94 research outputs found
The Slow Merger of Massive Stars
We study the complete merger of two massive stars inside a common envelope
and the subsequent evolution of the merger product, a rapidly rotating massive
supergiant. Three qualitatively different types of mergers have been identified
and investigated in detail, and the post-merger evolution has been followed to
the immediate presupernova stage. The ``quiet merger'' case does not lead to
significant changes in composition, and the star remains a red supergiant. In
the case of a ``moderate merger'', the star may become a blue supergiant and
end its evolution as a blue supergiant, depending on the core to total mass
ratio (as may be appropriate for the progenitor of SN 1987A). In the case of
the most effective ``explosive merger'', the merger product stays a red giant.
In last two cases, the He abundance in the envelope is increased drastically,
but significant s-processing is mainly expected in the ``explosive merger''
case.Comment: 4 pages, 1 figure, proc. ESO/MPA/MPE Workshop "From Twilight to
Highlight: The Physics of Supernovae
A Slow Merger History of Field Galaxies Since z~1
Using deep infrared observations conducted with the CISCO imager on the
Subaru Telescope, we investigate the field-corrected pair fraction and the
implied merger rate of galaxies in redshift survey fields with Hubble Space
Telescope imaging. In the redshift interval, 0.5 < z < 1.5, the fraction of
infrared-selected pairs increases only modestly with redshift to 7% +- 6% at
z~1. This is nearly a factor of three less than the fraction, 22% +- 8%,
determined using the same technique on HST optical images and as measured in a
previous similar study. Tests support the hypothesis that optical pair
fractions at z~1 are inflated by bright star-forming regions that are unlikely
to be representative of the underlying mass distribution. By determining
stellar masses for the companions, we estimate the mass accretion rate
associated with merging galaxies. At z~1, we estimate this to be 2x10^{9 +-
0.2} solar masses per galaxy per Gyr. Although uncertainties remain, our
results suggest that the growth of galaxies via the accretion of pre-existing
fragments remains as significant a phenomenon in the redshift range studied as
that estimated from ongoing star formation in independent surveys.Comment: 5 pages, accepted for publication in ApJ Letter
On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars
About a quarter of all post-asymptotic giant branch (AGB) stars are
hydrogen-deficient. Stellar evolutionary models explain the carbon-dominated
H-deficient stars by a (very) late thermal pulse scenario where the
hydrogen-rich envelope is mixed with the helium-rich intershell layer.
Depending on the particular time at which the final flash occurs, the entire
hydrogen envelope may be burned. In contrast, helium-dominated post-AGB stars
and their evolution are yet not understood. A small group of very hot,
helium-dominated stars is formed by O(He)-type stars. We performed a detailed
spectral analysis of ultraviolet and optical spectra of four O(He) stars by
means of state-of-the-art non-LTE model-atmosphere techniques. We determined
effective temperatures, surface gravities, and the abundances of H, He, C, N,
O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-loss
rates of the O(He) stars, we found that they do not exhibit enhanced mass-loss.
