101 research outputs found
Studying the Variation of the Fine Structure Constant Using Emission Line Multiplets
As an extension of the method by Bahcall et al. (2004) to investigate the
time dependence of the fine structure constant, we describe an approach based
on new observations of forbidden line multiplets from different ionic species.
We obtain optical spectra of fine structure transitions in [Ne III], [Ne V], [O
III], [OI], and [SII] multiplets from a sample of 14 Seyfert 1.5 galaxies in
the low-z range 0.035 < z < 0.281. Each source and each multiplet is
independently analyzed to ascertain possible errors. Averaging over our sample,
we obtain a conservative value alpha^2(t)/\alpha^2(0) = 1.0030+-0.0014.
However, our sample is limited in size and our fitting technique simplistic as
we primarily intend to illustrate the scope and strengths of emission line
studies of the time variation of the fine structure constant. The approach can
be further extended and generalized to a "many-multiplet emission line method"
analogous in principle to the corresponding method using absorption lines. With
that aim, we note that the theoretical limits on emission line ratios of
selected ions are precisely known, and provide well constrained selection
criteria. We also discuss several other forbidden and allowed lines that may
constitute the basis for a more rigorous study using high-resolution
instruments on the next generation of 8 m class telescopes.Comment: 20 pages, 4 figures, sumbitted to A
A vigorous activity cycle mimicking a planetary system in HD200466
Stellar activity can be a source of radial velocity (RV) noise and can
reproduce periodic RV variations similar to those produced by an exoplanet. We
present the vigorous activity cycle in the primary of the visual binary
HD200466, a system made of two almost identical solar-type stars with an
apparent separation of 4.6 arcsec at a distance of 44+/-2 pc. High precision RV
over more than a decade, adaptive optics (AO) images, and abundances have been
obtained for both components. A linear trend in the RV is found for the
secondary. We assumed that it is due to the binary orbit and once coupled with
the astrometric data, it strongly constrains the orbital solution of the binary
at high eccentricities (e~0.85) and quite small periastron of ~21 AU. If this
orbital motion is subtracted from the primary radial velocity curve, a highly
significant (false alarm probability <0.1%) period of about 1300 d is obtained,
suggesting in a first analysis the presence of a giant planet, but it turned
out to be due to the stellar activity cycle. Since our spectra do not include
the Ca~II resonance lines, we measured a chromospheric activity indicator based
on the Halpha line to study the correlation between activity cycles and
long-term activity variations. While the bisector analysis of the line profile
does not show a clear indication of activity, the correlation between the
Halpha line indicator and the RV measurements identify the presence of a strong
activity cycle.Comment: Accepted on Astronomy and Astrophysics Main Journal 2014, 16 pages,
18 figure
Metal abundances of RR Lyrae stars in the metal rich globular cluster NGC 6441
Low resolution spectra have been used to measure individual metal abundances
of RR Lyrae stars in NGC 6441, a Galactic globular cluster known to have very
unusual horizontal branch morphology and periods of the RR Lyrae stars for its
high metallicity. We find an average metal abundance of [Fe/H]=-0.69 +/- 0.06
(r.m.s.=0.33 dex) and [Fe/H]=-0.41 +/- 0.06 (r.m.s.=0.36 dex) on Zinn & West
and Carretta & Gratton metallicity scales, respectively, consistent with the
cluster metal abundance derived by Armandroff & Zinn. Most of the metallicities
were extrapolated from calibration relations defined for [Fe/H] < -1; however,
they are clearly high and contrast with the rather long periods of the NGC 6441
variables, thus confirming that the cluster does not fit in the general
Oosterhoff classification scheme. The r.m.s. scatter of the average is larger
than observational errors (0.15-0.16 dex) possibly indicating some spread in
metallicity. However, even the metal poor variables, if confirmed to be cluster
