiency of CE ejection.

Neutron stars: Formed, spun and kicked

One of the primary goals when studying stellar systems with neutron stars has been to reveal the physical properties of progenitors and understand how neutron star spins and birth kicks are determined. Over the years a consensus understanding had been developed, but recently some of the basic elements of this understanding are being challenged by current observations of some binary systems and their theoretical interpretation. In what follows we review such recent developments and highlight how they are interconnected; we particularly emphasize some of the assumptions and caveats of theoretical interpretations and examine their validity (e.g., in connection to the unknown radial velocities of pulsars or the nuances of multi‐dimensional statistical analyses). The emerging picture does not erase our earlier understanding; instead it broadens it as it reveals additional pathways for neutron star formation and evolution: neutron stars probably form at the end of both core collapse of Fe cores of massive stars...

IMPORTANCE OF TIDES FOR PERIASTRON PRECESSION IN ECCENTRIC NEUTRON STAR-WHITE DWARF BINARIES

Although not nearly as numerous as binaries with two white dwarfs, eccentric neutron star-white dwarf (NS-WD) binaries are important gravitational-wave (GW) sources for the next generation of space-based detectors sensitive to low frequency waves. Here we investigate periastron precession in these sources as a result of general relativistic, tidal, and rotational effects; such precession is expected to be detectable for at least some of the detected binaries of this type. Currently, two eccentric NS-WD binaries are known in the galactic field, PSR J1141-6545 and PSR B2303+46, both of which have orbits too wide to be relevant in their current state to GW observations. However, population synthesis studies predict the existence of a significant Galactic population of such systems. Though small in most of these systems, we find that tidally induced periastron precession becomes important when tides contribute to more than 3% of the total precession rate. For these systems, accounting for tides when analyzing periastron precession rate measurements can improve estimates of the WD component mass inferred and, in some cases, will prevent us from misclassifying the object. However, such systems are rare due to rapid orbital decay. To aid the inclusion of tidal effects when using periastron precession as a mass measurement tool, we derive a function that relates the WD radius and periastron precession constant to the WD mass.

The Intriguing Evolutionary History of the Massive Black Hole X‐ray Binary M33 X‐7

Black hole (BH) X‐ray binaries (XRBs) are X‐ray luminous binary systems comprising a BH accreting matter from a companion star. Understanding their origins sheds light on the still not well understood physics of BH formation. M33 X‐7 hosts one of the most massive stellar‐mass BH among all XRBs known to date, a 15.65M⊙ BH orbiting a 70M⊙ companion star in a 3.45 day orbit. The high masses of the two components and the tight orbit relative to the large H‐rich stellar component challenge our understanding of the typically invoked BH‐XRBs formation channels. The measured underluminosity of the optical component further complicates the picture. A solution to the evolutionary history of this system that can account for all its observed properties has yet to be presented, and here we propose the first scenario that is consistent with the complete set of current observational constraints. In our model, M33 X‐7 started its life hosting a 85–99M⊙ primary and a 28–32M⊙ companion in a Keplerian orbit of 2.8–3.1 days....

Constraining White Dwarf Masses Via Apsidal Precession in Eccentric Double White Dwarf Binaries

Galactic short period double white dwarfs (DWD) are guaranteed gravitational wave (GW) sources for the next generation of space-based interferometers sensitive to low-frequency GWs (10^{-4}- 1 Hz). Here we investigate the possibility of constraining the white dwarf (WD) properties through measurements of apsidal precession in eccentric binaries. We analyze the general relativistic (GR), tidal, and rotational contributions to apsidal precession by using detailed He WD models. We find that apsidal precession can lead to a detectable shift in the emitted GW signal, the effect being stronger (weaker) for binaries hosting hot (cool) WDs. We find that in hot (cool) DWDs tides dominate the precession at orbital frequencies above ~0.01 mHz (~1 mHz). Analyzing the apsidal precession of these sources only accounting for GR would potentially lead to an extreme overestimate of the component masses. Finally, we derive a relation that ties the radius and apsidal precession constant of cool WD components to their masses, therefore allowing tides to be used as an additional mass measurement tool.