494 research outputs found
Models for the 3-D axisymmetric gravitational potential of the Milky Way Galaxy - A detailed modelling of the Galactic disk
Aims. Galaxy mass models based on simple and analytical functions for the
density and potential pairs have been widely proposed in the literature. Disk
models constrained by kinematic data alone give information on the global disk
structure only very near the Galactic plane. We attempt to circumvent this
issue by constructing disk mass models whose three-dimensional structures are
constrained by a recent Galactic star counts model in the near-infrared and
also by observations of the hydrogen distribution in the disk. Our main aim is
to provide models for the gravitational potential of the Galaxy that are fully
analytical but also with a more realistic description of the density
distribution in the disk component. Methods. From the disk model directly based
on the observations (here divided into the thin and thick stellar disks and the
HI and H disks subcomponents), we produce fitted mass models by combining
three Miyamoto-Nagai disk profiles of any "model order" (1, 2, or 3) for each
disk subcomponent. The Miyamoto-Nagai disks are combined with models for the
bulge and "dark halo" components and the total set of parameters is adjusted by
observational kinematic constraints. A model which includes a ring density
structure in the disk, beyond the solar Galactic radius, is also investigated.
Results. The Galactic mass models return very good matches to the imposed
observational constraints. In particular, the model with the ring density
structure provides a greater contribution of the disk to the rotational support
inside the solar circle. The gravitational potential models and their
associated force-fields are described in analytically closed forms, and in
addition, they are also compatible with our best knowledge of the stellar and
gas distributions in the disk component. The gravitational potential models are
suited for investigations of orbits in the Galactic disk.Comment: 22 pages, 13 figures, 11 tables, accepted for publication in A&
A new model for gravitational potential perturbations in disks of spiral galaxies. An application to our Galaxy
We propose a new, more realistic, description of the perturbed gravitational
potential of spiral galaxies, with spiral arms having Gaussian-shaped groove
profiles. We investigate the stable stellar orbits in galactic disks, using the
new perturbed potential. The influence of the bulge mass on the stellar orbits
in the inner regions of a disk is also investigated. The new description offers
the advantage of easy control of the parameters of the Gaussian profile of its
potential. We find a range of values for the perturbation amplitude from 400 to
800 km^2 s^{-2} kpc^{-1} which implies a maximum ratio of the tangential force
to the axisymmetric force between 3% and 6%, approximately. Good
self-consistency of arm shapes is obtained between the Inner Lindblad resonance
(ILR) and the 4:1 resonance. Near the 4:1 resonance the response density starts
to deviate from the imposed logarithmic spiral form. This creates bifurcations
that appear as short arms. Therefore the deviation from a perfect logarithmic
spiral in galaxies can be understood as a natural effect of the 4:1 resonance.
Beyond the 4:1 resonance we find closed orbits which have similarities with the
arms observed in our Galaxy. In regions near the center, in the presence of a
massive bulge, elongated stellar orbits appear naturally, without imposing any
bar-shaped potential, but only extending the spiral perturbation a little
inward of the ILR. This suggests that a bar is formed with a half-size around 3
kpc by a mechanism similar to that of the spiral arms. The potential energy
perturbation that we adopted represents an important step in the direction of
self-consistency, compared to previous sine function descriptions of the
potential. Our model produces a realistic description of the spiral structure,
able to explain several details that were not yet understood.Comment: 12 pag., 11 fig. Accepted for publication in A&A, 2012 December 1
A new method for estimating the pattern speed of spiral structure in the Milky Way
In the last few decades many efforts have been made to understand the effect
of spiral arms on the gas and stellar dynamics in the Milky Way disc. One of
the fundamental parameters of the spiral structure is its angular velocity, or
pattern speed , which determines the location of resonances in the
disc and the spirals' radial extent. The most direct method for estimating the
pattern speed relies on backward integration techniques, trying to locate the
stellar birthplace of open clusters. Here we propose a new method based on the
interaction between the spiral arms and the stars in the disc. Using a sample
of around 500 open clusters from the {\it New Catalogue of Optically Visible
Open Clusters and Candidates}, and a sample of 500 giant stars observed by
APOGEE, we find km s kpc, for a local
standard of rest rotation ~km s and solar radius ~kpc.
Exploring a range in and within the acceptable values, 200-240 km
s and 7.5-8.5 kpc, respectively, results only in a small change in our
estimate of , that is within the error. Our result is in close
agreement with a number of studies which suggest values in the range 20-25 km
s kpc. An advantage of our method is that we do not need
knowledge of the stellar age, unlike in the case of the birthplace method,
which allows us to use data from large Galactic surveys. The precision of our
method will be improved once larger samples of disk stars with spectroscopic
information will become available thanks to future surveys such as 4MOST.Comment: 10 pages, 6 figures, 4 tables, accepted for publication in MNRA
Oxygen, -element and iron abundance distributions in the inner part of the Galactic thin disc. II
We have derived the abundances of 36 chemical elements in one Cepheid star,
ASAS 181024--2049.6, located R kpc from the Galactic center.
