245 research outputs found
Mid-infrared PL relations for Globular Cluster RR Lyrae
The period - metallicity - WISE W1- and W2-band luminosity relations are
derived for RR Lyrae stars based on WISE epoch photometry for 360 and 275 stars
in 15 and 9 Galactic globular clusters, respectively. Our final relations have
the form = gamma(W1) - (2.381 +/- 0.097) log PF + (0.096 +/- 0.021)[Fe/H]
and = gamma(W2)-(2.269 +/- 0.127)log PF + (0.108 +/- 0.021)[Fe/H], where
[Fe/H] values are on the scale of Carretta et al. (2009). We obtained two
appreciably discrepant estimates for the zero points gamma(W1) and gamma(W2) of
both relations: one based on a statistical-parallax analysis -- gamma(W1) =
-0.829 +/- 0.093 and gamma(W2)=-0.776 +/- 0.093 and another, significantly
brighter one, based on HST FGS trigonometric parallaxes -- gamma(W1, HST)
=-1.150 +/- 0.077 and gamma(W2, HST) =-1.105 +/- 0.077. The
period-metallicity-luminosity relations in the two bands yield highly
consistent distance moduli for the calibrator clusters and the distance moduli
computed using the W1- and W2-band relations with the HST zero points agree
well with those computed by \citet{sollima} based on their derived
period-metallicity-K-band luminosity relation whose zero point is tied to the
HST trigonometric parallax of RR Lyrae itself (Delta DM0 = +0.04 and +0.06,
respectively, with a scatter of only 0.06).Comment: 11 pages, 9 figures, 6 tables. Accepted for publication in MNRA
Effect of multiplicity of stellar encounters and the diffusion coefficients in the uniform stellar medium: no classical divergence ?
Agekyan lambda-factor that accounts for the effect of multiple distant
encounters with large impact factors is used for the first time to compute the
diffusion coefficients in the velocity space of a stellar system. It is shown
that in this case the cumulative effect - the total contribution of distant
encounters to the change in the velocity of the test star - is finite, and the
logarithmic divergence inherent to the classical description disappears, as
also was earlier noted by Kandrup (1981). At the same time, the formulas for
the diffusion coefficients, as before, contain the logarithm of the ratio of
two independent scale factors that fully characterize the state of the stellar
system: the average interparticle distance and the impact parameter of a close
encounter. However, the physical meaning of this factor is no longer associated
with the classical logarithmic divergence.Comment: 12 pages, 4 figures; Submitted to MNRA
Galactic masers: kinematics, spiral structure and the disk dynamic state
We applied the currently most comprehensive version of the
statistical-parallax technique to derive kinematical parameters of the maser
sample with 136 sources. Our kinematic model comprises the overall rotation of
the Galactic disk and the spiral density-wave effects. We take into account the
variation of radial velocity dispersion with Galactocentric distance. The best
description of the velocity field is provided by the model with constant radial
and vertical velocity dispersions, . We compute flat Galactic rotation curve over the
Galactocentric distance interval from 3 to 15 kpc and find the local circular
rotation velocity to be ~ km/s ~ km/s. We also
determine the parameters of the four-armed spiral pattern (pitch angle and the phase of the Sun ). The radial and tangential spiral perturbations are about ~km/s, ) ~km/s. The
kinematic data yield a solar Galactocentric distance of . Based on rotation curve parameters and the asymmetric drift we
Infer the exponential disk scale ~kpc under
assumption of marginal stability of the intermediate-age disk, and finally we
estimate the minimum local surface disk density, .Comment: 16 pages, 8 figures, 5 table
The Baade-Becker-Wesselink technique and the fundamental astrophysical parameters of Cepheids
The BBW method remains one of most demanded tool to derive full set of
Cepheid astrophysical parameters. Surface brightness version of the BBW
technique was preferentially used during last decades to calculate Cepheid
radii and to improve PLC relations. Its implementation requires a priory
knowledge of Cepheid reddening value. We propose a new version of the
Baade--Becker--Wesselink technique, which allows one to independently determine
the colour excess and the intrinsic colour of a radially pulsating star, in
addition to its radius, luminosity, and distance. It is considered to be a
