6 research outputs found
Measuring proper motions of isolated neutron stars with Chandra
The excellent spatial resolution of the Chandra observatory offers the
unprecedented possibility to measure proper motions at X-ray wavelength with
relatively high accuracy using as reference the background of extragalactic or
remote galactic X-ray sources. We took advantage of this capability to
constrain the proper motion of RX J0806.4-4123 and RX J0420.0-5022, two X-ray
bright and radio quiet isolated neutron stars (INSs) discovered by ROSAT and
lacking an optical counterpart. In this paper, we present results from a
preliminary analysis from which we derive 2 sigma upper limits of 76 mas/yr and
138 mas/yr on the proper motions of RX J0806.4-4123 and RX J0420.0-5022
respectively. We use these values together with those of other ROSAT discovered
INSs to constrain the origin, distance and evolutionary status of this
particular group of objects. We find that the tangential velocities of radio
quiet ROSAT neutron stars are probably consistent with those of 'normal'
pulsars. Their distribution on the sky and, for those having accurate proper
motion vectors, their possible birth places, all point to a local population,
probably created in the part of the Gould Belt nearest to the earth.Comment: 8 pages, 3 figures, to appear in Astrophysics and Space Science, in
the proceedings of "Isolated Neutron Stars: from the Interior to the
Surface", edited by D. Page, R. Turolla and S. Zan
Reddening law and interstellar dust properties along Magellanic sight-lines
This study establishes that SMC, LMC and Milky Way extinction curves obey the
same extinction law which depends on the 2200A bump size and one parameter, and
generalizes the Cardelli, Clayton and Mathis (1989) relationship. This suggests
that extinction in all three galaxies is of the same nature. The role of linear
reddening laws over all the visible/UV wavelength range, particularly important
in the SMC but also present in the LMC and in the Milky Way, is also
highlighted and discussed.Comment: accepted for publication in Astrophysics and Space Science. 16 pages,
12 figures. Some figures are colour plot
Multiwavelength Studies of Young OB Associations
We discuss how contemporary multiwavelength observations of young
OB-dominated clusters address long-standing astrophysical questions: Do
clusters form rapidly or slowly with an age spread? When do clusters expand and
disperse to constitute the field star population? Do rich clusters form by
amalgamation of smaller subclusters? What is the pattern and duration of
cluster formation in massive star forming regions (MSFRs)? Past observational
difficulties in obtaining good stellar censuses of MSFRs have been alleviated
in recent studies that combine X-ray and infrared surveys to obtain rich,
though still incomplete, censuses of young stars in MSFRs. We describe here one
of these efforts, the MYStIX project, that produced a catalog of 31,784
probable members of 20 MSFRs. We find that age spread within clusters are real
in the sense that the stars in the core formed after the cluster halo. Cluster
expansion is seen in the ensemble of (sub)clusters, and older dispersing
populations are found across MSFRs. Direct evidence for subcluster merging is
still unconvincing. Long-lived, asynchronous star formation is pervasive across
MSFRs.Comment: 22 pages, 9 figures. To appear in "The Origin of Stellar Clusters",
edited by Steven Stahler, Springer, 2017, in pres
Cluster Density and the IMF
Observed variations in the IMF are reviewed with an emphasis on environmental
density. The remote field IMF studied in the LMC by several authors is clearly
steeper than most cluster IMFs, which have slopes close to the Salpeter value.
Local field regions of star formation, like Taurus, may have relatively steep
IMFs too. Very dense and massive clusters, like super star clusters, could have
flatter IMFs, or inner-truncated IMFs. We propose that these variations are the
result of three distinct processes during star formation that affect the mass
function in different ways depending on mass range. At solar to intermediate
stellar masses, gas processes involving thermal pressure and supersonic
turbulence determine the basic scale for stellar mass, starting with the
observed pre-stellar condensations, and they define the mass function from
several tenths to several solar masses. Brown dwarfs require extraordinarily
high pressures for fragmentation from the gas, and presumably form inside the
pre-stellar condensations during mutual collisions, secondary fragmentations,
or in disks. High mass stars form in excess of the numbers expected from pure
turbulent fragmentation as pre-stellar condensations coalesce and accrete with
an enhanced gravitational cross section. Variations in the interaction rate,
interaction strength, and accretion rate among the primary fragments formed by
turbulence lead to variations in the relative proportions of brown dwarfs,
solar to intermediate mass stars, and high mass stars.Comment: 14 pages, 3 figures, to be published in ``IMF@50: A Fest-Colloquium
in honor of Edwin E. Salpeter,'' held at Abbazia di Spineto, Siena, Italy,
May 16-20, 2004. Kluwer Academic Publishers; edited by E. Corbelli, F. Palla,
and H. Zinnecke