16,361 research outputs found
The Chemical Evolution of the Milky Way: the Three Infall Model
We present a new chemical evolution model for the Galaxy that assumes three
main infall episodes of primordial gas for the formation of halo, thick and
thin disk, respectively. We compare our results with selected data taking into
account NLTE effects. The most important parameters of the model are (i) the
timescale for gas accretion, (ii) the efficiency of star formation and (iii) a
threshold in the gas density for the star formation process, for each Galactic
component. We find that, in order to best fit the features of the solar
neighbourhood, the halo and thick disk must form on short timescales (~0.2 and
~1.25 Gyr, respectively), while a longer timescale is required for the
thin-disk formation. The efficiency of star formation must be maximum (10
Gyr-1) during the thick-disk phase and minimum (1 Gyr-1) during the thin-disk
formation. Also the threshold gas density for star formation is suggested to be
different in the three Galactic components. Our main conclusion is that in the
framework of our model an independent episode of accretion of extragalactic
gas, which gives rise to a burst of star formation, is fundamental to explain
the formation of the thick disk. We discuss our results in comparison to
previous studies and in the framework of modern galaxy formation theories.Comment: 12 pages, 7 figures, accepted for publication in MNRA
Feedback from massive stars and gas expulsion from proto-globular clusters
© 2015. The American Astronomical Society. All rights reserved. Globular clusters (GCs) are considerably more complex structures than previously thought, harboring at least two stellar generations that present clearly distinct chemical abundances. Scenarios explaining the abundance patterns in GCs mostly assume that originally the clusters had to be much more massive than today, and that the second generation of stars originates from the gas shed by stars of the first generation (FG). The lack of metallicity spread in most GCs further requires that the supernova-enriched gas ejected by the FG is completely lost within ∼30 Myr, a hypothesis never tested by means of three-dimensional hydrodynamic simulations. In this paper, we use 3D hydrodynamic simulations including stellar feedback from winds and supernovae, radiative cooling and self-gravity to study whether a realistic distribution of OB associations in a massive proto-GC of initial mass M tot ∼ 10 7 M o is sufficient to expel its entire gas content. Our numerical experiment shows that the coherence of different associations plays a fundamental role: as the bubbles interact, distort, and merge, they carve narrow tunnels that reach deeper and deeper toward the innermost cluster regions, and through which the gas is able to escape. Our results indicate that after 3 Myr, the feedback from stellar winds is responsible for the removal of ∼40% of the pristine gas, and that after 14 Myr, 99% of the initial gas mass has been removed
Comment on the paper by Y.Komura and Y.Okabe [arXiv:1011.3321]
We point out that the claim of strong universality in the paper J.Phys. A 44,
015002, arXiv:1011.3321 is incorrect, as it contradicts known rigorous results.Comment: submitted to J.Phys.
Chemical abundances and properties of the ionized gas in NGC 1705
We obtained [O III] narrow-band imaging and multi-slit MXU spectroscopy of
the blue compact dwarf (BCD) galaxy NGC 1705 with FORS2@VLT to derive chemical
abundances of PNe and H II regions and, more in general, to characterize the
properties of the ionized gas. The auroral [O III]\lambda4363 line was detected
in all but one of the eleven analyzed regions, allowing for a direct estimate
of their electron temperature. The only object for which the [O III]\lambda4363
line was not detected is a possible low-ionization PN, the only one detected in
our data. For all the other regions, we derived the abundances of Nitrogen,
Oxygen, Neon, Sulfur and Argon out to ~ 1 kpc from the galaxy center. We detect
for the first time in NGC 1705 a negative radial gradient in the oxygen
metallicity of -0.24 \pm 0.08 dex kpc^{-1}. The element abundances are all
consistent with values reported in the literature for other samples of dwarf
irregular and blue compact dwarf galaxies. However, the average (central)
oxygen abundance, 12 + log(O/H)=7.96 \pm 0.04, is ~0.26 dex lower than previous
literature estimates for NGC 1705 based on the [O III]\lambda4363 line. From
classical emission-line diagnostic diagrams, we exclude a major contribution
from shock excitation. On the other hand, the radial behavior of the emission
line ratios is consistent with the progressive dilution of radiation with
increasing distance from the center of NGC 1705. This suggests that the
strongest starburst located within the central 150 pc is responsible for
the ionization of the gas out to at least 1 kpc. The gradual dilution of
the radiation with increasing distance from the center reflects the gradual and
continuous transition from the highly ionized H II regions in the proximity of
the major starburst into the diffuse ionized gas.Comment: Accepted for publication on A
Balmer Line Variations in the Radio-Loud AGN PG 1512+370
We present spectroscopic observations of the quasar PG~1512+370, covering the
Hbeta line spectral range and collected at moderate resolution (2-7 A FWHM)
from 1988 to 1996. The observations show that the blue wing of the Hbeta broad
profile component has changed significantly in flux and shape between 1988 and
1990 and between 1995 and 1996. A displaced blue peak on the Hbeta profile,
visible in 1988, but not in the 1990-1995 spectra, is revealed again in one of
the spectra obtained in 1996. The blue peak (in both the 1988 and 1996 spectra)
is centered at Delta v_r ~ -3000 +/- 500 km/s from the rest frame defined by
the narrow component of Hbeta, and the OIII lambda4959,5007 lines.
We discuss several conflicting interpretations of the data. We find that the
variability of the Hbeta blue wing is consistent with Balmer line emission from
regions whose motion is predominantly radial, if variations of the blue wing
are a response to continuum changes. Alternatively, we note that observed Hbeta
line profile variations are consistent with a variable line component as in a
``binary black hole'' scenario. More frequent observations of Hbeta are needed
to distinguish among these hypotheses.Comment: 19 pages, 1 embedded figure (eps), to appear in ApJ 49
Scalable Psychological Momentum Estimation in Esports
The world of competitive Esports and video gaming has seen and continues to experience steady growth in popularity and complexity. Correspondingly, more research on the topic is being published, ranging from social network analyses to the benchmarking of advanced artificial intelligence systems in playing against humans.In this paper, we present ongoing work on an intelligent agent recommendation engine that suggests actions to players in order to maximise success and enjoyment, both in the space of in-game choices, as well as decisions made around play session timing in the broader context. By leveraging temporal data and appropriate models, we show that a learned representation of player psychological momentum, and of tilt, can be used, in combination with player expertise, to achieve state-of-the-art performance in pre and post-draft win prediction. Our progress toward fulfilling the potential for deriving optimal recommendations is documented
Two years of monitoring Supergiant Fast X-ray Transients with Swift
We present two years of intense Swift monitoring of three SFXTs, IGR
J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007).
Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed
power laws with by hard photon indices (G~1-2). Their outburst broad-band
(0.3-150 keV) spectra can be fit well with models typically used to describe
the X-ray emission from accreting NSs in HMXBs. We assess how long each source
spends in each state using a systematic monitoring with a sensitive instrument.
These sources spend 3-5% of the total in bright outbursts. The most probable
flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to
luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of
magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19,
39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively.
We present a complete list of BAT on-board detections further confirming the
continued activity of these sources. This demonstrates that true quiescence is
a rare state, and that these transients accrete matter throughout their life at
different rates. X-ray variability is observed at all timescales and
intensities we can probe. Superimposed on the day-to-day variability is
intra-day flaring which involves variations up to one order of magnitude that
can occur down to timescales as short as ~1ks, and whichcan be explained by the
accretion of single clumps composing the donor wind with masses
M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table
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