477 research outputs found
Binary Evolutionary Models
In this talk, we present the general principles of binary evolution and give
two examples. The first example is the formation of subdwarf B stars (sdBs) and
their application to the long-standing problem of ultraviolet excess (also
known as UV-upturn) in elliptical galaxies. The second is for the progenitors
of type Ia supernovae (SNe Ia). We discuss the main binary interactions, i.e.,
stable Roche lobe overflow (RLOF) and common envelope (CE) evolution, and show
evolutionary channels leading to the formation of various binary-related
objects. In the first example, we show that the binary model of sdB stars of
Han et al. (2002, 2003) can reproduce field sdB stars and their counterparts,
extreme horizontal branch (EHB) stars, in globular clusters. By applying the
binary model to the study of evolutionary population synthesis, we have
obtained an ``a priori'' model for the UV-upturn of elliptical galaxies and
showed that the UV-upturn is most likely resulted from binary interactions.
This has major implications for understanding the evolution of the UV excess
and elliptical galaxies in general. In the second example, we introduce the
single degenerate channel and the double degenerate channel for the progenitors
of SNe Ia. We give the birth rates and delay time distributions for each
channel and the distributions of companion stars at the moment of SN explosion
for the single degenerate channel, which would help to search for the remnant
companion stars observationally.Comment: 8 pages, 4 figures, invited talk, to appear in the Proceedings of IAU
Symp. 252 "The Art of Modelling Stars in the 21st Century", Sanya, China,
6th-11th April 2008, (L. Deng, K.L. Chan & C. Chiosi, eds.
Evolutionary Population Synthesis for Binary Stellar Populations
We present integrated colours, integrated spectral energy distributions, and
absorption-line indices, for instantaneous burst solar-metallicity binary
stellar populations with ages in the range 1 - 15Gyr. By comparing the results
for populations with and without binary interactions we show that the inclusion
of binary interactions makes the appearance of the population substantially
bluer -- this is the case for each of the quantities we have considered. This
effect raises the derived age and metallicity of the population. Therefore it
is necessary to consider binary interactions in order to draw accurate
conclusions from evolutionary population synthesis work.Comment: 6 pages, 4 figure
The blue stragglers formed via mass transfer in old open clusters
In this paper, we present the simulations for the primordial blue stragglers
in the old open cluster M67 based on detailed modelling of the evolutionary
processes. The principal aim is to discuss the contribution of mass transfer
between the components of close binaries to the blue straggler population in
M67. First, we followed the evolution of a binary of 1.4M+0.9M.
The synthetic evolutionary track of the binary system revealed that a
primordial blue straggler had a long lifetime in the observed blue straggler
region of color-magnitude diagram. Second, a grid of models for close binary
systems experiencing mass exchange were computed from 1Gyr to 6Gyr in order to
account for primordial blue-straggler formation in a time sequence. Based on
such a grid, Monte-Carlo simulations were applied for the old open cluster M67.
Adopting appropriate orbital parameters, 4 primordial blue stragglers were
predicted by our simulations. This was consistent with the observational fact
that only a few blue stragglers in M67 were binaries with short orbital
periods. An upper boundary of the primordial blue stragglers in the
color-magnitude diagram (CMD) was defined and could be used to distinguish blue
stragglers that were not formed via mass exchange. Using the grid of binary
models, the orbital periods of the primordial BSs could be predicted. Compared
with the observations, it is clear that the mechanism discussed in this work
alone cannot fully predict the blue straggler population in M67. There must be
several other processes also involved in the formation of the observed blue
stragglers in M67.Comment: 11 pages, 6 figures, A&A accepte
Multi-seeded multi-mode formation of embedded clusters in the RMC: Clusters baked in swept-up shells
This is the first of a series of three papers on clustered star formation in
the Rosette Molecular Complex. Here we investigate star formation in the
interfacing layers between the expanding Rosette Nebula and its surrounding
cloud, based on an analysis of the spatially complete and unbiased 2MASS data.
