ABSTRACT
It is well known that measurements of H0 from gravitational lens time delays scale as H0 ∝ 1 − κE, where κE is the mean convergence at the Einstein radius RE but that all available lens data ...other than the delays provide no direct constraints on κE. The properties of the radial mass distribution constrained by lens data are RE and the dimensionless quantity ξ = REα″(RE)/(1 − κE), where α″(RE) is the second derivative of the deflection profile at RE. Lens models with too few degrees of freedom, like power-law models with densities ρ ∝ r−n, have a one-to-one correspondence between ξ and κE (for a power-law model, ξ = 2(n − 2) and κE = (3 − n)/2 = (2 − ξ)/4). This means that highly constrained lens models with few parameters quickly lead to very precise but inaccurate estimates of κE and hence H0. Based on experiments with a broad range of plausible dark matter halo models, it is unlikely that any current estimates of H0 from gravitational lens time delays are more accurate than ${\sim} 10{{\ \rm per\ cent}}$, regardless of the reported precision.
Tidal disruption event demographics Kochanek, C. S
Monthly notices of the Royal Astronomical Society,
09/2016, Letnik:
461, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We survey the properties of stars destroyed in tidal disruption events (TDEs) as a function of black hole (BH) mass, stellar mass and evolutionary state, star formation history and redshift. For
...$M_{{\rm BH}}\, {\mathrel {{<}{{\sim }}}\,10^7\ {\rm M}_{\odot }$
, the typical TDE is due to a M
* ∼ 0.3 M⊙ M-dwarf, although the mass function is relatively flat for
$M_{\ast }\,{ \mathrel {{<}{{\sim }}}\,{\rm M}_{\odot }$
. The contribution from older main-sequence stars and sub-giants is small but not negligible. From M
BH ≃ 107.5–108.5 M⊙, the balance rapidly shifts to higher mass stars and a larger contribution from evolved stars, and is ultimately dominated by evolved stars at higher BH masses. The star formation history has little effect until the rates are dominated by evolved stars. TDE rates should decline very rapidly towards higher redshifts. The volumetric rate of TDEs is very high because the BH mass function diverges for low masses. However, any emission mechanism which is largely Eddington-limited for low BH masses suppresses this divergence in any observed sample and leads to TDE samples dominated by M
BH ≃ 106.0–107.5 M⊙ BHs with roughly Eddington peak accretion rates. The typical fall-back time is relatively long, with 16 per cent having t
fb < 10−1 yr (37 d), and 84 per cent having longer time-scales. Many residual rate discrepancies can be explained if surveys are biased against TDEs with these longer t
fb, which seems very plausible if t
fb has any relation to the transient rise time. For almost any BH mass function, systematic searches for fainter, faster time-scale TDEs in smaller galaxies, and longer time-scale TDEs in more massive galaxies are likely to be rewarded.
The ∼10 per cent of tidal disruption events (TDEs) due to stars more massive than M
* ≳ M⊙ should show abundance anomalies due to stellar evolution in helium, carbon and nitrogen, but not oxygen. ...Helium is always enhanced, but only by up to ∼25 per cent on average because it becomes inaccessible once it is sequestered in the high-density core as the star leaves the main sequence. However, portions of the debris associated with the disrupted core of a main-sequence star can be enhanced in helium by factors of 2–3 for debris at a common orbital period. These helium abundance variations may be a contributor to the observed diversity of hydrogen and helium line strengths in TDEs. A still more striking anomaly is the rapid enhancement of nitrogen and the depletion of carbon due to the CNO cycle – stars with M
* ≳ M⊙ quickly show an increase in their average N/C ratio by factors of 3–10. Because low-mass stars evolve slowly and high-mass stars are rare, TDEs showing high N/C will almost all be due to ∼1–2 M⊙ stars disrupted on the main sequence. Like helium, portions of the debris will show still larger changes in C and N, and the anomalies decline as the star leaves the main sequence. The enhanced N/C abundance ratio of these TDEs provides the first natural explanation for the rare, nitrogen-rich quasars and may also explain the strong nitrogen emission seen in ultraviolet spectra of ASASSN-14li.
