308 research outputs found
Rebirth of Novae as Distance Indicators Due to Efficient Large Telescopes
Nova outbursts are the result of strong thermonuclear runaways on the surface
of a white dwarf accreting Hydrogen-rich material from a small mass companion.
These giant explosions cause the star to increase its brightness by hundreds of
thousands of times then making these objects powerful standard candles useful
to measure the extragalactic distances.We have used the Very Large Telescope,
located in the Chilean Atacama desert, to search for novae in NGC 1316--an
early type galaxy in the Fornax cluster. We discovered 4 novae with 3h of
observing time. The use of 8-10m class telescopes coupled with new detectors,
can dramatically improve the efficiency of nova searches in extragalactic
systems.Comment: 4 pages + 2 plot
On the white dwarf cooling sequence with extremely large telescopes
We present new diagnostics of white dwarf (WD) cooling sequences and
luminosity functions (LFs) in the near-infrared (NIR) bands that will exploit
the sensitivity and resolution of future extremely large telescopes. The
collision-induced absorption (CIA) of molecular hydrogen causes a clearly
defined blue turn-off along the WD (WDBTO) cooling sequences and a bright
secondary maximum in the WD LFs. These features are independent of age over a
broad age range and are minimally affected by metal abundance. This means that
the NIR magnitudes of the WDBTO are very promising distance indicators. The
interplay between the cooling time of progressively more massive WDs and the
onset of CIA causes a red turn-off along the WD (WDRTO) cooling sequences and a
well defined faint peak in the WD LFs. These features are very sensitive to the
cluster age, and indeed the K-band magnitude of the faint peak increases by
0.2--0.25 mag/Gyr for ages between 10 and 14 Gyr. On the other hand, the faint
peak in the optical WD LF increases in the same age range by 0.17 (V band) and
0.15 (I band) mag/Gyr. Moreover, we also suggest to use the difference in
magnitude between the main sequence turn-off and either the WDBTO (optical) or
the WDRTO (NIR). This age diagnostic is also independent of distance and
reddening. Once again the sensitivity in the K band (0.15-0.20 mag/Gyr) is on
average a factor of two higher than in the optical bands (0.10 [V band], 0.07
[I band] mag/Gyr). Finally, we also outline the use of the new diagnostics to
constrain the CO phase separation upon crystallization.Comment: 6 pages, 5 figures, accepted on A&
Using globular clusters to test gravity in the weak acceleration regime: NGC 7099
A test of Newton's law of gravity in the low acceleration regime using
globular clusters is presented. New results for the core collapsed globular
cluster NGC 7099 are given. The run of the gravitational potential as a
function of distance is probed studying the velocity dispersion profile of the
cluster, as derived from a set of 125 radial velocities with accuracy better
than 1 km/s. The velocity dispersion profile is traced up to ~18 pc from the
cluster center. The dispersion is found to be maximal at the center, then
decrease until 10+-2 pc from the center, well inside the cluster tidal radius
of 42 pc. After that the dispersion remains constant with average value
2.2+-0.3 km/s. Assuming for NGC 7099 a total V mag of M(V)=-7.43 mags and
mass-to-light ratio M/L=1, the acceleration at 10 pc from the center is 1.1e-8
cm/s/s. Thus, the flattening of the velocity dispersion profile occurs for a
value of the internal acceleration of gravity fully consistent with a_0=1.2e-8
cm/s/s observed in galaxies. This new result for NGC 7099 brings to 4 the
clusters with velocity dispersion profile probing acceleration below a_0. All
four have been found to have a flat dispersion profile at large radii where the
acceleration is below a_0, mimicking qualitatively and quantitatively
elliptical galaxies. Whether this indicates a failure of Newtonian dynamics in
the low acceleration limit or some more conventional dynamical effect (e.g.,
tidal heating) is still unclear. However, the similarities emerging between
very different globular clusters, as well as between globular clusters and
elliptical galaxies seem to favor the first of these two possibilities.Comment: Accepted for publication in A&A Letters. Four pages in tota
Using globular clusters to test gravity in the weak acceleration regime
We report on the results from an ongoing program aimed at testing Newton's
law of gravity in the low acceleration regime using globular clusters. It is
shown that all clusters studied so far do behave like galaxies, that is, their
velocity dispersion profile flattens out at large radii where the acceleration
of gravity goes below 1e-8 cm/s/s, instead of following the expected Keplerian
fall off. In galaxies this behavior is ascribed to the existence of a dark
matter halo. Globular clusters, however, do not contain dark matter, hence this
result might indicate that our present understanding of gravity in the weak
regime of accelerations is incomplete and somehow incorrect.Comment: As published on the European Southern Observatory "the Messenger",
Num. 128, June 2007. Seven pages, 4 figures, 2 table
The Lyman alpha forest of the high-z quasar 0000-263
Medium-resolution (delta(v) = 45 km/s) optical spectra of the bright, high-redshift (z = 4.1) quasar 0000-263 taken at the ESO 3.5-m NTT telescope were analyzed to determine the distribution of column densities, velocities and line widths of the Lyman-alpha forest absorption components. The values of NH, b, and z were determined by fitting Voigt profiles to the lines, and convolving with a Gaussian instrumental response function. Over 350 components with log N(sub H) greater than 13.2 were identified. An analysis of the dependence of the number of components with z reveals that the number evolution of components obeys the power law dN/dz varies as (1+z)(sup gamma), where gamma = 0.5 +/- 0.4 for the sample of 182 lines with log N(sub H) greater than 14.0. The distribution of component strengths is found to obey f(N(sub H)) varies as N(sub h)(sup -beta), where beta = -1.55 for components with log(N(sub H)) is greater than 14.7, and beta = -0.68 for the components with log(N(sub H)) greater than 13.5. A distinct break in the f(N(sub H)) histogram is also observed, at log(N(sub H)) is approximately 14.7. The results are briefly considered in the context of theoretical models of quasar Lyman alpha clouds and their evolution
Approximate nonnegative matrix factorization algorithm for the analysis of angular differential imaging data
The angular differential imaging (ADI) is used to improve contrast in high
resolution astronomical imaging. An example is the direct imaging of exoplanet
in camera fed by Extreme Adaptive Optics. The subtraction of the main dazzling
object to observe the faint companion was improved using Principal Component
Analysis (PCA). It factorizes the positive astronomical frames into positive
and negative components. On the contrary, the Nonnegative Matrix Factorization
(NMF) uses only positive components, mimicking the actual composition of the
long exposure images.Comment: 9 pages, 7 Figures, Proceeding of the SPIE Conference Adaptive Optics
Systems VI, SPIE Astronomical Telescopes + Instrumentation, Austin Convention
Center Austin, Texas, United States 10 - 15 June 201
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