7,201 research outputs found
The Outermost Ejecta of Type Ia Supernovae
The properties of the highest velocity ejecta of normal Type Ia supernovae
(SNe Ia) are studied via models of very early optical spectra of 6 SNe. At
epochs earlier than 1 week before maximum, SNe with a rapidly evolving Si II
6355 line velocity (HVG) have a larger photospheric velocity than SNe with a
slowly evolving Si II 6355 line velocity (LVG). Since the two groups have
comparable luminosities, the temperature at the photosphere is higher in LVG
SNe. This explains the different overall spectral appearance of HVG and LVG
SNe. However, the variation of the Ca II and Si II absorptions at the highest
velocities (v >~ 20,000 km/s) suggests that additional factors, such as
asphericity or different abundances in the progenitor white dwarf, affect the
outermost layers. The C II 6578 line is marginally detected in 3 LVG SNe,
suggesting that LVG undergo less intense burning. The carbon mass fraction is
small, only less than 0.01 near the photosphere, so that he mass of unburned C
is only <~ 0.01 Msun. Radioactive 56Ni and stable Fe are detected in both LVG
and HVG SNe. Different Fe-group abundances in the outer layers may be one of
the reasons for spectral diversity among SNe Ia at the earliest times. The
diversity among SNe Ia at the earliest phases could also indicate an intrinsic
dispersion in the width-luminosity relation of the light curve.Comment: 13 pages, 10 figures, Accepted for publication in The Astrophysical
Journa
High-J CO SLEDs in nearby infrared bright galaxies observed by Herschel-PACS
We report the detection of far-infrared (FIR) CO rotational emission from
nearby active galactic nuclei (AGN) and starburst galaxies, as well as several
merging systems and Ultra-Luminous Infrared Galaxies (ULIRGs). Using
Herschel-PACS, we have detected transitions in the J = 14 - 20 range
( 130 - 185 m, 1612 - 2300 GHz) with upper limits
on (and in two cases, detections of) CO line fluxes up to J = 30. The
PACS CO data obtained here provide the first well-sampled FIR extragalactic CO
SLEDs for this range, and will be an essential reference for future high
redshift studies. We find a large range in the overall SLED shape, even amongst
galaxies of similar type, demonstrating the uncertainties in relying solely on
high-J CO diagnostics to characterize the excitation source of a galaxy.
Combining our data with low-J line intensities taken from the literature, we
present a CO ratio-ratio diagram and discuss its potential diagnostic value in
distinguishing excitation sources and physical properties of the molecular gas.
The position of a galaxy on such a diagram is less a signature of its
excitation mechanism, than an indicator of the presence (or absence) of warm,
dense molecular gas. We then quantitatively analyze the CO emission from a
subset of the detected sources with Large Velocity Gradient (LVG) radiative
transfer models to fit the CO SLEDs. Using both single-component and
two-component LVG models to fit the kinetic temperature, velocity gradient,
number density and column density of the gas, we derive the molecular gas mass
and the corresponding CO-to-H conversion factor, , for each
respective source. For the ULIRGs we find values in the canonical
range 0.4 - 5 M/(K kmspc), while for the other objects,
varies between 0.2 and 14.} Finally, we compare our best-fit LVG model
..Comment: 39 pages, 3 figures; Accepted to Ap
The diversity of Type Ia Supernovae: evidence for systematics?