On the red supergiant problem Kochanek, C S
Monthly notices of the Royal Astronomical Society,
04/2020, Letnik:
493, Številka:
4
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
We examine the problem of estimating the mass range corresponding to the observed red supergiant (RSG) progenitors of Type IIP supernovae. Using Monte Carlo simulations designed to reproduce ...the properties of the observations, we find that the approach of Davies & Beasor significantly overestimates the maximum mass, yielding an upper limit of Mh/M⊙ = 20.5 ± 2.6 for an input population with Mh/M⊙ = 18. Our preferred Bayesian approach does better, with Mh/M⊙ = 18.6 ± 2.1 for the same input populations, but also tends to overestimate Mh. For the actual progenitor sample and a Salpeter initial mass function, we find $M_{\rm h}/\mathrm{M}_\odot = 19.01_{-2.04}^{+4.04}$ for the Eldridge & Tout mass–luminosity relation used by Smartt and Davies & Beasor, and $M_{\rm h}/\mathrm{M}_\odot = 21.28_{-2.28}^{+4.52}$ for the Sukhbold, Woosley & Heger mass–luminosity relation. Based on the Monte Carlo simulations, we estimate that these are overestimated by $(3.3\pm 0.8)\, \mathrm{M}_\odot$. The red supergiant problem remains.
We model the observed black hole mass function under the assumption that black hole formation is controlled by the compactness of the stellar core at the time of collapse. Low-compactness stars are ...more likely to explode as supernovae and produce neutron stars, while high-compactness stars are more likely to be failed supernovae that produce black holes with the mass of the helium core of the star. Using three sequences of stellar models and marginalizing over a model for the completeness of the black hole mass function, we find that the compactness ... above which 50% of core collapses produce black holes is ... The models also predict that f = 0.18 (0.09 < f < 0.39) of core collapses fail. We tested four other criteria for black hole formation based on ... and ..., the compactnesses at enclosed masses of 2.0 or 3.0 rather than 2.5 M..., the mass of the iron core MFe, and the mass inside the oxygen burning shell MO. We found that ... works as well as ..., while ..., MFe and MO are significantly worse. As expected from the high compactness of 20-25 M... stars, black hole formation in this mass range provides a natural explanation of the red supergiant problem. (ProQuest: ... denotes formulae/symbols omitted.)
We develop a general method for analyzing the light curves of microlensed quasars and apply it to the OGLE light curves of the four-image lens Q2237+0305. We simultaneously estimate the effective ...source velocity, the average stellar mass, the stellar mass function, and the size and structure of the quasar accretion disk. The light curves imply an effective source-plane velocity of 10,200 km ssuper-1 ~ vehang10M/Msuboang0super- 1/2 ~ 39,600 km ssuper-1 (68% confidence). Given an independent estimate for the source velocity, found by combining estimates for the peculiar velocity of the lens galaxy with its measured stellar velocity dispersion, we obtain a mean stellar mass of ang10Mang0 ~ 0.037 hsuper2 Msubo (0.0059 hsuper2 Msubo ~ ang10Mang0 ~ 0.20 hsuper2 Msubo). We were unable to distinguish a Salpeter mass function from one in which all stars had the same mass, but we do find a strong lower bound of Kappa sub*/ Kappa > ~ 0.5 on the fraction of the surface mass density represented by the microlenses. Our models favor a standard thin accretion disk model as the source structure over a simple Gaussian source. For a face-on, thin disk radiating as a blackbody with temperature profile Tsubs proportional Rsuper-3/4, the radius rsubs where the temperature matches the filter pass band 2000 Aa or Tsubs(rsubs) ~ 7 x 10super4 K is 1.4 x 10super15 hsuper-1 cm ~ rsubs ~ 4.5 x 10super15 hsuper-1 cm. The flux predicted by the disk model agrees with the observed flux of the quasar, so nonthermal or optically thin emission processes are not required. From the disk structure we estimate a black hole mass of MsubBH ~ 1.1image x 10super9 hsuper-3/2 eta image(L/LsubE)super-1/2 Msubo, consistent with the mass estimated under the assumption that the quasar is radiating at the Eddington luminosity (L/LsubE = 1).
