117 research outputs found
Improved spectral descriptions of planetary nebulae central stars
Context. At least 492 central stars of Galactic planetary nebulae (CSPNs) have been assigned spectral types. Since many CSPNs are faint, these classification efforts are frequently made at low spectral resolution. However, the stellar Balmer absorption lines are contaminated with nebular emission; therefore in many cases a low-resolution spectrum does not enable the determination of the H abundance in the CSPN photosphere. Whether or not the photosphere is H deficient is arguably the most important fact we should expect to extract from the CSPN spectrum, and should be the basis for an adequate spectral classification system.
Aims. Our purpose is to provide accurate spectral classifications and contribute to the knowledge of central stars of planetary nebulae and stellar evolution.
Methods. We have obtained and studied higher quality spectra of CSPNs described in the literature as weak emission-line star (WELS). We provide descriptions of 19 CSPN spectra. These stars had been previously classified at low spectral resolution. We used medium-resolution spectra taken with the Gemini Multi-Object Spectrograph (GMOS). We provide spectral types in the Morgan-Keenan (MK) system whenever possible.
Results. Twelve stars in our sample appear to have normal H rich photospheric abundances, and five stars remain unclassified. The rest (two) are most probably H deficient. Of all central stars described by other authors as WELS, we find that at least 26% of them are, in fact, H rich O stars, and at least 3% are H deficient. This supports the suggestion that the denomination WELS should not be taken as a spectral type, because, as a WELS is based on low-resolution spectra, it cannot provide enough information about the photospheric H abundance.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat
Dark matter content and internal dynamics of NGC 4697: nmagic particle models from slit data and planetary nebula velocities
We present a dynamical study of NGC 4697, an almost edge-on, intermediate-luminosity, E4 elliptical galaxy, combining new surface brightness photometry, new as well as published long-slit absorption-line kinematic data, and published planetary nebula (PN) velocity data. The combined kinematic data set extends out to ≃5 arcmin (≃4.5Re) and allows us to probe the galaxy's outer halo. For the first time, we model such a data set with the new and flexible χ2-made-to-measure particle code nmagic. We extend nmagic to include seeing effects, introduce an efficient scheme to estimate the mass-to-light ratio, and incorporate a maximum-likelihood technique to account for discrete velocity measurements. For modelling the PN kinematics, we use line-of-sight velocities and velocity dispersions computed on two different spatial grids, and we also use the individual velocity measurements with the likelihood method, in order to make sure that our results are not biased by the way we treat the PN measurements. We generate axisymmetric self-consistent models as well models including various dark matter haloes. These models fit all the mean velocity and velocity dispersion data with χ2/N < 1, both in the case with only luminous matter and in potentials including quite massive haloes. The likelihood analysis together with the velocity histograms suggest that models with low-density haloes such that the circular velocity vc≲ 200 km s−1 at 5Re are not consistent with the data. A range of massive haloes with vc≃ 250 km s−1 at 5Re fit the PN data best. To derive stronger results would require PN velocities at even larger radii. The best-fitting models are slightly radially anisotropic; the anisotropy parameter β≃ 0.3 at the centre, increasing to β≃ 0.5 at radii ≳2R
A kinematic study of planetary nebulae in the dwarf irregular galaxy IC10
We present positions, kinematics, and the planetary nebula luminosity
function (PNLF) for 35 planetary nebulae (PNe) in the nearest starburst galaxy
IC10 extending out to 3kpc from the galaxy's centre. We take advantage of the
deep imaging and spectroscopic capabilities provided by the spectrograph FOCAS
on the 8.2m Subaru telescope. The PN velocities were measured through the
slitless-spectroscopy technique, which allows us to explore the kinematics of
IC10 with high precision. Using these velocities, we conclude that there is a
kinematic connection between the HI envelope located around IC10 and the
galaxy's PN population. By assuming that the PNe in the central regions and in
the outskirts have similar ages, our results put strong observational
constraints on the past tidal interactions in the Local Group. This is so
because by dating the PN central stars, we, therefore, infer the epoch of a
major episode of star formation likely linked to the first encounter of the HI
extended envelope with the galaxy. Our deep [OIII] images also allow us to use
the PNLF to estimate a distance modulus of 24.1+/-0.25, which is in agreement
with recent results in the literature based on other techniques.Comment: 10 pages, 9 figures, 2 tables. Accepted for publication in MNRA
Improved spectral descriptions of planetary nebulae central stars
Context. At least 492 central stars of Galactic planetary nebulae (CSPNs) have been assigned spectral types. Since many CSPNs are faint, these classification efforts are frequently made at low spectral resolution. However, the stellar Balmer absorption lines are contaminated with nebular emission; therefore in many cases a low-resolution spectrum does not enable the determination of the H abundance in the CSPN photosphere. Whether or not the photosphere is H deficient is arguably the most important fact we should expect to extract from the CSPN spectrum, and should be the basis for an adequate spectral classification system.