The comparison with evolutionary models shows that the status of the O(He)
stars remains uncertain. Their abundances match predictions of a double helium
white dwarf merger scenario, suggesting that they might be the progeny of the
compact and of the luminous helium-rich sdO-type stars. The existence of
planetary nebulae that do not show helium enrichment around every other O(He)
star, precludes a merger origin for these stars. These stars must have formed
in a different way, for instance via enhanced mass-loss during their post-AGB
evolution or a merger within a common-envelope (CE) of a CO-WD and a red giant
or AGB star. A helium-dominated stellar evolutionary sequence exists, that may
be fed by different types of mergers or CE scenarios. It appears likely, that
all these pass through the O(He) phase just before they become white dwarfs.Comment: 29 pages, 27 figures, accepted for publication in A&
Matter Mixing in Aspherical Core-collapse Supernovae: Three-dimensional Simulations with Single Star and Binary Merger Progenitor Models for SN 1987A
We perform three-dimensional hydrodynamic simulations of aspherical core-collapse supernovae focusing on the matter mixing in SN 1987A. The impacts of four progenitor (pre-supernova) models and parameterized aspherical explosions are investigated. The four pre-supernova models include a blue supergiant (BSG) model based on a slow merger scenario developed recently for the progenitor of SN 1987A (Urushibata et al. 2018). The others are a BSG model based on a single star evolution and two red supergiant (RSG) models. Among the investigated explosion (simulation) models, a model with the binary merger progenitor model and with an asymmetric bipolar-like explosion, which invokes a jetlike explosion, best reproduces constraints on the mass of high velocity Ni, as inferred from the observed [Fe II] line profiles. The advantage of the binary merger progenitor model for the matter mixing is the flat and less extended profile of the C+O core and the helium layer, which may be characterized by the small helium core mass. From the best explosion model, the direction of the bipolar explosion axis (the strongest explosion direction), the neutron star (NS) kick velocity, and its direction are predicted. Other related implications and future prospects are also given
Jellyfish: The origin and distribution of extreme ram-pressure stripping events in massive galaxy clusters
We investigate the observational signatures and physical origin of ram-pressure stripping (RPS) in 63 massive galaxy clusters at z = 0.3–0.7, based on images obtained with the Hubble Space Telescope. Using a training set of a dozen ‘jellyfish’ galaxies identified earlier in the same imaging data, we define morphological criteria to select 211 additional, less obvious cases of RPS. Spectroscopic follow-up observations of 124 candidates so far confirmed 53 as cluster members. For the brightest and most favourably aligned systems, we visually derive estimates of the projected direction of motion based on the orientation of apparent compression shocks and debris trails. Our findings suggest that the onset of these events occurs primarily at large distances from the cluster core (>400 kpc), and that the trajectories of the affected galaxies feature high-impact parameters. Simple models show that such trajectories are highly improbable for galaxy infall along filaments but common for infall at high velocities, even after observational biases are accounted for, provided the duration of the resulting RPS events is ≲500 Myr. We thus tentatively conclude that extreme RPS events are preferentially triggered by cluster mergers, an interpretation that is supported by the disturbed dynamical state of many of the host clusters. This hypothesis implies that extreme RPS might occur also near the cores of merging poor clusters or even merging groups of galaxies. Finally, we present nine additional ‘jellyfish” galaxies at z > 0.3 discovered by us, thereby doubling the number of such systems known at intermediate redshift
Hydrodynamical Simulations of the Stream-Core Interaction in the Slow Merger of Massive Stars
We present detailed simulations of the interaction of a stream emanating from
a mass-losing secondary with the core of a massive supergiant in the slow
merger of the two stars inside a common envelope. The dynamics of the stream
can be divided into a ballistic phase, starting at the L_1 point, and a
hydrodynamical phase where the stream interacts strongly with the core.
Considering the merger of a 1 and 5Msun star with a 20Msun evolved supergiant,
we present two-dimensional hydrodynamical simulations using the PROMETHEUS code
to demonstrate how the penetration depth and post-impact conditions depend on
the initial properties of stream material (e.g. entropy, angular momentum,
stream width) and the properties of the core (e.g. density structure and
rotation rate). Using these results, we present a fitting formula for the
entropy generated in the stream--core interaction and a recipe for the
determination of the penetration depth based on a modified Bernoulli integral.Comment: 13 pages, 8 figures, submitted to MNRA
Systematic investigation of the expected gravitational wave signal from supermassive black hole binaries in the pulsar timing band
In this letter we carry out the first systematic investigation of the
expected gravitational wave (GW) background generated by supermassive black
hole (SMBH) binaries in the nHz frequency band accessible to pulsar timing
arrays (PTAs). We take from the literature several estimates of the redshift
dependent galaxy mass function and of the fraction of close galaxy pairs to
derive a wide range of galaxy merger rates. We then exploit empirical black
hole-host relations to populate merging galaxies with SMBHs. The result of our
procedure is a collection of a large number of phenomenological SMBH binary
merger rates consistent with current observational constraints on the galaxy
assembly at z<1.5. For each merger rate we compute the associated GW signal,
eventually producing a large set of estimates of the nHz GW background that we
use to infer confidence intervals of its expected amplitude. When considering
the most recent SMBH-host relations, accounting for ultra-massive black holes
in brightest cluster galaxies, we find that the nominal interval of
the expected GW signal is only a factor of 3-to-10 below current PTA limits,
implying a non negligible chance of detection in the next few years.Comment: 6 pages, 3 figures, submitted to MNRAS lette
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