4,081 research outputs found
The impact of metallicity and dynamics on the evolution of young star clusters
The early evolution of a dense young star cluster (YSC) depends on the
intricate connection between stellar evolution and dynamical processes. Thus,
N-body simulations of YSCs must account for both aspects. We discuss N-body
simulations of YSCs with three different metallicities (Z=0.01, 0.1 and 1
Zsun), including metallicity-dependent stellar evolution recipes and
metallicity-dependent prescriptions for stellar winds and remnant formation. We
show that mass-loss by stellar winds influences the reversal of core collapse.
In particular, the post-collapse expansion of the core is faster in metal-rich
YSCs than in metal-poor YSCs, because the former lose more mass (through
stellar winds) than the latter. As a consequence, the half-mass radius expands
more in metal-poor YSCs. We also discuss how these findings depend on the total
mass and on the virial radius of the YSC. These results give us a clue to
understand the early evolution of YSCs with different metallicity.Comment: to appear in "Massive Young Star Clusters Near and Far: From the
Milky Way to Reionization", 2013 Guillermo Haro Conference, Eds. Y. D. Mayya,
D. Rosa-Gonzalez & E. Terlevich, INAOE and AMC. 4 pages, 2 figure
Background radiation from sterile neutrino decay and reionization
Sterile neutrinos are one of the most promising Warm Dark Matter candidates.
By considering their radiative- and pion-decay channels, we derive the allowed
contribution of sterile neutrinos to the X-ray, optical and near-infrared
cosmic backgrounds. The X-ray background puts a strong constraint on the mass
of radiatively decaying neutrinos (m <= 14 keV), whereas the allowed mass range
for pion-decay neutrinos (for a particle lifetime > 4 X 10^17 s) is 150 <=
m/MeV <= 500. Taking into account these constraints, we find that sterile
neutrinos do not significantly contribute to the optical and near-infrared
background. We further consider the impact of sterile neutrinos on
reionization. We find that the Thomson optical depth due to sterile neutrinos
is tau_e = (0.4-3) X 10^-2 in the case of radiative decays, and it is ~10^-3
for the pion-decay channel. We conclude that these particles must have played
only a minor role in cosmic reionization history.Comment: 13 pages, 9 figures, replaced with revised version, accepted for
publication in MNRA
Extragalactic Background Light: new constraints from the study of the photon-photon absorption on blazar spectra
The study of the Extragalactic Background Light (EBL) is crucial to
understand many astrophysical problems (as the formation of first stars, the
evolution of galaxies and the role of dust emission). At present, one of the
most powerful ways to put constraints on EBL is represented by the study of the
photon-photon absorption on gamma-ray spectra of TeV blazars. Adopting this
method, we found that, if the only contribution to the optical and Near
Infrared (NIR) background is given by galaxies, the spectrum of the blazar
H1426+428 cannot be fitted. To reproduce the observational data of H1426+428 a
Near Infrared excess with respect to galaxy counts is required, with amplitude
consistent with both the Matsumoto et al. (2000) data with Kelsall's model of
zodiacal light (ZL) subtraction and the DIRBE data with Wright's model of ZL
subtraction. The derived constraints on the optical EBL are weaker, because the
experimental errors on blazar data are still bigger than the differences among
various optical EBL models. In the mid-infrared the SPITZER measurement at 24
micron provides the best fit of the blazar spectrum.Comment: 8 pages, 5 figures, to appear in Proceedings of "Baryons in Dark
Matter Halos", 5-9 October 2004, Novigrad, Croati
Forming circumnuclear disks and rings in galactic nuclei: a competition between supermassive black hole and nuclear star cluster
We investigate the formation of circumnuclear gas structures from the tidal
disruption of molecular clouds in galactic nuclei, by means of smoothed
particle hydrodynamics simulations. We model galactic nuclei as composed of a
supermassive black hole (SMBH) and a nuclear star cluster (NSC) and consider
different mass ratios between the two components. We find that the relative
masses of the SMBH and the NSC have a deep impact on the morphology of the
circumnuclear gas. Extended disks form only inside the sphere of influence of
the SMBH. In contrast, compact rings naturally form outside the SMBH's sphere
of influence, where the gravity is dominated by the NSC. This result is in
agreement with the properties of the Milky Way's circumnuclear ring, which
orbits outside the SMBH sphere of influence. Our results indicate that compact
circumnuclear rings can naturally form outside the SMBH sphere of influence.Comment: Accepted for publication in ApJ. 12 pages, 6 figures, 3 tables.
