57 research outputs found
Structure and Evolution of Low-Mass Stars: An Overview and Some Open Problems
A review is presented of some of the ingredients, assumptions and techniques
that are used in the computation of the structure and evolution of low-mass
stars. Emphasis is placed on several ingredients which are still subject to
considerable uncertainty. An overview of the evolution of low-mass stars is
also presented, from the cloud collapse phase all the way to the white dwarf
cooling curve.Comment: 52 pages, 6 figures. Invited review, to appear in "XIth Special
Courses at the National Observatory of Rio de Janeiro" (AIP Conf. Proc.), ed.
J. Alcaniz, R. de la Reza, F. Roig, & D. F. Lope
Sulfur Abundances in Orion B Stars
Sulfur abundances are derived for a sample of ten B MS star members of the Orion association. The analysis is based on LTE model atmospheres and non-LTE line formation theory by means of spectrum synthesis analysis of Sii and Siii lines. The abundance distribution obtained for the Orion targets is homogeneous within the errors in the analysis: A(S)=7.15±0.05. This abundance result is in agreement with the solar value and with results for the Orion nebula. The sulfur abundances for Orion combined with previous results for other OB-type stars produce a relatively shallow sulfur abundance gradient with a slope of â0.037±0.012 dex Kpcâ
Dark Matter and Galaxy Formation
The four lectures that I gave in the XIII Ciclo de Cursos Especiais at the
National Observatory of Brazil in Rio in October 2008 were (1) a brief history
of dark matter and structure formation in a LambdaCDM universe; (2) challenges
to LambdaCDM on small scales: satellites, cusps, and disks; (3) data on galaxy
evolution and clustering compared with simulations; and (4) semi-analytic
models. These lectures, themselves summaries of much work by many people, are
summarized here briefly.Comment: 37 pages, 20 figures, Lectures at XIII Special Courses at
Observatorio Nacional, Rio de Janeiro, Brazil, October 2008. To appear in the
Proceeding
Discovery of a Luminous Quasar in the Nearby Universe
In the course of the Pico dos Dias survey (PDS), we identified the stellar
like object PDS456 at coordinates alpha = 17h 28m 19.796s, delta = -14deg 15'
55.87'' (epoch 2000), with a relatively nearby (z = 0.184) and bright (B =
14.69) quasar. Its position at Galactic coordinates l_II = 10.4deg, b_II =
+11.2deg, near the bulge of the Galaxy, may explain why it was not detected
before. The optical spectrum of PDS456 is typical of a luminous quasar, showing
a broad (FWHM ~ 4000 km/s) H_\beta line, very intense FeII lines and a weak
[OIII]\lambda5007 line. PDS456 is associated to the infrared source IRAS
17254-1413 with a 60 \mum infrared luminosity L_{60} = 3.8 x 10^{45} erg/s. The
relatively flat slopes in the infrared (\alpha(25,60) = -0.33 and \alpha(12,25)
= -0.78) and a flat power index in the optical (F_{\nu} \propto \nu^{-0.72})
may indicate a low dust content. A good match between the position of PDS456
and the position of the X-ray source RXS J172819.3-141600 implies an X-ray
luminosity L_x = 2.8 x 10^{44} erg/s. The good correlation between the strength
of the emission lines in the optical and the X-ray luminosity, as well as the
steep optical to X-ray index estimated (\alpha_{ox} = -1.64) suggest that
PDS456 is radio quiet. A radio survey previously performed in this region
yields an upper limit for radio power at ~ 5 GHz of ~ 2.6 x 10^{30} erg/s/Hz.
We estimate the Galactic reddening in this line-of-sight to be A_B \simeq 2.0,
implying an absolute magnitude M_B = -26.7 (using H_0 = 75 km s^{-1} Mpc^{-1}
and q_0 = 0). In the optical, PDS456 is therefore 1.3 times more luminous than
3C 273 and the most luminous quasar in the nearby (z \leq 0.3) Universe.Comment: 12 pages, LaTeX (aasms4.sty) + 3 figures; accepted for publication in
the Astrophysical Journal Letter
On the Rotation of Post-T Tauri Stars in Associations
Nearby associations are excellent objects for the study of the initial spin
up phase during the PMS evolution. An empirical approach is adopted here to
infer their rotations properties and relations to X-ray emission. Three nearby
associations are considered. The TW Hya association with an age of 8 Myr, the
Beta Pictoris moving group with 12 Myr and a combination of Tucana and
Horologium associations (30 Myr). Two low and high rotation modes are
considered for each association with stellar masses less than 1.5M and greater
than 1.5M respectively. We infer representative equatorial rotation velocities
Vo from the observed distribution of projected rotational velocities vsini. A
spin up is found for the high rotation mode, whereas in the low rotation mode
the Vo do not increase significantly. This insufficient increase of Vo is
probably the cause of a decrease of the total mean specific angular momentum
for the low mass stars between 8 and 30 Myr. However, for the high mass stars,
where a sufficient spin up is present, the specific angular momentum is
practically conserved in this same time interval. By supposing that the
distribution of masses of these three associations follows a universal mass
function, we estimate the number of members of these associations that remain
to be detected. The analysis of rotation and stellar masses using luminosity
X-rays indicators present similar properties, as the dependence on stellar mass
and rotation, at least for the younger associations, to those obtained for T
Tauri stars in the ONC. A strong desaturation effect appears at 30 Myr. This
effect seems to be provoked by the minimum configuration of the stellar
convection layers, attained for the first time for the higher mass stars at 30
Myr. The desaturation appears to be independent of rotation at this stage.Comment: 37 pages, 10 figures, 4 tables. Accepted by the Astronomical Journa
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