Abstract
We present a detailed spectroscopic analysis of RR Lyrae (RRL) variables in the globular cluster NGC 5139 (
ω
Cen). We collected optical (4580–5330 Å), high-resolution (
R
∼ 34,000), high ...signal-to-noise ratio (∼200) spectra for 113 RRLs with the multifiber spectrograph M2FS at the
Magellan
/Clay Telescope at Las Campanas Observatory. We also analyzed high-resolution (
R
∼ 26,000) spectra for 122 RRLs collected with FLAMES/GIRAFFE at the Very Large Telescope, available in the ESO archive. The current sample doubles the literature abundances of cluster and field RRLs in the Milky Way based on high-resolution spectra. Equivalent-width measurements were used to estimate atmospheric parameters, iron, and abundance ratios for
α
(Mg, Ca, Ti), iron peak (Sc, Cr, Ni, Zn), and s-process (Y) elements. We confirm that
ω
Cen is a complex cluster, characterized by a large spread in the iron content: −2.58 ≤ Fe/H ≤ −0.85. We estimated the average cluster abundance as
, with
σ
= 0.33 dex. Our findings also suggest that two different RRL populations coexist in the cluster. The former is more metal-poor (Fe/H ≲ − 1.5), with almost solar abundance of Y. The latter is less numerous, more metal-rich, and yttrium enhanced (Y/Fe ≳ 0.4). This peculiar bimodal enrichment only shows up in the s-process element, and it is not observed among lighter elements, whose X/Fe ratios are typical for Galactic globular clusters.
ABSTRACT
We characterized the 67P/Churyumov–Gerasimenko’s dust activity, by analysing individual dust particle velocity and momentum measurements of Grain Impact Analyser and Dust Accumulator ...(GIADA), the dust detector onboard the ESA/Rosetta spacecraft, collecting dust from tens to hundreds of kilometres from the nucleus. Specifically, we developed a procedure to trace back the motion of dust particles down to the nucleus, identifying the surface’s region ejecting each dust particle. This procedure has been developed and validated for the first part of the mission by Longobardo et al. and was extended to the entire GIADA data set in this work. The results based on this technique allowed us to investigate the link between the dust porosity (fluffy/compact) and the morphology of the ejecting surface (rough/smooth). We found that fluffy and compact particles, despite the lack of correlation in their coma spatial distribution (at large nucleocentric distances) induced by their different velocities, have common ejection regions. In particular, the correlation between the distributions of fluffy and compact particles is maintained up to an altitude of about 10 km. Fluffy particles are more abundant in rough terrains. This could be the result of past cometary activity that resurfaced the smooth terrains and/or of the comet formation process that stored the fluffy particles inside the voids between the pebbles. The variation of fluffy particle concentration between rough and smooth terrains agrees with predictions of comet formation models. Finally, no correlation between dust distribution on the nucleus and surface thermal properties was found.
We gathered more than 1130 high-resolution optical spectra for more than 250 Galactic classical Cepheids. The spectra were collected with the optical spectrographs UVES at VLT, HARPS at 3.6 m, FEROS ...at 2.2 m MPG/ESO, and STELLA. To improve the effective temperature estimates, we present more than 150 new line depth ratio (LDR) calibrations that together with similar calibrations already available in the literature allowed us to cover a broad range in wavelength (5348 ≤ λ ≤ 8427 Å) and in effective temperature (3500 ≤ Teff ≤ 7700 K). This gives us the unique opportunity to cover both the hottest and coolest phases along the Cepheid pulsation cycle and to limit the intrinsic error on individual measurements at the level of ~100 K. As a consequence of the high signal-to-noise ratio of individual spectra, we identified and measured hundreds of neutral and ionized lines of heavy elements, and in turn, have the opportunity to trace the variation of both surface gravity and microturbulent velocity along the pulsation cycle. The accuracy of the physical parameters and the number of Fe I (more than one hundred) and Fe II (more than ten) lines measured allowed us to estimate mean iron abundances with a precision better than 0.1 dex. We focus on 14 calibrating Cepheids for which the current spectra cover either the entire or a significant portion of the pulsation cycle. The current estimates of the variation of the physical parameters along the pulsation cycle and of the iron abundances agree very well with similar estimates available in the literature. Independent homogeneous estimates of both physical parameters and metal abundances based on different approaches that can constrain possible systematics are highly encouraged.
ABSTRACT
We present a novel analysis of a young star cluster in the Large Magellanic cloud, R136-like, as seen by the Extremely Large Telescope (ELT). The main aim of this study is to quantify ...precision and accuracy of stellar proper motion measurements in crowded field when using an ELT working at its diffraction limit. This can serve as a reference study for future development of ELT scientific cases. In particular, we investigate our future ability to detect the dynamical signature of intermediate-mass black holes (IMBHs) with mass ∼104 M⊙ through detailed measurements of stellar proper motions. We have simulated two N-body dynamical cluster models with and without an IMBH. For each model, we have chosen two snapshots temporally spaced by 5 yr. Stellar fluxes in IJHK filters and star positions have been used to create ELT mock images for both single- and multiconjugate adaptive optics observing modes following the requierements given by ESO technical specifications for the first light imager. These images have been analysed using a classical software for seeing-limited data reduction, daophot/allstar. We make accurate photometry till the very faint pre-main-sequence stars, i.e. depending on the adaptive optics (AO) mode, magnitudes down to K ∼ 24 mag (single-conjugate AO) or K ∼ 22 mag (multiconjugate AO) in a total integration time of 20 min on target. Although daophot suite of programs is not devoted to precise astrometry, the astrometric accuracy is impressive, reaching few μas yr−1 or km s−1. In these assumptions, we are able to detect the IMBH signature at the centre of the cluster.