The photometric and spectroscopic properties of 26 well observed Type Ia
Supernovae (SNeIa) were analyzed with the aim to explore SNIa diversity. The
sample includes (Branch-)normal SNe as well as extreme events like SNe 1991T
and 1991bg, while the truly peculiar SNIa, SN2000cx and SN2002cx are not
included in our sample . A statistical treatment reveals the existence of three
different groups. The first group (FAINT) consists of faint SNeIa similar to
SN1991bg, with low expansion velocities and rapid evolution of SiII velocity. A
second group consists of ``normal'' SNeIa, also with high temporal velocity
gradient (HVG), but with brighter mean absolute magnitude =-19.3 and
higher expansion velocities than the FAINT SNe. The third group includes both
``normal'' and SN1991T-like SNeIa: these SNe populate a narrow strip in the
SiII velocity evolution plot, with a small velocity gradient (SVG), but have
absolute magnitudes similar to HVGs. While the FAINT and HVG SNeIa together
seem to define a relation between RSi(II) and Dm15(B), the SVG ones either do
not conform with that relation or define a new, looser one. The RSi(II)
pre-maximum evolution of HVGs is strikingly different from that of SVGs. The
impact of this evidence on the understanding of SNIa diversity, in terms of
explosion mechanisms, degree of ejecta mixing, and ejecta-CSM interaction, is
discussed.Comment: 9 pages, 3 figures, accepted for publication to ApJ; few referee's
comments adde
The molecular gas in Luminous Infrared Galaxies II: extreme physical conditions, and their effects on the X_{co} factor
In this work we conclude the analysis of our CO line survey of Luminous
Infrared Galaxies (LIRGs: L_{IR}>=10^{11}L_{sol}) in the local Universe
(Paper\,I), by focusing on the influence of their average ISM properties on the
total molecular gas mass estimates via the so-called X_{co}=M(H_2)/L_{co,1-0}
factor. One-phase radiative transfer models of the global CO Spectral Line
Energy Distributions (SLEDs) yield an X_{co} distribution with:
\sim(0.6+/-0.2) M_{sol}(K km s^{-1} pc^2)^{-1} over a significant range
of average gas densities, temperatures and dynamical states. The latter emerges
as the most important parameter in determining X_{co}, with unbound states
yielding low values and self-gravitating states the highest ones. Nevertheless
in many (U)LIRGs where available higher-J CO lines (J=3--2, 4--3, and/or
J=6--5) or HCN line data from the literature allow a separate assessment of the
gas mass at high densities (>=10^{4} cm^{-3}) rather than a simple one-phase
analysis we find that {\it near-Galactic X_{co} (3-6)\,
M_sol\,(K\,km^{-1}\,pc^2)^{-1} values become possible.} We further show that in
the highly turbulent molecular gas in ULIRGs a high-density component will be
common and can be massive enough for its high X_{co} to dominate the average
value for the entire galaxy. .........
...this may have thus resulted to systematic underestimates of molecular gas
mass in ULIRGs.Comment: 77 pages, 6 figures, one Table, accepted for publication at The
Astrophysical Journa
Radiative and mechanical feedback into the molecular gas of NGC 253
Starburst galaxies are undergoing intense periods of star formation.
Understanding the heating and cooling mechanisms in these galaxies can give us
insight to the driving mechanisms that fuel the starburst. Molecular emission
lines play a crucial role in the cooling of the excited gas. With SPIRE on the
Herschel Space Observatory we have observed the rich molecular spectrum towards
the central region of NGC 253. CO transitions from J=4-3 to 13-12 are observed
and together with low-J line fluxes from ground based observations, these lines
trace the excitation of CO. By studying the CO excitation ladder and comparing
the intensities to models, we investigate whether the gas is excited by UV
radiation, X-rays, cosmic rays, or turbulent heating. Comparing the CO
and CO observations to large velocity gradient models and PDR models we
find three main ISM phases. We estimate the density, temperature,and masses of
these ISM phases. By adding CO, HCN, and HNC line intensities, we are
able to constrain these degeneracies and determine the heating sources. The
first ISM phase responsible for the low-J CO lines is excited by PDRs, but the
second and third phases, responsible for the mid to high-J CO transitions,
require an additional heating source. We find three possible combinations of
models that can reproduce our observed molecular emission. Although we cannot
determine which of these are preferable, we can conclude that mechanical
heating is necessary to reproduce the observed molecular emission and cosmic
ray heating is a negligible heating source. We then estimate the mass of each
ISM phase; M for phase 1 (low-J CO lines), M for phase 2 (mid-J CO lines), and M for
phase 3 (high-J CO lines) for a total system mass of M
Warm gas in the rotating disk of the Red Rectangle: accurate models of molecular line emission
We aim to study the excitation conditions of the molecular gas in the
rotating disk of the Red Rectangle, the only post-Asymptotic-Giant-Branch
object in which the existence of an equatorial rotating disk has been
demonstrated. For this purpose, we developed a complex numerical code that
accurately treats radiative transfer in 2-D, adapted to the study of molecular
lines from rotating disks.
We present far-infrared Herschel/HIFI observations of the 12CO and 13CO
J=6-5, J=10-9, and J=16-15 transitions in the Red Rectangle. We also present
our code in detail and discuss the accuracy of its predictions, from comparison
with well-tested codes. Theoretical line profiles are compared with the
empirical data to deduce the physical conditions in the disk by means of model
fitting.
