11,962 research outputs found
A Near-Infrared L Band Survey of the Young Embedded Cluster NGC 2024
We present the results of the first sensitive L band (3.4 micron) imaging
study of the nearby young embedded cluster NGC 2024. Two separate surveys of
the cluster were acquired in order to obtain a census of the circumstellar disk
fraction in the cluster. From an analysis of the JHKL colors of all sources in
our largest area, we find an infrared excess fraction of > 86%. The JHKL colors
suggest that the infrared excesses arise in circumstellar disks, indicating
that the majority of the sources which formed in the NGC 2024 cluster are
currently surrounded by, and likely formed with circumstellar disks. The excess
fractions remain very high, within the errors, even at the faintest L
magnitudes from our deeper surveys suggesting that disks form around the
majority of the stars in very young clusters such as NGC 2024 independent of
mass. From comparison with published JHKL observations of Taurus, we find the K
- L excess fraction in NGC 2024 to be consistent with a high initial incidence
of circumstellar disks in both NGC 2024 and Taurus. Because NGC 2024 represents
a region of much higher stellar density than Taurus, this suggests that disks
may form around most of the YSOs in star forming regions independent of
environment. We find a relatively constant JHKL excess fraction with increasing
cluster radius, indicating that the disk fraction is independent of location in
the cluster. In contrast, the JHK excess fraction increases rapidly toward the
central region of the cluster, and is most likely due to contamination of the K
band measurements by bright nebulosity in the central regions of the cluster.
We identify 45 candidate protostellar sources in the central regions of the NGC
2024 cluster, and find a lower limit on the protostellar phase of early stellar
evolution of 0.4 - 1.4 X 10^5 yr, similar to that in Taurus.Comment: 37 pages, 8 figures, 3 tables, To appear in the Astronomical Journa
Renormalization-Scale Invariance, Minimal Sensitivity, and the Inclusive Hadronic Decays of a 115 GeV Higgs Particle
Known perturbative expressions for the decay rates of 115 GeV Higgs particle
into either two gluons or a pair are shown to exhibit
renormalization-scale-()-dependence that is largely removed via
renormalization-group/Pade-approximant estimates of these rates' next order
contributions. The extrema in characterizing both rates, as determined
from fully-known orders of perturbation theory, are very nearly equal to
corresponding -insensitive rates obtained via estimation of their next
order contributions, consistent with "minimal-sensitivity" expectations.Comment: 12 pages, 3 figures, LaTe
Explosion of a massive, He-rich star at z=0.16
We present spectroscopic and photometric data of the peculiar SN 2001gh,
discovered by the 'Southern inTermediate Redshift ESO Supernova Search'
(STRESS) at a redshift z=0.16. SN 2001gh has relatively high luminosity at
maximum (M_B = -18.55 mag), while the light curve shows a broad peak. An
early-time spectrum shows an almost featureless, blue continuum with a few weak
and shallow P-Cygni lines that we attribute to HeI. HeI lines remain the only
spectral features visible in a subsequent spectrum, obtained one month later. A
remarkable property of SN 2001gh is the lack of significant spectral evolution
over the temporal window of nearly one month separating the two spectra. In
order to explain the properties of SN 2001gh, three powering mechanism are
explored, including radioactive decays of a moderately large amount of 56Ni,
magnetar spin-down, and interaction of SN ejecta with circumstellar medium. We
favour the latter scenario, with a SN Ib wrapped in a dense, circumstellar
shell. The fact that no models provide an excellent fit with observations,
confirms the troublesome interpretation of the nature of SN 2001gh. A rate
estimate for SN 2001gh-like event is also provided, confirming the intrinsic
rarity of these objects.Comment: 11 pages, 8 figures, 3 tables. Accepted by MNRA
High Resolution Mid-Infrared Imaging of Ultraluminous Infrared Galaxies
Observations of ultraluminous infrared galaxies (ULIRGs) with an achieved
