629 research outputs found
Chemical evolution of the Galactic Center
In recent years, the Galactic Center (GC) region (200 pc in radius) has been
studied in detail with spectroscopic stellar data as well as an estimate of the
ongoing star formation rate. The aims of this paper are to study the chemical
evolution of the GC region by means of a detailed chemical evolution model and
to compare the results with high resolution spectroscopic data in order to
impose constraints on the GC formation history.The chemical evolution model
assumes that the GC region formed by fast infall of gas and then follows the
evolution of alpha-elements and Fe. We test different initial mass functions
(IMFs), efficiencies of star formation and gas infall timescales. To reproduce
the currently observed star formation rate, we assume a late episode of star
formation triggered by gas infall/accretion. We find that, in order to
reproduce the [alpha/Fe] ratios as well as the metallicity distribution
function observed in GC stars, the GC region should have experienced a main
early strong burst of star formation, with a star formation efficiency as high
as 25 Gyr^{-1}, occurring on a timescale in the range 0.1-0.7 Gyr, in agreement
with previous models of the entire bulge. Although the small amount of data
prevents us from drawing firm conclusions, we suggest that the best IMF should
contain more massive stars than expected in the solar vicinity, and the last
episode of star formation, which lasted several hundred million years, should
have been triggered by a modest episode of gas infall/accretion, with a star
formation efficiency similar to that of the previous main star formation
episode. This last episode of star formation produces negligible effects on the
abundance patterns and can be due to accretion of gas induced by the bar. Our
results exclude an important infall event as a trigger for the last starburst.Comment: 10 pages, 8 figures, accepted for publication in MNRA
Massive Young Stellar Objects in the Galactic Center. I. Spectroscopic Identification from Spitzer/IRS Observations
We present results from our spectroscopic study, using the Infrared
Spectrograph (IRS) onboard the Spitzer Space Telescope, designed to identify
massive young stellar objects (YSOs) in the Galactic Center (GC). Our sample of
107 YSO candidates was selected based on IRAC colors from the high spatial
resolution, high sensitivity Spitzer/IRAC images in the Central Molecular Zone
(CMZ), which spans the central ~300 pc region of the Milky Way Galaxy. We
obtained IRS spectra over 5um to 35um using both high- and low-resolution IRS
modules. We spectroscopically identify massive YSOs by the presence of a 15.4um
shoulder on the absorption profile of 15um CO2 ice, suggestive of CO2 ice mixed
with CH3OH ice on grains. This 15.4um shoulder is clearly observed in 16
sources and possibly observed in an additional 19 sources. We show that 9
massive YSOs also reveal molecular gas-phase absorption from CO2, C2H2, and/or
HCN, which traces warm and dense gas in YSOs. Our results provide the first
spectroscopic census of the massive YSO population in the GC. We fit YSO models
to the observed spectral energy distributions and find YSO masses of 8 - 23
Msun, which generally agree with the masses derived from observed radio
continuum emission. We find that about 50% of photometrically identified YSOs
are confirmed with our spectroscopic study. This implies a preliminary star
formation rate of ~0.07 Msun/yr at the GC.Comment: Accepted for publication in Ap
Massive Young Stellar Objects in the Galactic Center. II. Seeing Through the Ice-rich Envelopes
To study the demographics of interstellar ices in the Central Molecular Zone
(CMZ) of the Milky Way, we obtain near-infrared spectra of red point
sources using NASA IRTF/SpeX at Maunakea. We select the sample from near- and
mid-infrared photometry, including objects in the previous paper of this
series, to ensure that these sources trace a large amount of absorption through
clouds in each line of sight. We find that most of the sample ( objects)
show CO band-head absorption at m, tagging them as red (super-)
giants. Despite the photospheric signature, however, a fraction of the sample
with -band spectra () exhibit large HO ice column densities
(), and six of them also reveal CHOH ice
absorption. As one of such objects is identified as a young stellar object
(YSO) in our previous work, these ice-rich sight lines are likely associated
with background stars in projection to an extended envelope of a YSO or a dense
cloud core. The low frequency of such objects in the early stage of stellar
evolution implies a low star-formation rate ( yr),
reinforcing the previous claim on the suppressed star-formation activity in the
CMZ. Our data also indicate that the strong "shoulder" CO ice absorption at
m observed in YSO candidates in the previous paper arises from
CHOH-rich ice grains having a large CO concentration [].Comment: 28 pages, 12 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
Effect of quantum confinement on exciton-phonon interactions
We investigate the homogeneous linewidth of localized type-I excitons in
type-II GaAs/AlAs superlattices. These localizing centers represent the
intermediate case between quasi-two-dimensional (Q2D) and
quasi-zero-dimensional localizations. The temperature dependence of the
homogeneous linewidth is obtained with high precision from
micro-photoluminescence spectra. We confirm the reduced interaction of the
excitons with their environment with decreasing dimensionality except for the
coupling to LO-phonons. The low-temperature limit for the linewidth of these
localized excitons is five times smaller than that of Q2D excitons. The
coefficient of exciton-acoustic-phonon interaction is 5 ~ 6 times smaller than
that of Q2D excitons. An enhancement of the average exciton-LO-phonon
interaction by localization is found in our sample. But this interaction is
very sensitive to the detailed structure of the localizing centers.Comment: 6 pages, 4 figure
Point Sources from a Spitzer IRAC Survey of the Galactic Center
We have obtained Spitzer/IRAC observations of the central 2.0 x 1.4 degrees
(~280 x 200 pc) of the Galaxy at 3.6-8.0 microns. A point source catalog of
1,065,565 objects is presented. The catalog includes magnitudes for the point
sources at 3.6, 4.5, 5.8, and 8.0 microns, as well as JHK photometry from
2MASS. The point source catalog is confusion limited with average limits of
12.4, 12.1, 11.7, and 11.2 magnitudes for [3.6], [4.5], [5.8], and [8.0],
respectively. We find that the confusion limits are spatially variable because
of stellar surface density, background surface brightness level, and extinction
variations across the survey region. The overall distribution of point source
density with Galactic latitude and longitude is essentially constant, but
structure does appear when sources of different magnitude ranges are selected.