generalization of the Balona light curve modelling approach. The method also
allows the function F(CI_0) = BC + 10 log Teff for the class of pulsating stars
considered to be calibrated. We apply this technique to a number of classical
Cepheids with very accurate light and radial-velocity curves. The new technique
can also be applied to other pulsating variables, e.g. RR Lyraes. We discuss
also possible dependence of the projection factor on the pulsation phase.Comment: arXiv admin note: substantial text overlap with arXiv:1011.330
RR Lyrae variables: visual and infrared luminosities, intrinsic colours, and kinematics
We use UCAC4 proper motions and WISE W1-band apparent magnitudes
intensity-mean for almost 400 field RR Lyrae variables to determine the
parameters of the velocity distribution of Galactic RR Lyrae population and
constrain the zero points of the metallicity- relation and those of the
period-metallicity--band and period-metallicity--band luminosity
relations via statistical parallax. We find the mean velocities of the halo-
and thick-disc RR Lyrae populations in the solar neighbourhood to be (U0(Halo),
V0(Halo), W0(Halo)) = (-7 +/- 9, -214 +/- 10, -10 +/- 6) km/s and (U0(Disc),
V0(Disc), W0(Disc)) =(-13 +/- 7, -37 +/- 6, -17 +/- 4) km/s, respectively, and
the corresponding components of the velocity-dispersion ellipsoids, (sigma
VR(Halo), sigma Vphi(Halo), sigma Vtheta(Halo)) = (153 +/- 9, 101 +/- 6, 96 +/-
5) km/s and (sigma VR(Disc), sigma Vphi(Disc), sigma Vtheta(Disc)) = (46 +/- 7,
37 +/- 5, 27 +/- 4) km/s, respectively. The fraction of thick-disc stars is
estimated at 0.22 +/- 0.03. The corrected IR period-metallicity-luminosity
relations are = -0.769 +0.088 [Fe/H]- 2.33 mathoprm log PF and =
-0.825 + 0.088 [Fe/H] -2.33 mathoprm log PF, and the optical
metallicity-luminosity relation, [Fe/H]-, is = +1.094 + 0.232 [Fe/H],
with a standard error of +/- 0.089, implying an LMC distance modulus of 18.32
+/- 0.09, a solar Galactocentric distance of 7.73 +/- 0.36 kpc, and the M31 and
M33 distance moduli of DM(M31) = 24.24 +/- 0.09 (D = 705 +/- 30 kpc) and
DM(M33) = 24.36 +/- 0.09 (D = 745 +/- 31 kpc), respectively. Extragalactic
distances calibrated with our RR Lyrae star luminosity scale imply a Hubble
constant of ~80 km/s/Mpc. Our results suggest marginal prograde rotation for
the population of halo RR Lyraes in the Milky Way.Comment: 17 pages, 5 tables, 11 figures. To be published in the Monthly
Notices of the Royal Astronomical Societ
Visualization of radiotracers for SPECT imaging using a Timepix detector with a coded aperture
The work shows the ability to visualize radiotracers used in SPECT with a
system based on a coded aperture mask and a hybrid pixel Timepix detector with
the CdTe sensor. Characterization of the system using X-rays and radioactive
sources confirms that the spatial resolution of less than 1 mm with a field of
view 3 cm x 3 cm can be achieved. The results of a simulation study to
determine the expected spatial resolution of the system in the focal plane for
the various radionuclides is presented. The possibility of using this system
with a thin (1.5 mm) coded aperture mask for reconstructing images of gamma
emitters with the energy up to 180 keV is demonstrated
Star-forming regions at the periphery of the supershell surrounding the Cyg OB1 association. I. The star cluster vdB 130 and its ambient gas and dust medium
Stellar population and the interstellar gas-dust medium in the vicinity of
the open star cluster vdB 130 are analysed using optical observations taken
with the 6-m telescope of the SAO RAS and the 125-cm telescope of the SAI MSU
along with the data of Spitzer and Herschel. Based on proper motions and BV and
JHKs 2MASS photometric data, we select additional 36 stars as probable cluster
members. Some stars in vdB 130 are classified as B stars. Our estimates of
minimum colour excess, apparent distance modulus and the distance are
consistent with young age (from 5 to 10 Myrs) of the cluster vdB 130. We
suppose the large deviations from the conventional extinction law in the
cluster direction, with ~ 4 - 5. The cluster vdB 130 appears to be
physically related to the supershell around Cyg OB1, a cometary CO cloud,
ionized gas, and regions of infrared emission. There are a few regions of
bright mid-infrared emission in the vicinity of vdB 130. The largest of them is
also visible on H-alpha and [SII] emission maps. We suggest that the infrared