resolution approaching the diffraction limit in the mid-infrared from 8 - 25
m using the Keck Telescopes are reported. We find extremely compact
structures, with spatial scales of (diameter) in six of the seven
ULIRGs observed. These compact sources emit between 30% and 100% of the
mid-infrared energy from these galaxies. We have utilized the compact
mid-infrared structures as a diagnostic of whether an AGN or a compact (100 --
300 pc) starburst is the primary power source in these ULIRGs. In Markarian
231, the upper limit on the diameter of the 12.5 m source, 0.13, shows
that the size of the infrared source must increase with increasing wavelength,
consistent with AGN models. In IRAS 05189-2524 and IRAS 08572+3915 there is
strong evidence that the source size increases with increasing wavelength. This
suggests heating by a central source rather than an extended luminosity source,
consistent with the optical classification as an AGN. The compact mid-infrared
sources seen in the other galaxies cannot be used to distinguish the ultimate
luminosity source. If these ULIRGs are powered by compact starbursts, the star
formation rates seen in the central few hundred parsecs far exceed the global
rates seen in nearby starburst galaxies, and approach the surface brightness of
individual clusters in nearby starburst galaxies.Comment: 33pages, 6 tables, 5 figures, Accepted for publication in A
The eruptive history and magmatic evolution of Aluto volcano: new insights into silicic peralkaline volcanism in the Ethiopian rift
The silicic peralkaline volcanoes of the East African Rift are some of the least studied volcanoes on Earth. Here we bring together new constraints from fieldwork, remote sensing, geochronology and geochemistry to present the first detailed account of the eruptive history of Aluto, a restless silicic volcano located in a densely populated section of the Main Ethiopian Rift. Prior to the growth of the Aluto volcanic complex (before 500 ka) the region was characterized by a significant period of fault development and mafic fissure eruptions. The earliest volcanism at Aluto built up a trachytic complex over 8 km in diameter. Aluto then underwent large-volume ignimbrite eruptions at 316 ± 19 ka and 306 ± 12 ka developing a ~ 42 km2 collapse structure. After a hiatus of ~ 250 ka, a phase of post-caldera volcanism initiated at 55 ± 19 ka and the most recent eruption of Aluto has a radiocarbon age of 0.40 ± 0.05 cal. ka BP. During this post-caldera phase highly-evolved peralkaline rhyolite lavas, ignimbrites and pumice fall deposits have erupted from vents across the complex. Geochemical modelling is consistent with rhyolite genesis from protracted fractionation (> 80%) of basalt that is compositionally similar to rift-related basalts found east of the complex. Based on the style and volume of recent eruptions we suggest that silicic eruptions occur at an average rate of 1 per 1000 years, and that future eruptions of Aluto will involve explosive emplacement of localised pumice cones and effusive obsidian coulees of volumes in the range 1–100 × 106 m3
Moderately Luminous type II Supernovae
Core-collapse Supernovae (CC-SNe) descend from progenitors more massive than
about 8 Msun. Because of the young age of the progenitors, the ejecta may
eventually interact with the circumstellar medium (CSM) via highly energetic
processes detectable in the radio, X-ray, ultraviolet (UV) and, sometimes, in
the optical domains. In this paper we present ultraviolet, optical and near
infrared observations of five type II SNe, namely SNe 2009dd, 2007pk, 2010aj,
1995ad, and 1996W. Together with few other SNe they form a group of moderately
luminous type II events. We collected photometry and spectroscopy with several
telescopes in order to construct well-sampled light curves and spectral
evolutions from the photospheric to the nebular phases. Both photometry and
spectroscopy indicate a degree of heterogeneity in this sample. The light
curves have luminous peak magnitudes (). The ejected
masses of ^56\ni for three SNe span a wide range of values
(MsunM(\ni)Msun), while for a fourth
(SN2010aj) we could determine a stringent upper limit (Msun).
