1,141 research outputs found
Constraining the mass of the GRB 030329 progenitor
The long-duration gamma-ray burst (GRB) 030329, associated with supernova
(SN) 2003dh, occurred inside a star-forming dwarf galaxy at redshift
. The low redshift, and a rich set of archival Hubble Space Telescope
(HST) images, makes this GRB well-suited for a detailed study of the stellar
population in the immediate vicinity of the explosion. Since the lifetime of a
star is directly tied to its mass, the age of the stellar population can be
used to put constraints on the GRB and SN progenitor mass. From the HST images
we extract the colours of the precise site from which the GRB originated, and
find that the colours are significantly different from those of the overall
host galaxy and the surrounding starburst environment. We have used spectral
evolutionary models, including nebular emission, to carefully constrain the age
of the stellar population, and hence the progenitor, at the very explosion
site. For instantaneous burst models we find that a population age of 5 Myr
best matches the data, suggesting a very massive (M > 50 M_sun) star as the
progenitor, with an upper limit of 8 Myr (M > 25 M_sun). For more extended star
formation scenarios, the inferred progenitor age is in most cases still very
young (age 25 M_sun), with an upper limit of 20 Myr (M >
12 M_sun). These age estimates are an order of magnitude lower than the ages
inferred from the overall host galaxy colours, indicating that progenitor mass
estimates based on data for spatially unresolved GRB host galaxies will in
general be of limited use. Our results are consistent with the collapsar
scenario.Comment: Accepted to MNRA
The Abundance Scatter in M33 from HII Regions: Is There Any Evidence for Azimuthal Metallicity Variations?
Optical spectra of 25 H II regions in the inner two kpc of the M33 disk have
been obtained with the GMOS spectrograph at the Gemini North telescope. The
oxygen abundance gradient measured from the detection of the [O III]4363
auroral line displays a scatter of approximately 0.06 dex, a much smaller value
than recently reported by Rosolowsky & Simon in this galaxy. The analysis of
the abundances for a large sample of H II regions derived from the R23
strong-line indicator confirms that the scatter is small over the full disk of
M33, consistent with the measuring uncertainties, and comparable to what is
observed in other spiral galaxies. No evidence is therefore found for
significant azimuthal variations in the present-day metallicity of the
interstellar medium in this galaxy on spatial scales from ~100 pc to a few kpc.
A considerable fraction of M33 H II regions with auroral line detections show
spectral features revealing sources of hard ionizing radiation (such as He II
emission and large [Ne III], [O III] line fluxes). Since R23 is shown to
severely underestimate the oxygen abundances in such cases, care must be taken
in chemical abundance studies of extragalactic H II regions based on this
strong-line indicator.Comment: 9 pages, accepted for publication in the Astrophysical Journa
Artificial Intelligence as a Means to Moral Enhancement
This paper critically assesses the possibility of moral enhancement with ambient intelligence technologies and artificial intelligence presented in Savulescu and Maslen (2015). The main problem with their proposal is that it is not robust enough to play a normative role in users’ behavior. A more promising approach, and the one presented in the paper, relies on an artifi-cial moral reasoning engine, which is designed to present its users with moral arguments grounded in first-order normative theories, such as Kantianism or utilitarianism, that reason-responsive people can be persuaded by. This proposal can play a normative role and it is also a more promising avenue towards moral enhancement. It is more promising because such a system can be designed to take advantage of the sometimes undue trust that people put in automated technologies. We could therefore expect a well-designed moral reasoner system to be able to persuade people that may not be persuaded by similar arguments from other people. So, all things considered, there is hope in artificial intelli-gence for moral enhancement, but not in artificial intelligence that relies solely on ambient intelligence technologies
SN 1999ga: a low-luminosity linear type II supernova?