Aims. Our purpose is to provide accurate spectral classifications and contribute to the knowledge of central stars of planetary nebulae and stellar evolution.
Methods. We have obtained and studied higher quality spectra of CSPNs described in the literature as weak emission-line star (WELS). We provide descriptions of 19 CSPN spectra. These stars had been previously classified at low spectral resolution. We used medium-resolution spectra taken with the Gemini Multi-Object Spectrograph (GMOS). We provide spectral types in the Morgan-Keenan (MK) system whenever possible.
Results. Twelve stars in our sample appear to have normal H rich photospheric abundances, and five stars remain unclassified. The rest (two) are most probably H deficient. Of all central stars described by other authors as WELS, we find that at least 26% of them are, in fact, H rich O stars, and at least 3% are H deficient. This supports the suggestion that the denomination WELS should not be taken as a spectral type, because, as a WELS is based on low-resolution spectra, it cannot provide enough information about the photospheric H abundance.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat
Intracluster Planetary Nebulae in Virgo: Photometric selection, spectroscopic validation and cluster depth
We have imaged an empty area of 34'x34' one and a half degree north of the
Virgo cluster core to survey for intracluster planetary nebula candidates. We
have implemented and tested a fully automatic procedure for the selection of
emission line objects in wide-field images, based on the on-off technique from
Ciardullo and Jacoby. Freeman et al. have spectroscopically confirmed a sample
of intracluster planetary nebulae in one Virgo field. We use the photometric
and morphological properties of this sample to test our selection procedure. In
our newly surveyed Virgo field, 75 objects were identified as best candidates
for intracluster PNe.
The luminosity function of the spectroscopically confirmed PNe shows a
brighter cut-off than the planetary nebula luminosity function for the inner
regions of M87. Such a brighter cut-off is also observed in the newly surveyed
field and indicates a smaller distance modulus, implying that the front end of
the Virgo cluster is closer to us by a significant amount: 14% closer (2.1 Mpc)
than M87 for the spectroscopic field, using the PN luminosity function distance
of 14.9 Mpc to M87, and 19% closer (2.8 Mpc) than M87 for the newly surveyed
field. Independent distance indicators (Tully-Fisher relation for Virgo spirals
and surface brightness fluctuations for Virgo ellipticals) agree with these
findings.
From these two Virgo cluster fields there is no evidence that the surface
luminosity density for the diffuse stellar component in the cluster decreases
with radius. The luminosity surface density of the diffuse stellar population
is comparable to that of the galaxies.Comment: 30 pages, 8 figures, accepted for pubblication in A
Exact inhomogeneous cosmologies whose source is a radiation-matter mixture with consistent thermodynamics
We derive a new class of exact solutions of Einstein's equations providing a
physically plausible hydrodynamical description of cosmological matter in the
radiative era (), between nucleosynthesis and decoupling.
The solutions are characterized by the Lema\^{\i}tre-Tolman -Bondi metric with
a viscous fluid source, subjected to the following conditions: (a) the
equilibrium state variables satisfy the equation of state of a mixture of an
ultra-relativistic and a non-relativistic ideal gases, where the internal
energy of the latter has been neglected, (b) the particle numbers of the
mixture components are independently conserved, (c) the viscous stress is
consistent with the transport equation and entropy balance law of Extended
Irreversible Thermodynamics, with the coefficient of shear viscosity provided
by Kinetic Theory for the `radiative gas' model. The fulfilment of (a), (b) and
(c) restricts initial conditions in terms of an initial value function,
, related to the average of spatial gradients of the
fluctuations of photon entropy per baryon in the initial hypersurface.
Constraints on the observed anisotropy of the microwave cosmic radiation and
the condition that decoupling occurs at K yield
an estimated value: which can be associated
with a bound on promordial entropy fluctuations. The Jeans mass at decoupling
is of the same order of magnitude as that of baryon dominated perturbation
models ()Comment: LaTeX with revtex (PRD macros). Contains 9 figures (ps). To be
published in Physics Review
BVRI Light Curves for 29 Type Ia Supernovae
BVRI light curves are presented for 27 Type Ia supernovae discovered during
the course of the Calan/Tololo Survey and for two other SNe Ia observed during
the same period. Estimates of the maximum light magnitudes in the B, V, and I
bands and the initial decline rate parameter m15(B) are also given.Comment: 17 pages, figures and tables are not included (contact first author
if needed), to appear in the Astronomical Journa
The Mice at play in the CALIFA survey: A case study of a gas-rich major merger between first passage and coalescence
We present optical integral field spectroscopy (IFS) observations of the
Mice, a major merger between two massive (>10^11Msol) gas-rich spirals NGC4676A
and B, observed between first passage and final coalescence. The spectra
provide stellar and gas kinematics, ionised gas properties and stellar
population diagnostics, over the full optical extent of both galaxies. The Mice
provide a perfect case study highlighting the importance of IFS data for
improving our understanding of local galaxies. The impact of first passage on
the kinematics of the stars and gas has been significant, with strong bars
likely induced in both galaxies. The barred spiral NGC4676B exhibits a strong
twist in both its stellar and ionised gas disk. On the other hand, the impact
of the merger on the stellar populations has been minimal thus far: star
formation induced by the recent close passage has not contributed significantly
to the global star formation rate or stellar mass of the galaxies. Both
galaxies show bicones of high ionisation gas extending along their minor axes.