Two medium-mass infrared clusters with ages of around 1 Myr are identified in
the south and south-east arcs of the fragmented shell. The majority of the
candidate cluster members in these radiation and pressure-confined regions are
found to be almost uniformly distributed, roughly following the compression
layers traced by the distribution of optical depth at 100 m, and may well
develop into gravitationally unbound systems upon their emergence from the
parental cloud. These expanding shells are believed to be playing important
roles in impeding the emerging young open cluster NGC 2244 from intruding
immediately and deeply into the ambient molecular cloud, where sequential
formation of massive clusters is taking place.Comment: 6 figure
VLT/SPHERE deep insight of NGC 3603's core: Segregation or confusion?
We present new near-infrared photometric measurements of the core of the
young massive cluster NGC 3603 obtained with extreme adaptive optics. The data
were obtained with the SPHERE instrument mounted on ESO Very Large Telescope,
and cover three fields in the core of this cluster. We applied a correction for
the effect of extinction to our data obtained in the J and K broadband filters
and estimated the mass of detected sources inside the field of view of
SPHERE/IRDIS, which is 13.5"x13.5". We derived the mass function (MF) slope for
each spectral band and field. The MF slope in the core is unusual compared to
previous results based on Hubble space telescope (HST) and very large telescope
(VLT) observations. The average slope in the core is estimated as
-1.06^{+0.26}_{-0.26} for the main sequence stars with 3.5 Msun < M < 120
Msun.Thanks to the SPHERE extreme adaptive optics, 814 low-mass stars were
detected to estimate the MF slope for the pre-main sequence stars with 0.6
Msun< M < 3.5 Msun , Gamma = -0.54^{+0.11}_{-0.11} in the K-band images in two
fields in the core of the cluster. For the first time, we derive the mass
function of the very core of the NGC 3603 young cluster for masses in the range
0.6 - 120 Msun. Previous studies were either limited by crowding, lack of
dynamic range, or a combination of both
Cepheid Mass-Luminosity Relations from the Magellanic Clouds
The OGLE data base is used in conjunction with Kurucz atmosphere models to
generate sets of period, effective temperature and luminosity for fundamental
and overtone Magellanic Cloud Cepheids. The Florida pulsation code (with linear
turbulent convection) is then used to compute masses for these stars, assuming
an average composition of (=0.716, =0.010) for the LMC and of (=0.726,
=0.004) for the SMC. The average -- relation for the fundamental
Cepheids matches closely that for the first overtone Cepheids for each
Magellanic Cloud. Neither the SMC nor the LMC average \Log M--\Log L
relations are straight, but have a noticeable curvature.
In view of the uncertainties in distance and reddening we have adopted three
different choices for these quantities. The results based on the 'long'
distance scale to the clouds give a better agreement between theory and and
observations than the 'short' one.
All the current evolutionary tracks predict systematically larger masses for
given luminosities than our observationally derived ones, especially at the
high end. Moreover, our study confirms that the evolutionary tracks of the low
mass stars in SMC are not in agreement with the observations as they do not
extend sufficiently blueward and do not penetrate deep enough into the
instability strip, or not at all.
The inference of masses directly from the observational database yields a
novel and strong constraint on evolutionary calculations.Comment: 9 pages, Astronomy and Astrophysics, in pres
Different Evolutionary Stages in the Massive Star Forming Region S255 Complex
To understand evolutionary and environmental effects during the formation of
high-mass stars, we observed three regions of massive star formation at
different evolutionary stages that reside in the same natal molecular cloud.
Methods. The three regions S255IR, S255N and S255S were observed at 1.3 mm with
the Submillimeter Array (SMA) and followup short spacing information was
obtained with the IRAM 30m telescope. Near infrared (NIR) H + K-band spectra
and continuum observations were taken for S255IR with VLT-SINFONI to study the
different stellar populations in this region. The combination of millimeter
(mm) and near infrared data allow us to characterize different stellar
populations within the young forming cluster in detail. While we find multiple
mm continuum sources toward all regions, their outflow, disk and chemical
properties vary considerably. The most evolved source S255IR exhibits a
collimated bipolar outflow visible in CO and H2 emission, the outflows from the
youngest region S255S are still small and rather confined in the regions of the
mm continuum peaks. Also the chemistry toward S255IR is most evolved exhibiting
strong emission from complex molecules, while much fewer molecular lines are
detected in S255N, and in S255S we detect only CO isotopologues and SO lines.