Type II-linear supernovae are thought to arise from progenitors that have
lost most of their H envelope by the time of the explosion, and they are poorly
understood because they are only occasionally discovered. It is possible that
they are intrinsically rare, but selection effects due to their rapid
luminosity evolution may also play an important role in limiting the number of
detections. In this context, the discovery of a subluminous type II-linear
event is even more interesting. We investigate the physical properties and
characterise the explosion site of the type II SN 1999ga, which exploded in the
nearby spiral galaxy NGC 2442. Spectroscopic and photometric observations of SN
1999ga allow us to constrain the energetics of the explosion and to estimate
the mass of the ejected material, shedding light on the nature of the
progenitor star in the final stages of its life. The study of the environment
in the vicinity of the explosion site provides information on a possible
relation between these unusual supernovae and the properties of the galaxies
hosting them. Despite the lack of early-time observations, we provide
reasonable evidence that SN 1999ga was probably a type II-linear supernova that
ejected a few solar masses of material, with a very small amount of radioactive
elements of the order of 0.01 solar masses.Comment: 11 pages, 9 figures. Accepted for publication in A&A (March 28, 2009
Tracking magnetic bright point motions through the solar atmosphere
High-cadence, multiwavelength observations and simulations are employed for the analysis of solar photospheric magnetic bright points (MBPs) in the quiet Sun. The observations were obtained with the Rapid Oscillations in the Solar Atmosphere (ROSA) imager and the Interferometric Bidimensional Spectrometer at the Dunn Solar Telescope. Our analysis reveals that photospheric MBPs have an average transverse velocity of approximately 1 km s−1, whereas their chromospheric counterparts have a slightly higher average velocity of 1.4 km s−1. Additionally, chromospheric MBPs were found to be around 63 per cent larger than the equivalent photospheric MBPs. These velocity values were compared with the output of numerical simulations generated using the MURAM code. The simulated results were similar, but slightly elevated, when compared to the observed data. An average velocity of 1.3 km s−1 was found in the simulated G-band images and an average of 1.8 km s−1 seen in the velocity domain at a height of 500 km above the continuum formation layer. Delays in the change of velocities were also analysed. Average delays of ∼4 s between layers of the simulated data set were established and values of ∼29 s observed between G-band and Ca II K ROSA observations. The delays in the simulations are likely to be the result of oblique granular shock waves, whereas those found in the observations are possibly the result of a semi-rigid flux tube
Automated Detection and Tracking of Solar Magnetic Bright Points
Magnetic Bright Points (MBPs) in the internetwork are among the smallest
objects in the solar photosphere and appear bright against the ambient
environment. An algorithm is presented that can be used for the automated
detection of the MBPs in the spatial and temporal domains. The algorithm works
by mapping the lanes through intensity thresholding. A compass search, combined
with a study of the intensity gradient across the detected objects, allows the
disentanglement of MBPs from bright pixels within the granules. Object growing
is implemented to account for any pixels that might have been removed when
mapping the lanes. The images are stabilized by locating long-lived objects
that may have been missed due to variable light levels and seeing quality.
Tests of the algorithm employing data taken with the Swedish Solar Telescope
(SST), reveal that ~90% of MBPs within a 75"x 75" field of view are detected
The Wolf-Rayet Content of M33
Wolf-Rayet stars (WRs) are evolved massive stars, and the relative number of
WC-type and WN-type WRs should vary with metallicity, providing a sensitive
test of stellar evolutionary theory. The observed WC/WN ratio is much higher
than that predicted by theory in some galaxies but this could be due to
observational incompleteness for WN-types, which have weaker lines. Previous
studies of M33's WR content show a galactocentric gradient in the relative
numbers of WCs and WNs, but only small regions have been surveyed with
sufficient sensitivity to detect all of the WNs. Here we present a sensitive
survey for WRs covering all of M33, finding 55 new WRs, mostly of WN type. Our
spectroscopy also improves the spectral types of many previously known WRs,
establishing in one case that the star is actually a background quasar. The
total number of spectroscopically confirmed WRs in M33 is 206, a number we
argue is complete to approximately 5%, with most WRs residing in OB
associations, although approximately 2% are truly isolated. The WC/WN ratio in
the central regions (<2 kpc) of M33 is much higher than that predicted by the
current Geneva evolutionary models, while the WC/WN ratios in the outer regions
are in good accord, as are the values in the SMC and LMC. The WC/WN ratio and
the WC subtype distribution both argue that the oxygen abundance gradient in
M33 is significantly larger than found by some recent studies, but are