This star falls within a region of the inner thin disc poorly sampled in
Cepheids. Our spectral analysis shows that iron, magnesium, silicon, calcium
and titanium LTE abundances in that star support the presence of a plateau-like
abundance distribution in the thin disc within 5 kpc of the Galactic center, as
previously suggested by \cite{Maret15}. If confirmed, the flattening of the
abundance gradient within that region could be the result of a decrease in the
star formation rate due to dynamic effects, possibly from the central Galactic
bar.Comment: 5 pages, 3 figure
HI aperture synthesis and optical observations of the pair of galaxies NGC 6907 and 6908
NGC 6908, a S0 galaxy situated in direction of NGC 6907, was only recently
recognized as a distinct galaxy, instead of only a part of NGC 6907. We present
21 cm radio synthesis observations obtained with the GMRT and optical images
and spectroscopy obtained with the Gemini North telescope of this pair of
interacting galaxies. From the radio observations we obtained the velocity
field and the HI column density map of the whole region containing the NGC
6907/8 pair, and by means of the Gemini multi-object spectroscopy we obtained
high quality photometric images and resolution spectra sampling the
two galaxies. By comparing the rotation curve of NGC 6907 obtained from the two
opposite sides around the main kinematic axis, we were able to distinguish the
normal rotational velocity field from the velocity components produced by the
interaction between the two galaxies. Taking into account the rotational
velocity of NGC 6907 and the velocity derived from the absorption lines for NGC
6908, we verified that the relative velocity between these systems is lower
than 60 km s. The emission lines observed in the direction of NGC 6908,
not typical of S0 galaxies, have the same velocity expected for the NGC 6907
rotation curve. Some of them, superimposed on the absorption profiles, which
reinforces the idea that they were not formed in NGC 6908. Finally, the HI
profile exhibits details of the interaction, showing three components: one for
NGC 6908, another for the excited gas in the NGC 6907 disk and a last one for
the gas with higher relative velocities left behind NGC 6908 by dynamical
friction, used to estimate the time when the interaction started in years ago.Comment: 11 pages, 5 tables, 13 figures. Corrected typos. Accepted for
publication in MNRAS. The definitive version will be available at
http://www.blackwell-synergy.co
WR 110: A Single Wolf-Rayet Star With Corotating Interaction Regions In Its Wind?
A 30-day contiguous photometric run with the MOST satellite on the WN5-6b
star WR 110 (HD 165688) reveals a fundamental periodicity of P = 4.08 +/- 0.55
days along with a number of harmonics at periods P/n, with n ~ 2,3,4,5 and 6,
and a few other possible stray periodicities and/or stochastic variability on
timescales longer than about a day. Spectroscopic RV studies fail to reveal any
plausible companion with a period in this range. Therefore, we conjecture that
the observed light-curve cusps of amplitude ~ 0.01 mag that recur at a 4.08 day
timescale may arise in the inner parts, or at the base of, a corotating
interaction region (CIR) seen in emission as it rotates around with the star at
constant angular velocity. The hard X-ray component seen in WR 110 could then
be a result of a high velocity component of the CIR shock interacting with the
ambient wind at several stellar radii. Given that most hot, luminous stars
showing CIRs have two CIR arms, it is possible that either the fundamental
period is 8.2 days or, more likely in the case of WR 110, there is indeed a
second weaker CIR arm for P = 4.08 days, that occurs ~ two thirds of a rotation
period after the main CIR. If this interpretation is correct, WR 110 therefore
joins the ranks with three other single WR stars, all WN, with confirmed CIR
rotation periods (WR 1, WR 6, and WR 134), albeit with WR 110 having by far the
lowest amplitude photometric modulation. This illustrates the power of being
able to secure intense, continuous high-precision photometry from space-based
platforms such as MOST. It also opens the door to revealing low-amplitude
photometric variations in other WN stars, where previous attempts have failed.
If all WN stars have CIRs at some level, this could be important for revealing
sources of magnetism or pulsation in addition to rotation periods.Comment: 25 pages, 8 figures, 2 tables, accepted in Ap
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
Formation scenarios for the young stellar associations between galactic longitudes l = 280-360 deg
We investigate the spatial distribution, the space velocities and age
distribution of the pre-main sequence (PMS) stars belonging to Ophiuchus, Lupus
and Chamaeleon star-forming regions (SFRs), and of the young early-type star
members of the Scorpius-Centaurus OB association. These young stellar
associations extend over the galactic longitude range from 280 deg. to 360
deg., and are at a distance interval of around 100 and 200 pc. This study is
based on a compilation of distances, proper motions and radial velocities from
the literature for the kinematic properties, and of basic stellar data for the
construction of Hertzsprung-Russel diagrams. Although there was no well-known
OB association in Chamaeleon, the distances and the proper motions of a group
of 21 B- and A-type stars, taken from the Hipparcos Catalogue, lead us to
propose that they form a young association. We show that the young early-type
stars of the OB associations and the PMS stars of the SFRs follow a similar
spatial distribution, i.e., there is no separation between the low and the
high-mass young stars. We find no difference in the kinematics nor in the ages
of these two populations studied. We analyze the different scenarios for the
triggering of large-scale star-formation that have been proposed up to now, and
argue that most probably we are observing a spiral arm that passes close to the
Sun. The alignment of young stars and molecular clouds and the average velocity
of the stars in the opposite direction to the Galactic rotation agree with the
expected behavior of star formation in nearby spiral arms.Comment: 14 pages, 14 postscript figures, accepted for publication in A&
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