Bright stars show a steep decreasing gradient with Galactic latitude, and a
slow decreasing gradient with Galactic longitude, with a peak at the position
of the Galactic center. From IRAC color-magnitude and color-color diagrams, we
conclude that most of the point sources in our catalog have IRAC magnitudes and
colors characteristic of red giant and AGB stars.Comment: 44 pages, 13 figures, ApJS in pres
Discovery of new Milky Way star cluster candidates in the 2MASS point source catalog III. Follow-up observations of cluster candidates in the Galactic Center region
This paper is part of a project to search the inner Milky Way for hidden
massive clusters and to address the question of whether our Galaxy still forms
clusters similar to the progenitors of the present-day globular clusters.
We report high angular resolution deep near-infrared imaging of 21 cluster
candidates selected from the catalogues of Bica et al. (2003) and Dutra et
al.(2003) in a region around the Galactic Center. These catalogues were created
from visual inspection of the 2MASS images. Seven objects appear to be genuine
clusters, and for these objects we present estimates of extinction, distance
and in some cases age and mass.
Our estimated masses range from 1200 to 5500 solar masses. These clusters are
thus significantly smaller than any Galactic globular cluster, and indicate
that the formation of massive young clusters such as Arches and Quintuplet is
not common in the present-day Milky Way.
The remaining 14 objects are either not clusters or cannot be classified
based on our data.Comment: 8 pages, 19 figures, accepted for publication in A&
Free induction signal from biexcitons and bound excitons
A theory of the free induction signal from biexcitons and bound excitons is
presented. The simultaneous existence of the exciton continuum and a bound
state is shown to result in a new type of time dependence of the free
induction. The optically detected signal increases in time and oscillates with
increasing amplitude until damped by radiative or dephasing processes.
Radiative decay is anomalously fast and can result in strong picosecond pulses.
The expanding area of a coherent exciton polarization (inflating antenna),
produced by the exciting pulse, is the underlying physical mechanism. The
developed formalism can be applied to different biexciton transients.Comment: RevTeX, 20 p. + 2 ps fig. To appear in Phys. Rev. B1
Photometric redshifts for the CFHTLS T0004 Deep and Wide fields
We compute photometric redshifts based on the template-fitting method in the
fourth public release of the Canada-France-Hawaii Telescope Legacy Survey. This
unique multi-colour catalogue comprises u*,g',r',i',z' photometry in four deep
fields of 1 deg2 each and 35 deg2 distributed over three Wide fields. Our
photometric redshifts are calibrated with and compared to 16,983 high-quality
spectroscopic redshifts from several surveys. We find a dispersion of 0.028 and
an outlier rate of 3.5% in the Deep field at i'AB < 24 and a dispersion of
0.036 and an outlier rate of 2.8% in the Wide field at i'AB < 22.5. Beyond i'AB
= 22.5 in the Wide field the number of outliers rises from 5% to 10% at i'AB<23
and i'AB<24 respectively. For the Wide sample, we find the systematic redshift
bias keeps below 1% to i'AB < 22.5, whereas we find no significant bias in the
Deep field. We investigated the effect of tile-to-tile photometric variations
and demonstrate that the accuracy of our photometric redshifts is reduced by at
most 21%. We separate stars from galaxies using both the size and colour
information, reducing the contamination by stars in our catalogues from 50% to
8% at i'AB < 22.5 in fields with the highest stellar density while keeping a
complete galaxy sample. Our CFHTLS T0004 photometric redshifts are distributed
to the community. Our release include 592,891 (i'AB < 22.5) and 244,701 (i'AB <
24) reliable galaxy photometric redshifts in the Wide and Deep fields,
respectively.Comment: 18 pages, 17 figure
Chemical abundances and ages of the bulge stars in APOGEE high-velocity peaks
A cold high-velocity (HV, 200 km/s) peak was first reported in several
Galactic bulge fields based on the APOGEE commissioning observations. Both the
existence and the nature of the high-velocity peak are still under debate. Here
we revisit this feature with the latest APOGEE DR13 data. We find that most of
the low latitude bulge fields display a skewed Gaussian distribution with a HV
shoulder. However, only 3 out of 53 fields show distinct high-velocity peaks
around 200 km/s. The velocity distribution can be well described by
Gauss-Hermite polynomials, except the three fields showing clear HV peaks. We
find that the correlation between the skewness parameter () and the mean
velocity (), instead of a distinctive HV peak, is a strong indicator
of the bar. It was recently suggested that the HV peak is composed of
preferentially young stars. We choose three fields showing clear HV peaks to
test this hypothesis using the metallicity, [/M] and [C/N] as age
proxies. We find that both young and old stars show HV features. The similarity
between the chemical abundances of stars in the HV peaks and the main component
indicates that they are not systematically different in terms of chemical
abundance or age. In contrast, there are clear differences in chemical space
between stars in the Sagittarius dwarf and the bulge stars. The strong HV peaks
off-plane are still to be explained properly, and could be different in nature.Comment: 13 pages, 10 figures, published in ApJ. Updated to match the final
ApJ published version. Minor revisions to the text and Figure
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