consistent with the two-component model proposed by Magrini et al.Comment: ApJ, in pres
Soft X-ray emission from the inner disk of M33
We present a study, based on archival XMM-Newton observations, of the
extended X-ray emission associated with the inner disk of M33. After the
exclusion of point sources with L_X > 2 x 10^{35} erg/s (0.3-6 keV), we
investigate the morphology and spectrum of the residual X-ray emission. This
residual emission has a soft X-ray spectrum which can be fitted with a
two-temperature thermal model, with kT = 0.2 keV and 0.6 keV. The soft X-ray
surface brightness distribution shows a strong correlation with FUV emission,
indicative of a close connection between recent star-formation activity and the
production of soft X-rays. Within 3.5 kpc of the nucleus of M33, the soft X-ray
and FUV surface brightness distributions exhibit similar radial profiles. This
implies that the ratio of the soft X-ray luminosity (0.3-2.0 keV) to the star
formation rate (SFR) per unit disk area remains fairly constant within this
inner disk region. We derive a value for this ratio of 1-1.5 x 10^{39}
(erg/s)/(M_sun/yr), consistent with previous studies. In the same region, the
ratio of soft X-ray luminosity to stellar mass (derived from K-band photometry)
is 4 x 10^{28} erg/s/M_sun, a factor of 5-10 higher than is typical of dwarf
elliptical galaxies, suggesting that 10-20% of the unresolved emission seen in
M33 may originate in its old stellar population. The remainder of the soft
X-ray emission is equally split between two spatial components, one which
closely traces the spiral arms of the galaxy and the other more smoothly
distributed across the inner disk of M33. The former must represent a highly
clumped low-filling factor component linked to sites of recent or ongoing star
formation, whereas the distribution of the latter gives few clues as to its
exact origin.Comment: 13 pages, 4 figures, 5 tables. Accepted for publication in MNRA
Keck HIRES Spectroscopy of Extragalactic HII Regions: C and O Abundances from Recombination Lines
We present very deep spectrophotometry of 14 bright extragalactic HII regions
belonging to spiral, irregular, and blue compact galaxies. The data for 13
objects were taken with the HIRES echelle spectrograph on the Keck I telescope.
We have measured CII recombination lines in 10 of the objects and OII
recombination lines in 8 of them. We have determined electron temperatures from
line ratios of several ions, specially of low ionization potential ones. We
have found a rather tight linear empirical relation between Te([NII]) and
Te([OIII]). We have found that OII lines give always larger abundances than
[OIII] lines. Moreover, the difference of both O++ abundance determinations
--the so-called abundance discrepancy factor-- is very similar in all the
objects, with a mean value of 0.26+/-0.09 dex, independently of the properties
of the HII region and of the parent galaxy. Using the observed recombination
lines, we have determined the O, C, and C/O radial abundance gradients for 3
spiral galaxies: M33, M101, and NGC2403, finding that C abundance gradients are
always steeper than those of O, producing negative C/O gradients accross the
galactic disks. This result is similar to that found in the Milky Way and has
important implications for chemical evolution models and the nucleosynthesis of
C.Comment: Accepted for publication in ApJ, 54 pages, 4 figure
High-resolution radio continuum survey of M33 II. Thermal and nonthermal emission
We determine the variation in the nonthermal radio spectral index in the
nearby spiral galaxy M33 at a linear resolution of 360 pc. We separate the
thermal and nonthermal components of the radio continuum emission without the
assumption of a constant nonthermal spectral index. Using the Spitzer FIR data
at 70 and 160 m and a standard dust model, we deredden the H
emission. The extinction corrected H emission serves as a template for
the thermal free-free radio emission. Subtracting from the observed 3.6 cm and
20 cm emission (Effelsberg and the VLA) this free-free emission, we obtain the
nonthermal maps. A constant electron temperature used to obtain the thermal
radio intensity seems appropriate for M~33 which, unlike the Milky Way, has a
shallow metallicity gradient. For the first time, we derive the distribution of
the nonthermal spectral index across a galaxy, M33. We detect strong nonthermal
emission from the spiral arms and star-forming regions. Wavelet analysis shows
that at 3.6 cm the nonthermal emission is dominated by contributions from
star-forming regions, while it is smoothly distributed at 20 cm. For the whole
galaxy, we obtain thermal fractions of 51% and 18% at 3.6 cm and 20 cm,
respectively. The thermal emission is slightly stronger in the southern than in
the northern half of the galaxy. We find a clear radial gradient of mean
extinction in the galactic plane. The nonthermal spectral index map indicates
that the relativistic electrons suffer energy-loss when diffusing from their
origin in star-forming regions towards interarm regions and the outer parts of
the galaxy. We also conclude that the radio emission is mostly nonthermal at R
5 kpc in M33.Comment: 15 pages, 14 figures, accepted for publication in the Astronomy and
Astrophysics journa
- …