2,370 research outputs found
Using machine learning to classify the diffuse interstellar bands
Using over a million and a half extragalactic spectra we study the
correlations of the Diffuse Interstellar Bands (DIBs) in the Milky Way. We
measure the correlation between DIB strength and dust extinction for 142 DIBs
using 24 stacked spectra in the reddening range E(B-V) < 0.2, many more lines
than ever studied before. Most of the DIBs do not correlate with dust
extinction. However, we find 10 weak and barely studied DIBs with correlations
that are higher than 0.7 with dust extinction and confirm the high correlation
of additional 5 strong DIBs. Furthermore, we find a pair of DIBs, 5925.9A and
5927.5A which exhibits significant negative correlation with dust extinction,
indicating that their carrier may be depleted on dust. We use Machine Learning
algorithms to divide the DIBs to spectroscopic families based on 250 stacked
spectra. By removing the dust dependency we study how DIBs follow their local
environment. We thus obtain 6 groups of weak DIBs, 4 of which are tightly
associated with C2 or CN absorption lines.Comment: minor changes, MNRAS accepte
A survey of diffuse interstellar bands in the Andromeda galaxy: optical spectroscopy of M31 OB stars
We present the largest sample to-date of intermediate-resolution blue-to-red
optical spectra of B-type supergiants in M31 and undertake the first survey of
diffuse interstellar bands (DIBs) in this galaxy. Spectral classifications,
radial velocities and interstellar reddenings are presented for 34 stars in
three regions of M31. Radial velocities and equivalent widths are given for the
5780 and 6283 DIBs towards 11 stars. Equivalent widths are also presented for
the following DIBs detected in three sightlines in M31: 4428, 5705, 5780, 5797,
6203, 6269, 6283, 6379, 6613, 6660, and 6993. All of these M31 DIB carriers
reside in clouds at radial velocities matching those of interstellar Na I
and/or H I. The relationships between DIB equivalent widths and reddening
(E(B-V)) are consistent with those observed in the local ISM of the Milky Way.
Many of the observed sightlines show DIB strengths (per unit reddening) which
lie at the upper end of the range of Galactic values. DIB strengths per unit
reddening are found (with 68% confidence), to correlate with the interstellar
UV radiation field strength. The strongest DIBs are observed where the
interstellar UV flux is lowest. The mean Spitzer 8/24 micron emission ratio in
our three fields is slightly lower than that measured in the Milky Way, but we
identify no correlation between this ratio and the DIB strengths in M31.
Interstellar oxygen abundances derived from the spectra of three M31 H II
regions in one of the fields indicate that the average metallicity of the ISM
in that region is 12 + log[O/H] = 8.54 +- 0.18, which is approximately equal to
the value in the solar neighbourhood
Spectropolarimetry of Galactic stars with anomalous extinction sightlines
Highly reddened type Ia Supernovae (SNe Ia) with low total-to-selective
visual extinction ratio values, , also show peculiar linear polarization
wavelength dependencies with peak polarizations at short wavelengths
(). It is not clear why sightlines to SNe Ia
display such different continuum polarization profiles from interstellar
sightlines in the Milky Way with similar values. We investigate
polarization profiles of a sample of Galactic stars with low values,
along anomalous extinction sightlines, with the aim to find similarities to the
polarization profiles that we observe in SN Ia sightlines. We undertook
spectropolarimetry of 14 stars, and used archival data for three additional
stars, and run dust extinction and polarization simulations to infer a simple
dust model that can reproduce the observed extinction and polarization curves.
Our sample of Galactic stars with low values and anomalous extinction
sightlines displays normal polarization profiles with an average , and is consistent within 3 to a larger coherent
sample of Galactic stars from literature. Despite the low values of dust
towards the stars in our sample, the polarization curves do not show any
similarity to the continuum polarization curves observed towards SNe Ia with
low values. There is a correlation between the best-fit Serkowski
parameters and , but we did not find any significant
correlation between and . Our simulations show that the
relationship is an intrinsic property of polarization.
Furthermore, we have shown that in order to reproduce polarization curves with
normal and low values, a population of large (a ) interstellar silicate grains must be contained in the dust's
composition.Comment: accepted for publication in A&
SpS5 - III. Matter ejection and feedback
The last part of SpS5 dealt with the circumstellar environment. Structures are indeed found around several types of massive stars, such as blue and red supergiants, as well as WRs and LBVs. As shown in the last years, the potential of IR for their study is twofold: first, IR can help discover many previously unknown nebulae, leading to the identification of new massive stars as their progenitors; second, IR can help characterize the nebular features. Current and new IR facilities thus pave the way to a better understanding of the feedback from massive star
NGC 2770 - a supernova Ib factory?