members, are still more metal rich than those commonly found in the Oosterhoff
type II globular clusters.Comment: Accepted for publication on ApJ Letter
Abundance differences between components of wide binaries
In this paper we present iron abundance analysis for 23 wide binaries with
main sequence components in the temperture range 4900-6300 K, taken from the
sample of the pairs currently included in the radial velocity planet search on
going at the TNG. The use of a line-by-line differential analysis technique
between the components of each pair allows to reach errors of about 0.02 dex in
the iron content difference. Most of the pairs have abundance differences lower
than 0.02 dex and there are no pairs with differences larger than 0.07 dex. The
four cases of differences larger than 0.02 dex may be spouriuos because of the
larger error bars affecting pairs with large temperature difference, cold stars
and rotating stars. The pair HD 219542, previously reported by us to have
different composition, results instead normal. For non-rotating stars warmer
than 5500 K, we are able to exclude in most cases the ingestion of more than 1
Earth Mass of iron (about 5 Earth masses of meteoritic material) during the
main sequence lifetime of the stars, placing more stringent limits (about 0.4
Earth masses of iron) in five cases of warm stars. This latter limit is similar
to the estimates of rocky material accreted by the Sun during its main sequence
lifetime. Combining the results of the present analysis with those for the
Hyades and Pleiades, we conclude that the hypothesis that pollution by
planetary material is the only responsible for the highest metallicity of the
stars with planets may be rejected at more than 99% level of confidence if the
incidence of planets in these samples is as high as 8% and similar to the field
stars included in current radial velocity surveys. However, the significance of
this result drops considerably if the incidence of planets around stars in
binary systems and clusters is less than a half than around normal field stars.Comment: 14 pages, 10 figures, A&A, in press, minor changes (language editing,
typos
The Fine-structure Constant as a Probe of Chemical Evolution and AGB Nucleosynthesis in Damped Lyman-alpha Systems
Evidence from a large sample of quasar absorption-line spectra in damped
Lyman-alpha systems has suggested a possible time variation of the fine
structure constant alpha. The most statistically significant portion of this
sample involves the comparison of Mg and Fe wavelength shifts using the
many-multiplet (MM) method. However, the sensitivity of this method to the
abundance of heavy isotopes, especially Mg, is enough to imitate an apparent
variation in alpha in the redshift range 0.5 < z < 1.8. We implement recent
yields of intermediate mass (IM) stars into a chemical evolution model and show
that the ensuing isotope distribution of Mg can account for the observed
variation in alpha provided the early IMF was particularly rich in intermediate
mass stars (or the heavy Mg isotope yields from AGB stars are even higher than
in present-day models). As such, these observations of quasar absorption
spectra can be used to probe the nucleosynthetic history of low-metallicity
damped Lyman-alpha systems in the redshift range 0.5 < z < 1.8. This analysis,
in conjunction with other abundance measurements of low-metallicity systems,
reinforces the mounting evidence that star formation at low metallicities may
have been strongly influenced by a population of IM stars. Such IM stars have a
significant influence on other abundances, particularly nitrogen. We constrain
our models with independent measurements of N, Si, and Fe in damped Lyman-alpha
systems as well as C/O in low-metallicity stars. In this way, we obtain
consistent model parameters for this chemical-evolution interpretation of the
MM method results.Comment: 38 pages, latex, 25 ps figures, as accepted in Ap
The HADES RV Programme with HARPS-N at TNG XI. GJ 685 b: a warm super-Earth around an active M dwarf
Small rocky planets seem to be very abundant around low-mass M-type stars.
Their actual planetary population is however not yet precisely understood.