The Mass of the Cepheid V350 Sgr Evans, Nancy Remage; Proffitt, Charles; Carpenter, Kenneth G. ...
The Astrophysical journal,
10/2018, Letnik:
866, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
V350 Sgr is a classical Cepheid suitable for mass determination. It has a hot companion which is prominent in the ultraviolet (UV) and which is not itself a binary. We have obtained two ...high-resolution echelle spectra of the companion at orbital velocity maximum and minimum with the Space Telescope Imaging Spectrograph on the
Hubble Space Telescope
in the 1320 to 1510 Å region. By cross-correlating these spectra we obtained the orbital velocity amplitude of the companion with an uncertainty in the companion amplitude of 1.9 km s
−1
. This provides a mass ratio of the Cepheid to the companion of 2.1. The UV energy distribution of the companion provides the mass of the companion, yielding a Cepheid mass of 5.2 ± 0.3
M
⊙
. This mass requires some combination of moderate main sequence core convective overshoot and rotation to match evolutionary tracks.
ABSTRACT
The ESA mission Comet Interceptor will target an Oort or interstellar comet during its first approach to the Sun. Meanwhile, the Vera Rubin LSST Survey will observe hundreds of active comets ...per month beyond 4 au from the Sun, where water vapour pressure is expected to be too low to eject dust. We discuss observations of dust tails at heliocentric distances larger than 4 au in order to retrieve the physical parameters driving cometary activity beyond Jupiter by means of a probabilistic tail model, which is consistent with the activity model defining the gas coma parameters due to the sublimation of carbon monoxide, molecular oxygen, methane, ethane, and carbon dioxide since the activity onset at 85 au from the Sun. We find that: (i) All the observed dust tails are consistent with the adopted activity model; (ii) The tail fits depend on three free parameters only, all correlated to the nucleus size; (iii) Tail fits are always improved by anisotropic dust ejection, suggesting activity of Oort nuclei dominated by seasons; (iv) Inbound seasons suggest cometary activity before the ejection of protocomets into the Oort cloud, as predicted by the activity model; (v) Oort nuclei larger than 1 km may be characterized by a fallout up to ≈100 m thick deposited during ≈60 yr inbound; (vi) On the other side, Oort nuclei smaller than 1 km may appear more pristine than Jupiter Family Comets when observed at 1 au from the Sun.
We collected over 6000 high-resolution spectra of four dozen field RR Lyrae (RRL) variables pulsating either in the fundamental (39 RRab) or in the first overtone (9 RRc) mode. We measured radial ...velocities (RVs) of four strong metallic and four Balmer lines along the entire pulsational cycle and derived RV amplitudes with accuracies better than 1-2 km s−1. The new amplitudes were combined with literature data for 23 RRab and 3 RRc stars (total sample of 74 RRLs), which allowed us to investigate the variation of the Bailey diagram (photometric amplitude versus period) when moving from optical to mid-infrared bands and to recast the Bailey diagram in terms of RV amplitudes. We found that RV amplitudes for RRab are minimally affected by nonlinear phenomena (shocks) and multiperiodicity (Blazhko effect). The RV slope (logP-A(Vr)) when compared with the visual slope (logP-A(V)) is shallower, and the dispersion, at fixed period, decreases by a factor of two. We constructed homogeneous sets of horizontal branch evolutionary models and nonlinear, convective pulsation models of RRLs to constrain the impact of evolutionary effects on their pulsation properties. Evolution causes, on the Bailey diagram based on RV amplitudes, a modest variation in pulsation period and a large dispersion in amplitude. The broad dispersion in period of the Bailey diagram is mainly caused by variation in RRL intrinsic parameters (stellar mass, chemical composition). Empirical evidence indicates that RV amplitudes are an optimal diagnostic for tracing the mean effective temperature across the RRab instability strip.
Context. A robust classification of Cepheids into their different sub-classes and, in particular, between classical and Type II Cepheids, is necessary to properly calibrate the period-luminosity ...relations and for populations studies in the Galactic disc. Type II Cepheids are, however, very diverse, and classifications based either on intrinsic (period, light curve) or external parameters (e.g., Fe/FH, z ) do not provide a unique classification. Aims. We want to ascertain the classification of two Cepheids, HQ Car and DD Vel, that are sometimes classified as classical Cepheids and sometimes as Type II Cepheids. Methods. To achieve this goal, we examine both their chemical composition and the presence of specific features in their spectra. Results. We find emission features in the Ho and in the 5875.64 Angstrom He I lines that are typical of W Vir stars. The Na/Fe (or Na/Zn) abundances are typical of thick-disc stars, while BL Her stars are Na-overabundant (Na/Fe > +0.5 dex). Finally, the two Cepheids show a possible (HQ Car) or probable (DD Vel) signature of mild dust-gas separation that is usually observed only in long-period type II Cepheids and RV Tau stars. Conclusions. These findings clearly indicate that HQ Car and DD Vel are both Type II Cepheids from the W Vir sub-class. Several studies have reported an increase in the Cepheids' abundance dispersion towards the outer (thin) disc. A detailed inspection of the Cepheid classification, in particular for those located in the outer disc, will indicate whether this feature is real or simply an artefact of the inclusion of type II Cepheids belonging to the thick disc in the current samples.
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