In NGC4676A the high gas velocity dispersion and Seyfert-like line ratios at
large scaleheight indicate a powerful outflow. Fast shocks extend to ~6.6kpc
above the disk plane. The measured ram pressure and mass outflow rate
(~8-20Msol/yr) are similar to superwinds from local ULIRGs, although NGC4676A
has only a moderate infrared luminosity of 3x10^10Lsol. Energy beyond that
provided by the mechanical energy of the starburst appears to be required to
drive the outflow. We compare the observations to mock kinematic and stellar
population maps from a merger simulation. The models show little enhancement in
star formation during and following first passage, in agreement with the
observations. We highlight areas where IFS data could help further constrain
the models.Comment: 23 pages, 13 figures, accepted to A&A. A version with a complete set
of high resolution figures is available here:
http://www-star.st-and.ac.uk/~vw8/resources/mice_v8_astroph.pd
Kinematic Signatures of Bulges Correlate with Bulge Morphologies and S\'ersic Index
We use the Marcario Low Resolution Spectrograph (LRS) at the
Hobby-Eberly-Telescope (HET) to study the kinematics of pseudobulges and
classical bulges in the nearby universe. We present major-axis rotational
velocities, velocity dispersions, and h3 and h4 moments derived from
high-resolution (sigma ~ 39 km/s) spectra for 45 S0 to Sc galaxies; for 27 of
the galaxies we also present minor axis data. We combine our kinematics with
bulge-to-disk decompositions. We demonstrate for the first time that purely
kinematic diagnostics of the bulge dichotomy agree systematically with those
based on S\'ersic index. Low S\'ersic index bulges have both increased
rotational support (higher v/sigma values) and on average lower central
velocity dispersions. Furthermore, we confirm that the same correlation also
holds when visual morphologies are used to diagnose bulge type. The previously
noted trend of photometrically flattened bulges to have shallower velocity
dispersion profiles turns to be significant and systematic if the S\'ersic
index is used to distinguish between pseudobulges and classical bulges. The
correlation between h3 and v/sigma observed in elliptical galaxies is also
observed in intermediate type galaxies, irrespective of bulge type. Finally, we
present evidence for formerly undetected counter rotation in the two systems
NGC 3945 and NGC 4736.
Based on observations obtained with the Hobby-Eberly Telescope, which is a
joint project of the University of Texas at Austin, the Pennsylvania State
University, Stanford University, Ludwig-Maximilians-Universit\"at M\"unchen,
and Georg-August-Universit\"at G\"ottingen.Comment: 49 pages, 16 figures. Accepted for publication in Ap
An RNAi in silico approach to find an optimal shRNA cocktail against HIV-1
<p>Abstract</p> <p>Background</p> <p>HIV-1 can be inhibited by RNA interference <it>in vitro </it>through the expression of short hairpin RNAs (shRNAs) that target conserved genome sequences. <it>In silico </it>shRNA design for HIV has lacked a detailed study of virus variability constituting a possible breaking point in a clinical setting. We designed shRNAs against HIV-1 considering the variability observed in naïve and drug-resistant isolates available at public databases.</p> <p>Methods</p> <p>A Bioperl-based algorithm was developed to automatically scan multiple sequence alignments of HIV, while evaluating the possibility of identifying dominant and subdominant viral variants that could be used as efficient silencing molecules. Student t-test and Bonferroni Dunn correction test were used to assess statistical significance of our findings.</p> <p>Results</p> <p>Our <it>in silico </it>approach identified the most common viral variants within highly conserved genome regions, with a calculated free energy of ≥ -6.6 kcal/mol. This is crucial for strand loading to RISC complex and for a predicted silencing efficiency score, which could be used in combination for achieving over 90% silencing. Resistant and naïve isolate variability revealed that the most frequent shRNA per region targets a maximum of 85% of viral sequences. Adding more divergent sequences maintained this percentage. Specific sequence features that have been found to be related with higher silencing efficiency were hardly accomplished in conserved regions, even when lower entropy values correlated with better scores. We identified a conserved region among most HIV-1 genomes, which meets as many sequence features for efficient silencing.</p> <p>Conclusions</p> <p>HIV-1 variability is an obstacle to achieving absolute silencing using shRNAs designed against a consensus sequence, mainly because there are many functional viral variants. Our shRNA cocktail could be truly effective at silencing dominant and subdominant naïve viral variants. Additionally, resistant isolates might be targeted under specific antiretroviral selective pressure, but in both cases these should be tested exhaustively prior to clinical use.</p
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