84 research outputs found
A New Reddening Law for M4
We have used a combination of broad-band near-infrared and optical
Johnson-Cousins photometry to study the dust properties in the line of sight to
the Galactic globular cluster M4. We have investigated the reddening effects in
terms of absolute strength and variation across the cluster field, as well as
the form of the reddening law defined by the type of dust. Here, we determine
the ratio of absolute to selective extinction (R_V) in the line of sight
towards M4, which is known to be a useful indicator for the type of dust and
therefore characterizes the applicable reddening law. Our method is independent
of age assumptions and appears to be significantly more precise and accurate
than previous approaches. We obtain A_V/E(B-V)=3.76\pm0.07 (random error) for
the dust in the line of sight to M4 for our set of filters. With this value,
the distance to M4 is found to be 1.80\pm0.05 kpc (random error). A reddening
map for M4 has been created, which reveals a spatial differential reddening of
delta E(B-V)>0.2 mag across a field within 10' around the cluster centre and a
total mean reddening of E(B-V)=0.37\pm0.01. In order to provide accurate zero
points for the extinction coefficients of our photometric filters, we
investigated the impact of stellar parameters such as temperature, surface
gravity and metallicity on the extinction properties in different bandpasses.
Using both synthetic ATLAS9 spectra and observed spectral energy distributions,
we found similar sized effects for the range of temperature, surface gravity,
and metallicity typical of globular cluster stars: each causes a change of
about 3% in the necessary correction factor for each filter combination. From
our calculations, we provide extinction zero points for Johnson-Cousins and
2MASS filters, spanning a wide range of stellar parameters and dust types,
suited for accurate, object-specific extinction corrections.Comment: 24 pages, 19 figures, 13 table
Variable stars in the ultra-faint dwarf spheroidal galaxy Ursa Major I
We have performed the first study of the variable star population of Ursa
Major I (UMa I), an ultra-faint dwarf satellite recently discovered around the
Milky Way by the Sloan Digital Sky Survey. Combining time series observations
in the B and V bands from four different telescopes, we have identified seven
RR Lyrae stars in UMa I, of which five are fundamental-mode (RRab) and two are
first-overtone pulsators (RRc). Our V, B-V color-magnitude diagram of UMa I
reaches V~23 mag (at a signal-to-noise ratio of ~ 6) and shows features typical
of a single old stellar population. The mean pulsation period of the RRab stars
= 0.628, {\sigma} = 0.071 days (or = 0.599, {\sigma} = 0.032 days,
if V4, the longest period and brightest variable, is discarded) and the
position on the period-amplitude diagram suggest an Oosterhoff-intermediate
classification for the galaxy. The RR Lyrae stars trace the galaxy horizontal
branch at an average apparent magnitude of = 20.43 +/- 0.02 mag
(average on 6 stars and discarding V4), giving in turn a distance modulus for
UMa I of (m-M)0 = 19.94 +/- 0.13 mag, distance d= 97.3 +6.0/-5.7 kpc, in the
scale where the distance modulus of the Large Magellanic Cloud is 18.5 +/- 0.1
mag. Isodensity contours of UMa I red giants and horizontal branch stars
(including the RR Lyrae stars identified in this study) show that the galaxy
has an S-shaped structure, which is likely caused by the tidal interaction with
the Milky Way. Photometric metallicities were derived for six of the UMa I RR
Lyrae stars from the parameters of the Fourier decomposition of the V-band
light curves, leading to an average metal abundance of [Fe/H] = -2.29 dex
({\sigma} = 0.06 dex, average on 6 stars) on the Carretta et al. metallicity
scale.Comment: Accepted for publication in Ap
Pulsating Variable Stars in the Coma Berenices dwarf spheroidal galaxy
We present B, V, I time-series photometry of the Coma Berenices dwarf
spheroidal galaxy, a faint Milky Way satellite, recently discovered by the
Sloan Digital Sky Survey. We have obtained V, B-V and V, V-I color-magnitude
diagrams that reach V~23.0-23.2 mag showing the galaxy turnoff at V~21.7 mag,
and have performed the first study of the variable star population of this new
Milky Way companion. Two RR Lyrae stars (a fundamental-mode -RRab- and a first
overtone -RRc- pulsator) and a short period variable with period P=0.12468 days
were identified in the galaxy. The RRab star has a rather long period of
P_ab=0.66971 days and is about 0.2 mag brighter than the RRc variable and other
non-variable stars on the galaxy horizontal branch. In the period-amplitude
diagram the RRab variable falls closer to the loci of Oosterhoff type-II
systems and evolved fundamental-mode RR Lyrae stars in the Galactic globular
cluster M3. The average apparent magnitude of the galaxy horizontal branch,
=18.64+-0.04 mag, leads to a distance modulus for the Coma dSph
mu_0=18.13+-0.08 mag, corresponding to a distance d=42^{+2}_{-1} kpc, by
adopting a reddening E(B-V) = 0.045 +- 0.015 mag and a metallicity [Fe/H]=-2.53
+- 0.05 dex.Comment: 14 pages, 3 figures, Accepted for publication in ApJ
Observations of field and cluster RR Lyræ with Spitzer. Towards high precision distances with Population II stellar tracers
Indexación: Scopus.We present our project to calibrate the RR Lyræ period-luminosity-metallicity
relation using a sample of Galactic calibrators in the halo and globular clusters.https://www.epj-conferences.org/articles/epjconf/pdf/2017/21/epjconf_puls2017_07004.pd
Dark halo structure in the Carina dwarf spheroidal galaxy: joint analysis of multiple stellar components
Photometric and spectroscopic observations of the Carina dSph revealed that
this galaxy contains two dominant stellar populations of different age and
kinematics. The co-existence of multiple populations provides new constraints
on the dark halo structure of the galaxy, because different populations should
be in equilibrium in the same dark matter potential well. We develop
non-spherical dynamical models including such multiple stellar components and
attempt to constrain the properties of the non-spherical dark halo of Carina.
