759 research outputs found
Irradiated Interfaces in the Ara OB1, Carina, Eagle Nebula, and Cyg OB2 Massive Star Formation Regions
Regions of massive star formation offer some of the best and most
easily-observed examples of radiation hydrodynamics. Boundaries where
fully-ionized H II regions transition to neutral/molecular photodissociation
regions (PDRs) are of particular interest because marked temperature and
density contrasts across the boundaries lead to evaporative flows and fluid
dynamical instabilities that can evolve into spectacular pillar-like
structures. When detached from their parent clouds, pillars become ionized
globules that often harbor one or more young stars. H2 molecules at the
interface between a PDR and an H II region absorb ultraviolet light from
massive stars, and the resulting fluoresced infrared emission lines are an
ideal way to trace this boundary independent of obscuring dust. This paper
presents H2 images of four regions of massive star formation that illustrate
different types of PDR boundaries. The Ara OB1 star formation region contains a
striking long wall that has several wavy structures which are present in H2,
but the emission is not particularly bright because the ambient UV fluxes are
relatively low. In contrast, the Carina star formation region shows strong H2
fluorescence both along curved walls and at the edges of spectacular pillars
that in some cases have become detached from their parent clouds. The
less-spectacular but more well-known Eagle Nebula has two regions that have
strong fluorescence in addition to its pillars. While somewhat older than the
other regions, Cyg OB2 has the highest number of massive stars of the regions
surveyed and contains many isolated, fluoresced globules that have head-tail
morphologies which point towards the sources of ionizing radiation. These
images provide a collection of potential astrophysical analogs that may relate
to ablated interfaces observed in laser experiments of radiation hydrodynamics
Geysers in the Lagoon: new Herbig-Haro objects in M8
Aims: We search for direct evidence of ongoing star formation by accretion in
the Lagoon Nebula (M8), using optical wide-field narrow-band imaging obtained
at La Silla Observatory.
Methods: We examine [SII] and Halpha images for line-emission features that
could be interpreted as signatures of outflow activity of the exciting sources.
Results: We discover five new Herbig-Haro objects, study in detail their
morphology and attempt to identify their potential driving sources among the
population of T Tauri stars and embedded sources in the surroundings.
Conclusions: The results reported here conclusively demonstrate the existence
of very young stars going through the accreting phase in the M8 region.Comment: 9 pages, 6 postscript figures (one in color). Accepted, Astronomy and
Astrophysic
A Spectroscopic Survey of Subarcsecond Binaries in the Taurus-Auriga Dark Cloud with the Hubble Space Telescope
We report the results of a spectroscopic survey of 20 close T Tauri binaries
in the Taurus-Auriga dark cloud where the separations between primaries and
their secondaries are less than the typical size of a circumstellar disk around
a young star. Analysis of low-resolution and medium-resolution STIS spectra
yields the stellar luminosities, reddenings, ages, masses, mass accretion
rates, IR excesses, and emission line luminosities for each star in each pair.
We examine the ability of IR color excesses, H-alpha equivalent widths, [O I]
emission, and veiling to distinguish between weak emission and classical T
Tauri stars. Four pairs have one cTTs and one wTTs; the cTTs is the primary in
three of these systems. This frequency of mixed pairs among the close T Tauri
binaries is similar to the frequency of mixed pairs in wider young binaries.
Extinctions within pairs are usually similar; however, the secondary is more
heavily reddened than the primary in some systems, where it may be viewed
through the primary's disk. Mass accretion rates of primaries and secondaries
are strongly correlated, and H-alpha luminosities, IR excesses, and ages also
correlate within pairs. Primaries tend to have somewhat larger accretion rates
than their secondaries do, and are typically slightly older than their
secondaries according to three different sets of modern pre-main-sequence
evolutionary tracks. Age differences for XZ Tau and FS Tau, systems embedded in
reflection nebulae, are striking; the secondary in each pair is less massive
but more luminous than the primary. The stellar masses of the UY Aur and GG Tau
binaries measured from their rotating molecular disks are about 30% larger than
the masses inferred from the spectra and evolutionary tracks
Asymptotic Learning Curve and Renormalizable Condition in Statistical Learning Theory
Bayes statistics and statistical physics have the common mathematical
structure, where the log likelihood function corresponds to the random
Hamiltonian. Recently, it was discovered that the asymptotic learning curves in
Bayes estimation are subject to a universal law, even if the log likelihood
function can not be approximated by any quadratic form. However, it is left
unknown what mathematical property ensures such a universal law. In this paper,
we define a renormalizable condition of the statistical estimation problem, and
show that, under such a condition, the asymptotic learning curves are ensured
to be subject to the universal law, even if the true distribution is
unrealizable and singular for a statistical model. Also we study a
nonrenormalizable case, in which the learning curves have the different
asymptotic behaviors from the universal law
Reopening the TNOs Color Controversy: Centaurs Bimodality and TNOs Unimodality
We revisit the Trans-Neptunian Objects (TNOs) color controversy allegedly
solved by Tegler and Romanishin 2003. We debate the statistical approach of the
quoted work and discuss why it can not draw the claimed conclusions, and
reanalyze their data sample with a more adequate statistical test. We find
evidence for the existence of two color groups among the Centaurs. Therefore,
mixing both centaurs and TNOs populations lead to the erroneous conclusion of a
global bimodality, while there is no evidence for two color groups in the TNOs
population alone. We use quasi-simultaneous visible color measurements
published for 20 centaurs (corresponding to about half of the identified
objects of this class), and conclude on the existence of two groups. With the
surface evolution model of Delsanti et al. (2003) we discuss how the existence
of two groups of Centaurs may be compatible with a continuous TNOs color
distribution.Comment: 4 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics Letter
Star Spot Induced Radial Velocity Variability in LkCa 19
We describe a new radial velocity survey of T Tauri stars and present the
first results. Our search is motivated by an interest in detecting massive
young planets, as well as investigating the origin of the brown dwarf desert.
