122 research outputs found
High-resolution simulations of clump-clump collisions using SPH with Particle Splitting
We investigate, by means of numerical simulations, the phenomenology of star
formation triggered by low-velocity collisions between low-mass molecular
clumps. The simulations are performed using an SPH code which satisfies the
Jeans condition by invoking On-the-Fly Particle Splitting. Clumps are modelled
as stable truncated (non-singular) isothermal, i.e. Bonnor-Ebert, spheres.
Collisions are characterised by M_0 (clump mass), b (offset parameter, i.e.
ratio of impact parameter to clump radius), and M (Mach Number, i.e. ratio of
collision velocity to effective post-shock sound speed). The gas subscribes to
a barotropic equation of state, which is intended to capture (i) the scaling of
pre-collision internal velocity dispersion with clump mass, (ii) post-shock
radiative cooling, and (iii) adiabatic heating in optically thick protostellar
fragments. The efficiency of star formation is found to vary between 10% and
30% in the different collisions studied and it appears to increase with
decreasing M_0, and/or decreasing b, and/or increasing M. For b<0.5 collisions
produce shock compressed layers which fragment into filaments. Protostellar
objects then condense out of the filaments and accrete from them. The resulting
accretion rates are high, 1 to 5 x 10^{-5} M_sun yr^{-1}, for the first 1 to 3
x 10^4 yrs. The densities in the filaments, n >~ 5 x 10^5 cm^{-3}, are
sufficient that they could be mapped in NH_3 or CS line radiation, in nearby
star formation regions.Comment: Accepted for publication in MNRAS; 21 pages; 25 figures. Four figures
are provided separately in reduced jpg format due to their large original ps
size: click on "PostScript" to have direct access to the 4 jpg figures; full
size ps files for these 4 figures can be found at
http://www.aip.de/People/skitsionas/papers
SPH simulations of star/planet formation triggered by cloud-cloud collisions
We present results of hydrodynamic simulations of star formation triggered by
cloud-cloud collisions. During the early stages of star formation, low-mass
objects form by gravitational instabilities in protostellar discs. A number of
these low-mass objects are in the sub-stellar mass range, including a few
objects of planetary mass. The disc instabilities that lead to the formation of
low-mass objects in our simulations are the product of disc-disc interactions
and/or interactions between the discs and their surrounding gas.Comment: 8 pages, 7 figures; accepted for publication in the proceedings of
IAU Symposium 249: Exoplanets: Detection, Formation and Dynamics, Y.-S. Sun,
S. Ferraz-Mello & J.-L. Zhou (eds.), Cambridge University Pres
Gravitational fragmentation and the formation of brown dwarfs in stellar clusters
We investigate the formation of brown dwarfs and very low-mass stars through
the gravitational fragmentation of infalling gas into stellar clusters. The
gravitational potential of a forming stellar cluster provides the focus that
attracts gas from the surrounding molecular cloud. Structures present in the
gas grow, forming filaments flowing into the cluster centre. These filaments
attain high gas densities due to the combination of the cluster potential and
local self-gravity. The resultant Jeans masses are low, allowing the formation
of very low-mass fragments. The tidal shear and high velocity dispersion
present in the cluster preclude any subsequent accretion thus resulting in the
formation of brown dwarfs or very low-mass stars. Ejections are not required as
the brown dwarfs enter the cluster with high relative velocities, suggesting
that their disc and binary properties should be similar to that of low-mass
stars. This mechanism requires the presence of a strong gravitational potential
due to the stellar cluster implying that brown dwarf formation should be more
frequent in stellar clusters than in distributed populations of young stars.
Brown dwarfs formed in isolation would require another formation mechanism such
as due to turbulent fragmentation.Comment: 8 pages, 7 figures. MNRAS, in pres
XMM and Chandra measurements of the AGN intrinsic absorption: dependence on luminosity and redshift
We combine bright XMM data with the Chandra Deep Field South observations in
order to explore the behavior of the intrinsic AGN absorption, as a function of
redshift and luminosity.Our sample consists of 359 sources selected in the hard
2-8 keV band, spanning the flux range 6\times10^{-16}-$3\times10^{-13} erg s^-1
cm^-2 with a high rate of spectroscopic or photometric redshift completeness
(100 and 85 per cent respectively for the Chandra and XMM data. We derive the
column density values using X-ray spectral fits. We find that the fraction of
obscured AGN falls with increasing luminosity in agreement with previous
findings. The fraction of obscured AGN shows an apparent increase at high
redshifts (z>2). Simulations show that this effect can be most probably
attributed to the fact that at high redshifts the column densities are
overestimated.Comment: 14 pages, 9 figures, A&A accepte
The efficiency of star formation in clustered and distributed regions
We investigate the formation of both clustered and distributed populations of
young stars in a single molecular cloud. We present a numerical simulation of a
10,000 solar mass elongated, turbulent, molecular cloud and the formation of
over 2500 stars. The stars form both in stellar clusters and in a distributed
mode which is determined by the local gravitational binding of the cloud. A
density gradient along the major axis of the cloud produces bound regions that
form stellar clusters and unbound regions that form a more distributed
population. The initial mass function also depends on the local gravitational
binding of the cloud with bound regions forming full IMFs whereas in the
unbound, distributed regions the stellar masses cluster around the local Jeans
mass and lack both the high-mass and the low-mass stars. The overall efficiency
of star formation is ~ 15 % in the cloud when the calculation is terminated,
but varies from less than 1 % in the the regions of distributed star formation
to ~ 40 % in regions containing large stellar clusters. Considering that large
scale surveys are likely to catch clouds at all evolutionary stages, estimates
of the (time-averaged) star formation efficiency for the giant molecular cloud
reported here is only ~ 4 %. This would lead to the erroneous conclusion of
'slow' star formation when in fact it is occurring on a dynamical timescale.Comment: 9 pages, 8 figures, MNRAS in pres
The Serendipitous XMM-Newton Cluster Athens Survey (SEXCLAS): Sample selection and the cluster log N - log S
In this paper we serendipitously identify X-ray cluster candidates using
XMM-Newton archival observations complemented by 5-band optical photometric
follow-up observations (r~23 mag) as part of the X-ray Identification (XID)
programme. Our sample covers an area of ~2.1 sq. deg (15 XMM-Newton fields) and
comprises a total of 21 (19 serendipitous + 2 target) extended X-ray sources to
the limit f(0.5-2keV) ~ 6x10^{-15} cgs with a high probability (> 99.9%) of
being extended on the XMM-Newton images. Of the 21 cluster candidates 7 are
spectroscopically confirmed in the literature. Exploiting the optical data
available for these fields we discover that 68% of the X-ray cluster candidates
are associated with optical galaxy overdensities. We also attempt to constrain
the redshifts of our cluster candidates using photometric methods. We thus
construct the photometric redshift distribution of galaxies in the vicinity of
each X-ray selected cluster candidate and search for statistically significant
redshift peaks against that of the background distribution of field galaxies.