Also, rotational structures are found toward S255N and S255IR. Furthermore, a
comparison of the NIR SINFONI and mm data from S255IR clearly reveal two
different (proto) stellar populations with an estimated age difference of
approximately 1 Myr. A multi-wavelength spectroscopy and mapping study reveals
different evolutionary phases of the star formation regions. We propose the
triggered outside-in collapse star formation scenario for the bigger picture
and the fragmentation scenario for S255IR.Comment: 23 pages,25 figures, accepted by A&
A binary model for the UV-upturn of elliptical galaxies (MNRAS version)
The discovery of a flux excess in the far-ultraviolet (UV) spectrum of
elliptical galaxies was a major surprise in 1969. While it is now clear that
this UV excess is caused by an old population of hot helium-burning stars
without large hydrogen-rich envelopes, rather than young stars, their origin
has remained a mystery. Here we show that these stars most likely lost their
envelopes because of binary interactions, similar to the hot subdwarf
population in our own Galaxy. We have developed an evolutionary population
synthesis model for the far-UV excess of elliptical galaxies based on the
binary model developed by Han et al (2002, 2003) for the formation of hot
subdwarfs in our Galaxy. Despite its simplicity, it successfully reproduces
most of the properties of elliptical galaxies with a UV excess: the range of
observed UV excesses, both in and , and their evolution
with redshift. We also present colour-colour diagrams for use as diagnostic
tools in the study of elliptical galaxies. The model has major implications for
understanding the evolution of the UV excess and of elliptical galaxies in
general. In particular, it implies that the UV excess is not a sign of age, as
had been postulated previously, and predicts that it should not be strongly
dependent on the metallicity of the population, but exists universally from
dwarf ellipticals to giant ellipticals.Comment: accepted for publication in MNRAS, 24 pages, 15 figures, 2 table
The COSMOS-WIRCam near-infrared imaging survey: I: BzK selected passive and star forming galaxy candidates at z>1.4
(abridged) We present a new near-infrared survey covering the 2 deg sq COSMOS
field. Combining our survey with Subaru B and z images we construct a deep,
wide-field optical-infrared catalogue. At Ks<23 (AB magnitudes) our survey
completeness is greater than 90% and 70% for stars and galaxies respectively
and contains 143,466 galaxies and 13,254 stars. At z~2 our catalogues contain
3931 quiescent and 25,757 star-forming BzK-selected galaxies representing the
largest and most secure sample of these objects to date. Our counts of
quiescent galaxies turns over at Ks~22 an effect which we demonstrate cannot be
due to sample incompleteness. In our survey both the number of faint and bright
quiescent objects exceeds the predictions of a semi-analytic galaxy formation
model, indicating potentially the need for further refinements in the amount of
merging and AGN feedback at z~2 in these models. We measure the angular
correlation function for each sample and find that at small scales the
correlation function for passive BzK galaxies exceeds the clustering of dark
matter. We use 30-band photometric redshifts to derive the spatial correlation
length and the redshift distributions for each object class. At Ks<22 we find
r_0^{\gamma/1.8}=7.0 +/-0.5h^{-1} Mpc for the passive BzK candidates and
4.7+/-0.8h^{-1} Mpc for the star-forming BzK galaxies. Our pBzK galaxies have
an average photometric redshift of z_p~1.4, in approximate agreement with the
limited spectroscopic information currently available. The stacked Ks image
will be made publicly available from IRSA.Comment: Accepted for publication in Astrophysical Journal. 17 pages, 17
figures, minor revisions to match published version available at
http://adsabs.harvard.edu/abs/2010ApJ...708..202
Observation and simulation of space-charge effects in a radio-frequency photoinjector using a transverse multibeamlet distribution
- …