As part of this survey, we discovered large-amplitude, periodic, radial
velocity variations in the spectrum of the weak line T Tauri star LkCa 19.
Using line bisector analysis and a new simulation of the effect of star spots
on the photometric and radial velocity variability of T Tauri stars, we show
that our measured radial velocities for LkCa19 are fully consistent with
variations caused by the presence of large star spots on this rapidly rotating
young star. These results illustrate the level of activity-induced radial
velocity noise associated with at least some very young stars. This
activity-induced noise will set lower limits on the mass of a companion
detectable around LkCa 19, and similarly active young stars.Comment: ApJ accepted, 27 pages, 12 figures, aaste
Regional coherence evaluation in mild cognitive impairment and Alzheimer's disease based on adaptively extracted magnetoencephalogram rhythms
This study assesses the connectivity alterations caused by Alzheimer's disease (AD) and mild cognitive impairment (MCI) in magnetoencephalogram (MEG) background activity. Moreover, a novel methodology to adaptively extract brain rhythms from the MEG is introduced. This methodology relies on the ability of empirical mode decomposition to isolate local signal oscillations and constrained blind source separation to extract the activity that jointly represents a subset of channels. Inter-regional MEG connectivity was analysed for 36 AD, 18 MCI and 26 control subjects in δ, θ, α and β bands over left and right central, anterior, lateral and posterior regions with magnitude squared coherence—c(f). For the sake of comparison, c(f) was calculated from the original MEG channels and from the adaptively extracted rhythms. The results indicated that AD and MCI cause slight alterations in the MEG connectivity. Computed from the extracted rhythms, c(f) distinguished AD and MCI subjects from controls with 69.4% and 77.3% accuracies, respectively, in a full leave-one-out cross-validation evaluation. These values were higher than those obtained without the proposed extraction methodology
Pre-main sequence stars in the Lagoon Nebula (M8)
We report the discovery of new pre-main sequence (PMS) stars in the Lagoon
Nebula (M8) at a distance of 1.25 kpc, based on intermediate resolution spectra
obtained with the Boller & Chivens spectrograph at the 6.5-m Magellan I
telescope (Las Campanas Observatory, Chile). According to the spectral types,
the presence of emission lines and the lithium 6708A absorption line, we are
able to identify 27 classical T Tauri stars, 7 weak-lined T Tauri stars and 3
PMS emission objects with spectral type G, which we include in a separated
stellar class denominated "PMS Fe/Ge class". Using near-infrared photometry
either from 2MASS or from our own previous work we derive effective
temperatures and luminosities for these stars and locate them in the
Hertzsprung-Russell diagram, in order to estimate their masses and ages. We
find that almost all of our sample stars are younger than 3 10^6 years and span
over a range of masses between 0.8 and 2.5 Msun. A cross-correlation between
our spectroscopic data and the X-ray sources detected with the Chandra ACIS
instrument is also presented.Comment: 18 pages, 15 figures, MNRAS, in pres
A new look at globular cluster colors in NGC 3311 and the case for exclusively metal-rich globular cluster systems
NGC 3311, the central cD galaxy in the Hydra cluster, was previously thought
to host the most metal-rich globular cluster system known. Ground-based
Washington photometry had indicated the almost complete absence of the
population of globular clusters near [Fe/H] ~ -1 dex, normally dominant in the
metallicity distribution functions of giant elliptical galaxies. Lacking the
normal metal-poor globular cluster population, NGC 3311 was an outstanding
exception among galaxies, not easily understood under any of the current
globular cluster formation scenarios. Our HST/WFPC2 data yield normal globular
cluster colors and hence metallicities for this galaxy. We find a bi-modal
color distribution with peaks at (V-I)=0.91 +/- 0.03 and 1.09 +/- 0.03,
corresponding to [Fe/H] ~ -1.5 and -0.75 dex (somewhat dependent on the choice
of the conversion relation between color and metallicity). We review the
evidence for exclusively metal-rich globular cluster systems in other galaxies
and briefly discuss the implications for our understanding of globular cluster
and galaxy formation.Comment: 11 pages, 2 figures, accepted for publication in ApJ Letter
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