Comparison of the photometric with spectroscopic redshift estimates for the
confirmed clusters suggest that our simple method is robust out to z~0.5. For
clusters at higher z, deeper optical data are required to estimate reliable
photometric redshifts. Finally, using the sample of the 19 serendipitous X-ray
selected cluster candidates we estimate their surface density down to
f(0.5-2keV) ~ 6x10^{-15} cgs and find it to be in fair agreement with previous
and recent studies.Comment: Submitted to the MNRAS, 8 page
Properties of hierarchically forming star clusters
We undertake a systematic analysis of the early (< 0.5 Myr) evolution of
clustering and the stellar initial mass function in turbulent fragmentation
simulations. These large scale simulations for the first time offer the
opportunity for a statistical analysis of IMF variations and correlations
between stellar properties and cluster richness. The typical evolutionary
scenario involves star formation in small-n clusters which then progressively
merge; the first stars to form are seeds of massive stars and achieve a
headstart in mass acquisition. These massive seeds end up in the cores of
clusters and a large fraction of new stars of lower mass is formed in the outer
parts of the clusters. The resulting clusters are therefore mass segregated at
an age of 0.5 Myr, although the signature of mass segregation is weakened
during mergers. We find that the resulting IMF has a smaller exponent
(alpha=1.8-2.2) than the Salpeter value (alpha=2.35). The IMFs in subclusters
are truncated at masses only somewhat larger than the most massive stars (which
depends on the richness of the cluster) and an universal upper mass limit of
150 Msun is ruled out. We also find that the simulations show signs of the
IGIMF effect proposed by Weidner & Kroupa, where the frequency of massive stars
is suppressed in the integrated IMF compared to the IMF in individual clusters.
We identify clusters through the use of a minimum spanning tree algorithm which
allows easy comparison between observational survey data and the predictions of
turbulent fragmentation models. In particular we present quantitative
predictions regarding properties such as cluster morphology, degree of mass
segregation, upper slope of the IMF and the relation between cluster richness
and maximum stellar mass. [abridged]Comment: 21 Pages, 25 Figure
On the use of photometric redshifts for X-ray selected AGNs
(Abridged) In this paper we present photometric redshift estimates for a
sample of X-ray selected sources detected in the wide field (~2 deg^2), bright
[f_{X} (0.5-8 keV)~10^{-14} cgs] XMM-Newton/2dF survey. Unlike deeper X-ray
samples comprising a large fraction of sources with colours dominated by the
host galaxy, our bright survey primarily probes the QSO X-ray population.
Therefore photometric redshift methods employing both galaxy and QSO templates
need to be used. We employ the photometric redshift technique of
Hatziminaoglou, Mathez & Pello (2000) using 5-band photometry from the SDSS. We
separate our X-ray sources according to their optical profile to point-like and
extended. We apply QSO and galaxy templates to the point-like and extended
sources respectively. X-ray sources associated with Galactic stars are
identified and discarded from our point-like sample on the basis of their low
X-ray--to--optical flux ratio and their broad band colours that are best fit by
stellar templates. Comparison of our results with spectroscopic redshifts
available, allows calibration of our method and estimation of the photometric
redshift accuracy. For ~70 per cent of the point-like sources photometric
redshifts are correct within dz <= 0.3 (or ~75 per cent have dz/(1+z) <= 0.2),
and the rms scatter is estimated to be sigma_z = 0.30. For the optically
extended objects the photometric redshifts work only in the case of red (g - r
> 0.5 mag) sources yielding dz <= 0.15 and dz/(1+z) <= 0.2 for 73 and 93 per
cent respectively. However, we find that the above photometric redshift
technique does not work in the case of extended sources with blue colours (g -
r < 0.5): such sources cannot be fit successfully by QSO or galaxy templates,
or any linear combination of the two.Comment: Replaced due to extended revision; 11 pages, 4 figures; Accepted in
A&
- …