Comments welcom
Constraining DM through 21 cm observations
Beyond reionization epoch cosmic hydrogen is neutral and can be directly
observed through its 21 cm line signal. If dark matter (DM) decays or
annihilates the corresponding energy input affects the hydrogen kinetic
temperature and ionized fraction, and contributes to the Ly_alpha background.
The changes induced by these processes on the 21 cm signal can then be used to
constrain the proposed DM candidates, among which we select the three most
popular ones: (i) 25-keV decaying sterile neutrinos, (ii) 10-MeV decaying light
dark matter (LDM) and (iii) 10-MeV annihilating LDM. Although we find that the
DM effects are considerably smaller than found by previous studies (due to a
more physical description of the energy transfer from DM to the gas), we
conclude that combined observations of the 21 cm background and of its gradient
should be able to put constrains at least on LDM candidates. In fact, LDM
decays (annihilations) induce differential brightness temperature variations
with respect to the non decaying/annihilating DM case up to Delta_delta T_b=8
(22) mK at about 50 (15) MHz. In principle this signal could be detected both
by current single dish radio telescopes and future facilities as LOFAR;
however, this assumes that ionospheric, interference and foreground issues can
be properly taken care of.Comment: 9 pages, submitted to MNRA
Ultra-luminous X-ray sources and remnants of massive metal-poor stars
Massive metal-poor stars might form massive stellar black holes (BHs), with
mass 25<=mBH/Msun<=80, via direct collapse. We derive the number of massive BHs
(NBH) that are expected to form per galaxy through this mechanism. Such massive
BHs might power most of the observed ultra-luminous X-ray sources (ULXs). We
select a sample of 64 galaxies with X-ray coverage, measurements of the star
formation rate (SFR) and of the metallicity. We find that NBH correlates with
the number of observed ULXs per galaxy (NULX) in this sample. We discuss the
dependence of our model on the SFR and on the metallicity. The SFR is found to
be crucial, consistently with previous studies. The metallicity plays a role in
our model, since a lower metallicity enhances the formation of massive BHs.
Consistently with our model, the data indicate that there might be an
anticorrelation between NULX, normalized to the SFR, and the metallicity. A
larger and more homogeneous sample of metallicity measurements is required, in
order to confirm our results.Comment: 21 pages, 8 figures, accepted for publication in MNRA
Ultraluminous X-ray Sources forming in low metallicity natal environments
In the last few years multiwavelength observations have boosted our
understanding of Ultraluminous X-ray Sources (ULXs). Yet, the most fundamental
questions on ULXs still remain to be definitively answered: do they contain
stellar or intermediate mass black holes? How do they form? We investigate the
possibility that the black holes hosted in ULXs originate from massive (40-120
) stars in low metallicity natal environments. Such black holes have a
typical mass in the range and may account for the
properties of bright (above erg s) ULXs. More than massive black holes might have been generated in this way in the metal
poor Cartwheel galaxy during the last years and might power most of the
ULXs observed in it. Support to our interpretation comes from NGC 1313 X-2, the
first ULX with a tentative identification of the orbital period in the optical
band, for which binary evolution calculations show that the system is most
likely made by a massive donor dumping matter on a black hole.Comment: 4 pages. To appear in the Proceedings of the Conference "X-Ray
Astronomy 2009: Present Status, Multiwavelength Approach and Future
Perspectives", Bologna, Italy, September 2009, Eds. A. Comastri, M. Cappi, L.
Angelini, 2010 AIP (in press)
A ring in a shell: the large-scale 6D structure of the Vela OB2 complex
The Vela OB2 association is a group of 10 Myr stars exhibiting a complex
spatial and kinematic substructure. The all-sky Gaia DR2 catalogue contains
proper motions, parallaxes (a proxy for distance) and photometry that allow us
to separate the various components of Vela OB2. We characterise the
distribution of the Vela OB2 stars on a large spatial scale, and study its
internal kinematics and dynamic history. We make use of Gaia DR2 astrometry and
published Gaia-ESO Survey data. We apply an unsupervised classification
algorithm to determine groups of stars with common proper motions and
parallaxes. We find that the association is made up of a number of small
groups, with a total current mass over 2330 Msun. The three-dimensional
distribution of these young stars trace the edge of the gas and dust structure
known as the IRAS Vela Shell across 180 pc and shows clear signs of expansion.
We propose a common history for Vela OB2 and the IRAS Vela Shell. The event
that caused the expansion of the shell happened before the Vela OB2 stars
formed, imprinted the expansion in the gas the stars formed from, and most
likely triggered star formation.Comment: Accepted by A&A (02 November 2018), 13 pages, 9+2 figure
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