We conclude that our code is very efficient and produces reliable results.
The comparison of the theoretical predictions with our observations reveals
that the temperature of the Red Rectangle disk is typically ~ 100-150 K, about
twice as high as previously deduced from mm-wave observations of lower-J lines.
We discuss the relevance of these new temperature estimates for understanding
the thermodynamics and dynamics of this prototype object, as well as for
interpreting observations of other rarely studied post-AGB disks. Despite our
sophisticated treatment of the line formation, our model cannot explain the
relatively strong line-wing emission for intermediate-J transitions. We argue
that a model including a rotating disk only cannot reproduce these data and
suggest that there is an additional extended (probably bipolar) structure
expanding at about 7--15 km/s.Comment: 18 pages, 21 figure
Gas and dust in a z=2.8 obscured quasar
We present new detections of the CO(5-4), CO(7-6), [CI](1-0) and [CI](2-1)
molecular and atomic line transitions towards the unlensed, obscured quasar
AMS12 (z=2.7672), observed with the IRAM PdBI. This is the first unlensed, high
redshift source to have both [CI] transitions detected. Continuum measurements
between 70 m and 3 mm are used to constrain the FIR SED, and we find a
best fit FIR luminosity of log[Lfir/Lsol] = 13.5+/-0.1, dust temperature T_d =
88+/-8 K and emissivity index {\beta} = 0.6+/-0.1. The highly-excited molecular
gas probed by CO(3-2), (5-4) and (7-6), is modelled with large velocity
gradient (LVG) models. The gas kinetic temperature T_g, density n(H2), and the
characteristic size r0, are determined using the dust temperature from the FIR
SED as a prior for the gas temperature. The best fitting parameters are T_g =
90+/-8 K, n(H2) = 10^(3.9+/-0.1) cm^(-3) and r0 = 0.8+/-0.04 kpc. The ratio of
the [CI] lines gives a [CI] excitation temperature of 43+/-10 K, indicating the
[CI] and the high-excitation CO are not in thermal equilibrium. The [CI]
excitation temperature is below that of T_d and T_g of the high-excitation CO,
perhaps because [CI] lies at a larger radius where there may also be a large
reservoir of CO at a cooler temperature, perhaps detectable through the
CO(1-0). Using the [CI](1-0) line we can estimate the strength of the CO(1-0)
line and hence the gas mass. This suggests that a significant fraction (~30%)
of the molecular gas is missed from the high-excitation line analysis. The
Eddington limited black hole mass is found from the bolometric luminosity to be
Mbh >~ 1.5x10^9 Msol. Along with the stellar mass of 3x10^11 Msol, these give a
black hole - bulge mass ratio of Mbh/Mbulge >~ 0.005. This is in agreement with
studies on the evolution of the Mbh/Mbulge relationship at high redshifts,
which find a departure from the local value ~0.002.Comment: Accepted by MNRAS, 17 pages, 9 figure
A spectroscopic survey of Orion KL between 41.5 and 50 GHz
Orion KL is one of the most frequently observed sources in the Galaxy, and
the site where many molecular species have been discovered for the first time.
With the availability of powerful wideband backends, it is nowadays possible to
complete spectral surveys in the entire mm-range to obtain a spectroscopically
unbiased chemical picture of the region. In this paper we present a sensitive
spectral survey of Orion KL, made with one of the 34m antennas of the Madrid
Deep Space Communications Complex in Robledo de Chavela, Spain. The spectral
range surveyed is from 41.5 to 50 GHz, with a frequency spacing of 180 kHz
(equivalent to about 1.2 km/s, depending on the exact frequency). The rms
achieved ranges from 8 to 12 mK. The spectrum is dominated by the J=1-0 SiO
maser lines and by radio recombination lines (RRLs), which were detected up to
Delta_n=11. Above a 3-sigma level, we identified 66 RRLs and 161 molecular
lines corresponding to 39 isotopologues from 20 molecules; a total of 18 lines
remain unidentified, two of them above a 5-sigma level. Results of radiative
modelling of the detected molecular lines (excluding masers) are presented. At
this frequency range, this is the most sensitive survey and also the one with
the widest band. Although some complex molecules like CH_3CH_2CN and CH_2CHCN
arise from the hot core, most of the detected molecules originate from the low
temperature components in Orion KL.Comment: Accepted for Astronomy and Astrophysics. 29 pages, 5 tables, 6
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