Stellar mergers are common Kochanek, C. S; Adams, Scott M; Belczynski, Krzysztof
Monthly notices of the Royal Astronomical Society,
09/2014, Letnik:
443, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The observed Galactic rate of stellar mergers or the initiation of common envelope phases brighter than M
V
= −3 (M
I
= −4) is of the order of ∼0.5 (0.3) yr−1 with 90 per cent confidence ...statistical uncertainties of 0.24–1.1 (0.14–0.65) and factor of ∼2 systematic uncertainties. The (peak) luminosity function is roughly
$\text{d}N/\text{d} L \propto L^{-1.4\pm 0.3}$
, so the rates for events more luminous than V1309 Sco (M
V
≃ −7 mag) or V838 Mon (M
V
≃ −10 mag) are lower at r ∼ 0.1 and ∼0.03/year, respectively. The peak luminosity is a steep function of progenitor mass, L ∝ M
2 − 3. This very roughly parallels the scaling of luminosity with mass on the main sequence, but the transients are ∼2000–4000 times more luminous at peak. Combining these, the mass function of the progenitors, dN/dM ∝ M
−2.0 ± 0.8, is consistent with the initial mass function, albeit with broad uncertainties. These observational results are also broadly consistent with the estimates of binary population synthesis models. While extragalactic variability surveys can better define the rates and properties of the high-luminosity events, systematic, moderate depth (I ≳ 16 mag) surveys of the Galactic plane are needed to characterize the low-luminosity events. The existing Galactic samples are only ∼20 per cent complete, and Galactic surveys are (at best!) reaching a typical magnitude limit of ≲ 13 mag.
Transients obscured by dusty discs Kochanek, C S
Monthly notices of the Royal Astronomical Society,
03/2024, Letnik:
529, Številka:
3
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Dust absorption is invoked in a number of contexts for hiding a star that has survived some sort of transient event from view. Dust formed in a transient is expanding away from the star and, ...in spherical models, the mass and energy budgets implied by a high optical depth at late times make such models untenable. Concentrating the dust in a disc or torus can in principle hide a source from an equatorial observer using less mass and so delay this problem. However, using axisymmetric dust radiation transfer models with a range of equatorial dust concentrations, we find that this is quite difficult to achieve in practice. The polar optical depth must be either low or high to avoid scattering optical photons to equatorial observers. Most of the emission remains at wavelengths easily observed by JWST. The equatorial brightness can be significantly suppressed for very discy configurations with little polar optical depth – but only by a factor of ∼2 for polar optical depths of τp = 1 and ∼5 for τp = 0.1 even for a very high optical depth disc (τe = 1000) viewed edge-on. It is particularly difficult to hide a source with silicate dusts because the absorption feature near 10 µm frequently leads to the emission being concentrated just bluewards of the feature, near 8 µm.
Abstract
The majority of massive stars are in binaries, which implies that many core collapse supernovae should be binaries at the time of the explosion. Here we show that the three most recent, ...local (visual) SNe (the Crab, Cas A and SN 1987A) were not stellar binaries at death, with limits on the initial mass ratios of q = M2/M1 ≲ 0.1. No quantitative limits have previously been set for Cas A and the Crab, while for SN 1987A we merely updated existing limits in view of new estimates of the dust content. The lack of stellar companions to these three ccSNe implies a 90 per cent confidence upper limit on the q ≳ 0.1 binary fraction at death of fb < 44 per cent. In a passively evolving binary model (meaning no binary interactions), with a flat mass ratio distribution and a Salpeter IMF, the resulting 90 per cent confidence upper limit on the initial binary fraction of F < 63 per cent is in tension with observed massive binary statistics. Allowing a significant fraction fM ≃ 25 per cent of stellar binaries to merge reduces the tension, with $F < 63({1-f}_{M})^{-1}{\,\rm per\,cent} \simeq 81{\,\rm per\,cent}$, but allowing for the significant fraction in higher order systems (triples, etc.) reintroduces the tension. That Cas A was not a stellar binary at death also shows that a surviving massive binary companion at the time of the explosion is not necessary for producing a Type IIb SNe. Much larger surveys for binary companions to Galactic SNe will become feasible with the release of the full Gaia proper motion and parallax catalogues providing a powerful probe of the statistics of such binaries and their role in massive star evolution, neutron star velocity distributions and runaway stars.
PDF