We find that Carina probably has a larger and denser dark halo than found in
previous works and a less cuspy inner dark matter density profile, even though
the uncertainties of dark halo parameters are still large due to small volume
of data sample. Using our fitting results, we evaluate astrophysical factors
for dark matter annihilation and decay and find that Carina should be one of
the most promising detectable targets among classical dSph galaxies. We also
calculate stellar velocity anisotropy profiles for both stellar populations and
find that they are both radially anisotropic in the inner regions, while in the
outer regions the older population becomes more tangentially biased than the
intermediate one. This is consistent with the anisotropy predicted from tidal
effects on the dynamical structure of a satellite galaxy and thereby can be
considered as kinematic evidence for the tidal evolution of Carina.Comment: 14 pages, 7 figures and 3 tables. Accepted for publication in MNRA
Supersolar Ni/Fe production in the Type IIP SN 2012ec
SN 2012ec is a Type IIP supernova (SN) with a progenitor detection and
comprehensive photospheric-phase observational coverage. Here, we present Very
Large Telescope and PESSTO observations of this SN in the nebular phase. We
model the nebular [O I] 6300, 6364 lines and find their strength to suggest a
progenitor main-sequence mass of 13-15 Msun. SN 2012ec is unique among
hydrogen-rich SNe in showing a distinct and unblended line of stable nickel [Ni
II] 7378. This line is produced by 58Ni, a nuclear burning ash whose abundance
is a sensitive tracer of explosive burning conditions. Using spectral synthesis
modelling, we use the relative strengths of [Ni II] 7378 and [Fe II] 7155 (the
progenitor of which is 56Ni) to derive a Ni/Fe production ratio of 0.20pm0.07
(by mass), which is a factor 3.4pm1.2 times the solar value. High production of
stable nickel is confirmed by a strong [Ni II] 1.939 micron line. This is the
third reported case of a core-collapse supernova producing a Ni/Fe ratio far
above the solar value, which has implications for core-collapse explosion
theory and galactic chemical evolution models.Comment: Published versio
Shapley Supercluster Survey: Construction of the photometric catalogues and i-band data release
The Shapley Supercluster Survey is a multi-wavelength survey covering an area of ~23 deg2 (~260 Mpc2 at z = 0.048) around the supercluster core, including nine Abell and two poor clusters, having redshifts in the range 0.045-0.050. The survey aims to investigate the role of the cluster-scale mass assembly on the evolution of galaxies, mapping the effects of the environment from the cores of the clusters to their outskirts and along the filaments. The optical (ugri) imaging acquired with OmegaCAM on the VLT Survey Telescope is essential to achieve the project goals providing accurate multi-band photometry for the galaxy population down to m * + 6. We describe the methodology adopted to construct the optical catalogues and to separate extended and point-like sources. The catalogues reach average 5s limitingmagnitudes within a 3 arcsec diameter aperture of ugri=[24.4,24.6,24.1,23.3] and are 93 per cent complete down to ugri = [23.8,23.8,23.5,22.0] mag, corresponding to ~mr * + 8.5. The data are highly uniform in terms of observing conditions and all acquired with seeing less than 1.1 arcsec full width at half-maximum. The median seeing in r band is 0.6 arcsec, corresponding to 0.56 kpc h70 -1 at z = 0.048. While the observations in the u, g and r bands are still ongoing, the i-band observations have been completed, and we present the i-band catalogue over the whole survey area. The latter is released and it will be regularly updated, through the use of the Virtual Observatory tools. This includes 734 319 sources down to i = 22.0 mag and it is the first optical homogeneous catalogue at such a depth, covering the central region of the Shapley supercluster
The Araucaria Project. The Distance to the Sculptor dwarf spheroidal galaxy from infrared photometry of RR Lyrae stars
We have obtained single-phase near-infrared magnitudes in the J and K bands
for a sample of 78 RR Lyrae stars in the Sculptor dSph galaxy. Applying
different theoretical and empirical calibrations of the
period-luminosity-metallicity relation for RR Lyrae stars in the infrared, we
find consistent results and obtain a true, reddening-corrected distance modulus
of 19.67 0.02 (statistical) 0.12 (systematic) mag for Sculptor from
our data. This distance value is consistent with the value of 19.68 0.08
mag which we obtain from earlier V-band data of RR Lyrae stars in Sculptor, and
the V magnitude-metallicity calibration of Sandage (1993). It is also in a very
good agreement with the results obtain by Rizzi (2002) based on tip of the red
giant branch (TRGB, 19.64 0.08 mag) and horizontal branch (HB, 19.66
0.15 mag).Comment: AJ in pres
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