NGC 2770 has been the host of three supernovae of Type Ib during the last 10
years, SN 1999eh, SN 2007uy and SN 2008D. SN 2008D attracted special attention
due to the serendipitous discovery of an associated X-ray transient. In this
paper, we study the properties of NGC 2770 and specifically the three SN sites
to investigate whether this galaxy is in any way peculiar to cause a high
frequency of SNe Ib. We model the global SED of the galaxy from broadband data
and derive a star-formation and SN rate comparable to the values of the Milky
Way. We further study the galaxy using longslit spectroscopy covering the major
axis and the three SN sites. From the spectroscopic study we find subsolar
metallicities for the SN sites, a high extinction and a moderate star-formation
rate. In a high resolution spectrum, we also detect diffuse interstellar bands
in the line-of-sight towards SN 2008. A comparison of NGC 2770 to the global
properties of a galaxy sample with high SN occurance (at least 3 SN in the last
100 years) suggests that NGC 2770 is not particularly destined to produce such
an enhancement of observed SNe observed. Its properties are also very different
from gamma-ray burst host galaxies. Statistical considerations on SN Ib
detection rates give a probability of ~1.5% to find a galaxy with three Ib SNe
detected in 10 years. The high number of rare Ib SNe in this galaxy is
therefore likely to be a coincidence rather than special properties of the
galaxy itself. NGC 2770 has a small irregular companion, NGC 2770B, which is
highly starforming, has a very low mass and one of the lowest metallicities
detected in the nearby universe as derived from longslit spectroscopy. In the
most metal poor part, we even detect Wolf-Rayet features, against the current
models of WR stars which require high metallicities.Comment: 15 pages, 10 figures, submitted to Ap
A massive proto-cluster of galaxies at a redshift of z {\approx} 5.3
Massive clusters of galaxies have been found as early as 3.9 Billion years
(z=1.62) after the Big Bang containing stars that formed at even earlier
epochs. Cosmological simulations using the current cold dark matter paradigm
predict these systems should descend from "proto-clusters" - early
over-densities of massive galaxies that merge hierarchically to form a cluster.
These proto-cluster regions themselves are built-up hierarchically and so are
expected to contain extremely massive galaxies which can be observed as
luminous quasars and starbursts. However, observational evidence for this
scenario is sparse due to the fact that high-redshift proto-clusters are rare
and difficult to observe. Here we report a proto-cluster region 1 billion years
(z=5.3) after the Big Bang. This cluster of massive galaxies extends over >13
Mega-parsecs, contains a luminous quasar as well as a system rich in molecular
gas. These massive galaxies place a lower limit of >4x10^11 solar masses of
dark and luminous matter in this region consistent with that expected from
cosmological simulations for the earliest galaxy clusters.Comment: Accepted to Nature, 16 Pages, 6 figure
A Survey of Honey Bee Colony Losses in the U.S., Fall 2007 to Spring 2008
Honey bees are an essential component of modern agriculture. A recently recognized ailment, Colony Collapse Disorder (CCD), devastates colonies, leaving hives with a complete lack of bees, dead or alive. Up to now, estimates of honey bee population decline have not included losses occurring during the wintering period, thus underestimating actual colony mortality. Our survey quantifies the extent of colony losses in the United States over the winter of 2007–2008.Surveys were conducted to quantify and identify management factors (e.g. operation size, hive migration) that contribute to high colony losses in general and CCD symptoms in particular. Over 19% of the country's estimated 2.44 million colonies were surveyed. A total loss of 35.8% of colonies was recorded; an increase of 11.4% compared to last year. Operations that pollinated almonds lost, on average, the same number of colonies as those that did not. The 37.9% of operations that reported having at least some of their colonies die with a complete lack of bees had a total loss of 40.8% of colonies compared to the 17.1% loss reported by beekeepers without this symptom. Large operations were more likely to have this symptom suggesting that a contagious condition may be a causal factor. Sixty percent of all colonies that were reported dead in this survey died without dead bees, and thus possibly suffered from CCD. In PA, losses varied with region, indicating that ambient temperature over winter may be an important factor.Of utmost importance to understanding the recent losses and CCD is keeping track of losses over time and on a large geographic scale. Given that our surveys are representative of the losses across all beekeeping operations, between 0.75 and 1.00 million honey bee colonies are estimated to have died in the United States over the winter of 2007–2008. This article is an extensive survey of U.S. beekeepers across the continent, serving as a reference for comparison with future losses as well as providing guidance to future hypothesis-driven research on the causes of colony mortality
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