Currently several surveys aim to expand the statistics with intensive detection
campaigns, both photometric and spectroscopic. We analyse 106 spectroscopic
HARPS-N observations of the active M0-type star GJ 685 taken over the past five
years. We combine these data with photometric measurements from different
observatories to accurately model the stellar rotation and disentangle its
signals from genuine Doppler planetary signals in the RV data. We run an MCMC
analysis on the RV and activity indexes time series to model the planetary and
stellar signals present in the data, applying Gaussian Process regression
technique to deal with the stellar activity signals. We identify three periodic
signals in the RV time series, with periods of 9, 24, and 18 d. Combining the
analyses of the photometry of the star with the activity indexes derived from
the HARPS-N spectra, we identify the 18 d and 9 d signals as activity-related,
corresponding to the stellar rotation period and its first harmonic
respectively. The 24 d signals shows no relations with any activity proxy, so
we identify it as a genuine planetary signal. We find the best-fit model
describing the Doppler signal of the newly-found planet, GJ 685\,b,
corresponding to an orbital period d and a
minimum mass M. We also study a
sample of 70 RV-detected M-dwarf planets, and present new statistical evidence
of a difference in mass distribution between the populations of single- and
multi-planet systems, which can shed new light on the formation mechanisms of
low-mass planets around late-type stars.Comment: 18 pages, 13 figures, accepted for publication in A&
The HADES RV Programme with HARPS-N@TNG II. Data treatment and simulations
The distribution of exoplanets around low-mass stars is still not well
understood. Such stars, however, present an excellent opportunity of reaching
down to the rocky and habitable planet domains. The number of current
detections used for statistical purposes is still quite modest and different
surveys, using both photometry and precise radial velocities, are searching for
planets around M dwarfs. Our HARPS-N red dwarf exoplanet survey is aimed at the
detection of new planets around a sample of 78 selected stars, together with
the subsequent characterization of their activity properties. Here we
investigate the survey performance and strategy. From 2700 observed spectra, we
compare the radial velocity determinations of the HARPS-N DRS pipeline and the
HARPS-TERRA code, we calculate the mean activity jitter level, we evaluate the
planet detection expectations, and we address the general question of how to
define the strategy of spectroscopic surveys in order to be most efficient in
the detection of planets. We find that the HARPS-TERRA radial velocities show
less scatter and we calculate a mean activity jitter of 2.3 m/s for our sample.
For a general radial velocity survey with limited observing time, the number of
observations per star is key for the detection efficiency. In the case of an
early M-type target sample, we conclude that approximately 50 observations per
star with exposure times of 900 s and precisions of about 1 m/s maximizes the
number of planet detections
The GAPS Programme with HARPS-N at TNG XV. A substellar companion around a K giant star identified with quasi-simultaneous HARPS-N and GIANO measurements
Context. Identification of planetary companions of giant stars is made
difficult because of the astrophysical noise, that may produce radial velocity
(RV) variations similar to those induced by a companion. On the other hand any
stellar signal is wavelength dependent, while signals due to a companion are
achromatic. Aims. Our goal is to determine the origin of the Doppler periodic
variations observed in the thick disk K giant star TYC 4282-605-1 by HARPS-N at
the Telescopio Nazionale Galileo (TNG) and verify if they can be due to the
presence of a substellar companion. Methods. Several methods have been used to
exclude the stellar origin of the observed signal including detailed analysis
of activity indicators and bisector and the analysis of the photometric light
curve. Finally we have conducted an observational campaign to monitor the near
infrared (NIR) RV with GIANO at the TNG in order to verify whether the NIR
amplitude variations are comparable with those observed in the visible.
Results. Both optical and NIR RVs show consistent variations with a period at
101 days and similar amplitude, pointing to the presence of a companion
orbiting the target. The main orbital properties obtained for our giant star
with a derived mass of M=0.97+-0.03M_sun are
M_Psini=10.78+-0.12MJ;P=101.54+-0.05days;e=0.28+-0.01 and a=0.422+-0.009AU. The
chemical analysis shows a significant enrichment in the abundance of Nai, Mgi,
Ali and S i while the rest of analyzed elements are consistent with the solar
value demonstrating that the chemical composition corresponds with an old K
giant (age = 10.1 Gyr) belonging to local thick disk. Conclusions. We conclude
that the substellar companion hypothesis for this K giant is the best
explanation for the observed periodic RV variation. This study also shows the
high potential of multi-wavelength RV observations for the validation of planet
candidates.Comment: Accepted in Journal reference A&A 14/06/201
- …