Clues of interaction, such as the presence of high velocity (HV) features of
the Balmer lines, are visible in the photospheric spectra of SNe 2009dd and
1996W. For SN2007pk we observe a spectral transition from a type IIn to a
standard type II SN. Modelling the observations of SNe 2009dd, 2010aj and
1995ad with radiation hydrodynamics codes, we infer kinetic plus thermal
energies of about 0.2-0.5 foe, initial radii of 2-5 cm and
ejected masses of 5.0-9.5 Msun. These values suggest moderate-mass,
super-asymptotic giant branch (SAGB) or red super-giants (RSG) stars as SN
precursors, in analogy with other luminous type IIP SNe 2007od and 2009bw.Comment: 28 pages, 27 fig, accepted by A&A, 3 pages of online material,
abstract abridged. revised significantly with respect to the previous versio
Probing the Electronic Structure of Bilayer Graphene by Raman Scattering
The electronic structure of bilayer graphene is investigated from a resonant
Raman study using different laser excitation energies. The values of the
parameters of the Slonczewski-Weiss-McClure model for graphite are measured
experimentally and some of them differ significantly from those reported
previously for graphite, specially that associated with the difference of the
effective mass of electrons and holes. The splitting of the two TO phonon
branches in bilayer graphene is also obtained from the experimental data. Our
results have implications for bilayer graphene electronic devices.Comment: 4 pages, 4 figure
Observation of Distinct Electron-Phonon Couplings in Gated Bilayer Graphene
A Raman study of a back gated bilayer graphene sample is presented. The
changes in the Fermi level induced by charge transfer splits the Raman G-band,
hardening its higher component and softening the lower one. These two
components are associated with the symmetric (S) and anti-symmetric vibration
(AS) of the atoms in the two layers, the later one becoming Raman active due to
inversion symmetry breaking. The phonon hardening and softening are explained
by considering the selective coupling of the S and AS phonons with interband
and intraband electron-hole pairs.Comment: 4 pages, 4 figure
Massive stars exploding in a He-rich circumstellar medium - VIII. PSN J07285387+3349106, a highly reddened supernova Ibn
We present spectroscopic and photometric observations for the Type Ibn
supernova (SN) dubbed PSN J07285387+3349106. Using data provided by amateur
astronomers, we monitored the photometric rise of the SN to maximum light,
occurred on 2015 February 18.8 UT (JD(max,V) = 2457072.0 +- 0.8). PSN
J07285387+3349106 exploded in the inner region of an infrared luminous galaxy,
and is the most reddened SN Ibn discovered so far. We apply multiple methods to
derive the total reddening to the SN, and determine a total colour excess
E(B-V)(tot) = 0.99 +- 0.48 mag. Accounting for the reddening correction, which
is affected by a large uncertainty, we estimate a peak absolute magnitude of
M(V) = -20.30 +- 1.50. The spectra are dominated by continuum emission at early
phases, and He I lines with narrow P-Cygni profiles are detected. We also
identify weak Fe III and N II features. All these lines show an absorption
component which is blue-shifted by about 900-1000 km/s. The spectra also show
relatively broad He I line wings with low contrast, which extend to above 3000
km/s. From about 2 weeks past maximum, broad lines of O I, Mg II and the Ca II
near-infrared triplet are identified. The composition and the expansion
velocity of the circumstellar material, and the presence of He I and
alpha-elements in the SN ejecta indicate that PSN J07285387+3349106 was
produced by the core-collapse of a stripped-envelope star. We suggest that the
precursor was WNE-type Wolf-Rayet star in its dense, He-rich circumstellar
cocoon.Comment: 12 pages, 7 figures, 2 tables. Accepted for publication in MNRA
The supernova impostor PSN J09132750+7627410 and its progenitor
We report the results of our follow-up campaign of the supernova impostor PSN
J09132750+7627410, based on optical data covering . From the
beginning, the transient shows prominent narrow Balmer lines with P-Cygni
profiles, with a blue-shifted absorption component becoming more prominent with
time. Along the of the spectroscopic monitoring, broad
components are never detected in the hydrogen lines, suggesting that these
features are produced in slowly expanding material. The transient reaches an
absolute magnitude at maximum, a typical
luminosity for supernova impostors. Amateur astronomers provided
of archival observations of the host galaxy, NGC 2748. The
detection of the quiescent progenitor star in archival images obtained with the
Hubble Space Telescope suggests it to be an \msun white-yellow
supergiant.Comment: 7 pages, 4 figures, supplemental material available in the source
file. Accepted for publication on Astrophysical Journal Letter
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