blobs that coincide in projection with the head of the molecular cloud are HII
regions, excited by the cluster B-stars. Some signatures of a shock front are
identified between these IR-bright regions.Comment: 17 pages, 14 figures, 2 tables. Accepted by MNRA
Globular clusters: absolute proper motions and Galactic orbits
We cross-match objects from several different astronomical catalogs to
determine the absolute proper proper motions of stars within the 30-arcmin
radius fields of 115 Milky-Way globular clusters with the accuracy of
1--2~mas/yr. The proper motions are based on positional data recovered from the
USNO-B1, 2MASS, URAT1, ALLWISE, UCAC5, and GAIA DR1 surveys with up to 10
positions spanning an epoch difference of up to ~65~years, and reduced to
GAIA DR1 TGAS frame using UCAC5 as the reference catalog. Cluster members are
photometrically identified by selecting horizontal- and red-giant branch stars
on color-magnitude diagrams, and the mean absolute proper motions of the
clusters with a typical formal error of ~0.4~mas/yr are computed by
averaging the proper motions of selected members. The inferred absolute proper
motions of clusters are combined with available radial-velocity data and
heliocentric distance estimates to compute the cluster orbits in terms of the
Galactic potential models based on Miyamoto and Nagai disk, Hernquist spheroid,
and modified isothermal dark-matter halo (axisymmetric model without a bar) and
the same model + rotating Ferre's bar (non-axisymmetric). Five distant clusters
have higher-than-escape velocities, most likely due to large errors of computed
transversal velocities, whereas the computed orbits of all other clusters
remain bound to the Galaxy. Unlike previously published results, we find the
bar to affect substantially the orbits of most of the clusters, even those at
large Galactocentric distances, bringing appreciable chaotization, especially
in the portions of the orbits close to the Galactic center, and stretching out
the orbits of some of the thick-disk clusters.Comment: 14 pages, 14 figures, 3 tables, to appear in Astrophysical Bulletin.
Table 1 replaced by corrected version (Tables 2 and 3 are based on correct
data and remain unchanged
The period-luminosity relation for Cepheids derived from multiphase temperature measurements and Cepheids kinematics based on GAIA DR2 data
Calibration of the period-luminosity relation (PLR) for Cepheids has always
been one of the biggest goals of stellar astronomy. Among a considerable number
of different approaches, the Baade-Becker-Wesselink (BBW) method stands in the
foreground as one of the most universal and precise methods. We present a new
realization of the BBW method which is considered to be the generalization of
surface brightness (\citealt{Barnes+Evans+1976}) and \cite{Balona+1977}
approaches first proposed by \cite{Rastorguev+Dambis+2010} and described in
\cite{Paper1}. One of the main features of this method is using measured
effective temperature variations to determine the main parameters of Cepheid,
such as distance, radius, luminosity, colour excess, intrinsic colour. We apply
this method to 45 Cepheids of Northern sky, for which multiphase temperature
data are available. We take into account the effect of shock waves, whose
presence in stellar atmosphere distorts the observational data and the
calibrations used in this work. Within phase interval we derived PL
relation . It was used to
calculate the distances, rotation curve and kinematical parameters of the
sample of 435 Cepheids with GAIA DR2 proper motions.Comment: 8 pages, 6 figure
Classical Cepheids and the spiral structure of the Milky Way
We use the currently most complete collection of reliable Cepheid positions
(565 stars) out to ~5 kpc based mostly on our photometric data to outline the
spiral pattern of our Galaxy. We find the pitch-angle to be equal to 9--10
degrees with the most accurate estimate (i=9.5 +/-0.1 degrees) obtained
assuming that the spiral pattern has a four-armed structure, and the solar
phase angle in the spiral pattern to be chi_0 = 121+/-3 degrees. The pattern
speed is found to be Omega_P=25.2+/-0.5km/s/kpc based on a comparison of the
positions of the spiral arms delineated by Cepheids and maser sources and the
age difference between these objects.Comment: 21 pages, 11 figures. To appear in Astronomy Letter
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