In this fourth part of the series presenting the Optical Gravitational Lensing Experiment (OGLE) microlensing studies of the dark matter halo compact objects (MACHOs), we describe results of the ...OGLE-III monitoring of the Small Magellanic Cloud (SMC). Three sound candidates for microlensing events were found and yielded the optical depth τSMC-OIII= 1.30 ± 1.01 × 10−7, consistent with the expected contribution from Galactic disc and SMC self-lensing. We report that event OGLE-SMC-03 is most likely a thick-disc lens candidate, the first of such type found towards the SMC. In this paper we also combined all OGLE Large Magellanic Cloud and SMC microlensing results in order to refine the conclusions on MACHOs. All but one of the OGLE events are most likely caused by the lensing by known populations of stars; therefore, we concluded that there is no need for introducing any special dark matter compact objects in order to explain the observed event rates. Potential black hole event indicates that similar lenses can contribute only about 2 per cent to the total mass of the halo, which is still in agreement with the expected number of such objects.
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ABSTRACT Low-resolution (4.5-5 ) spectra of 58 blue supergiant stars distributed over the disk of the Magellanic spiral galaxy NGC 55 in the Sculptor group are analyzed by means of non-LTE techniques ...to determine stellar temperatures, gravities, and metallicities (from iron peak and -elements). A metallicity gradient of −0.22 0.06 dex/R25 is detected. The central metallicity on a logarithmic scale relative to the Sun is Z = −0.37 0.03. A chemical evolution model using the observed distribution of column densities of the stellar and interstellar medium gas mass reproduces the observed metallicity distribution well and reveals a recent history of strong galactic mass accretion and wind outflows with accretion and mass-loss rates of the order of the star formation rate. There is an indication of spatial inhomogeneity in metallicity. In addition, the relatively high central metallicity of the disk confirms that two extraplanar metal-poor H ii regions detected in previous work 1.13 to 2.22 kpc above the galactic plane are ionized by massive stars formed in situ outside the disk. For a subsample of supergiants, for which Hubble Space Telescope photometry is available, the flux-weighted gravity-luminosity relationship is used to determine a distance modulus of 26.85 0.10 mag.
High-quality spectra of 90 blue supergiant stars in the Large Magellanic Cloud are analyzed with respect to effective temperature, gravity, metallicity, reddening, extinction, and extinction law. An ...average metallicity, based on Fe and Mg abundances, relative to the Sun of Z = −0.35 0.09 dex is obtained. The reddening distribution peaks at = 0.08 mag, but significantly larger values are also encountered. A wide distribution of the ratio of extinction to reddening is found ranging from = 2 to 6. The results are used to investigate the blue supergiant relationship between flux-weighted gravity, gf g/ , and absolute bolometric magnitude Mbol. The existence of a tight relationship, the Flux-weighted Gravity-Luminosity Relationship (FGLR), is confirmed. However, in contrast to previous work, the observations reveal that the FGLR is divided into two parts with a different slope. For flux-weighted gravities larger than 1.30 dex, the slope is similar to that found in previous work, but the relationship becomes significantly steeper for smaller values of the flux-weighted gravity. A new calibration of the FGLR for extragalactic distance determinations is provided.
Abstract We present the results from a complex study of an eclipsing O-type binary (Aa+Ab) with the orbital period of P A = 3.2254367 days that forms part of a higher-order multiple system in a ...configuration of (A+B)+C. We derived masses of the Aa+Ab binary of M 1 = 19.02 ± 0.12 and M 2 = 17.50 ± 0.13 M ⊙ , the radii of R 1 = 7.70 ± 0.05 and R 2 = 6.64 ± 0.06 R ⊙ , and temperatures of T 1 = 34,250 ± 500 K and T 2 = 33,750 ± 500 K. From the analysis of the radial velocities, we found a spectroscopic orbit of A in the outer A+B system with P A+B = 195.8 days ( P A+B / P A ≈ 61). In the O − C analysis, we confirmed this orbit and found another component orbiting the A+B system with P AB+C = 2550 days ( P AB+C / P A+B ≈ 13). From the total mass of the inner binary and its outer orbit, we estimated the mass of the third object, M B ≳ 10.7 M ⊙ . From the light travel time effect fit to the O − C data, we obtained the limit for the mass of the fourth component, M C ≳ 7.3 M ⊙ . These extra components contribute about 20%–30% (increasing with wavelength) to the total system light. From the comparison of model spectra with the multiband photometry, we derived a distance modulus of 18.59 ± 0.06 mag, a reddening of 0.16 ± 0.02 mag, and an R V of 3.2. This work is part of our ongoing project, which aims to calibrate the surface brightness–color relation for early-type stars.
Most known extrasolar planets (exoplanets) have been discovered using the radial velocity or transit methods. Both are biased towards planets that are relatively close to their parent stars, and ...studies find that around 17-30% (refs 4, 5) of solar-like stars host a planet. Gravitational microlensing, on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing. These planets are at least as numerous as the stars in the Milky Way. Here we report a statistical analysis of microlensing data (gathered in 2002-07) that reveals the fraction of bound planets 0.5-10 AU (Sun-Earth distance) from their stars. We find that 17(+6)(-9)% of stars host Jupiter-mass planets (0.3-10 M(J), where M(J) = 318 M(⊕) and M(⊕) is Earth's mass). Cool Neptunes (10-30 M(⊕)) and super-Earths (5-10 M(⊕)) are even more common: their respective abundances per star are 52(+22)(-29)% and 62(+35)(-37)%. We conclude that stars are orbited by planets as a rule, rather than the exception.
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The instability strip (IS) of classical Cepheids has been extensively studied theoretically. Comparing the theoretical IS edges with those obtained empirically, using the most recent Cepheids ...catalogs available, can provide us with insights into the physical processes that determine the position of the IS boundaries. We aim to investigate the empirical positions of the IS of the classical Cepheids in the Large Magellanic Cloud (LMC), considering any effect that increases its width, to obtain intrinsic edges that can be compared with theoretical models. We used data of classical fundamental-mode (F) and first-overtone (1O) LMC Cepheids from the OGLE-IV variable star catalog, together with a recent high-resolution reddening map from the literature. Our final sample includes 2058 F and 1387 1O Cepheids. We studied their position on the Hertzsprung-Russell diagram and determined the IS borders by tracing the edges of the color distribution along the strip. We obtained the blue and red edges of the IS in V- and I-photometric bands, in addition to $ eff $ and $ L$. The results obtained show a break located at the Cepheids' period of about three days, which was not reported before. We compared our empirical borders with theoretical ones published in the literature, obtaining a good agreement for specific parameter sets. The break in the IS borders is most likely explained by the depopulation of second- and third-crossing classical Cepheids in the faint part of the IS, since blue loops of evolutionary tracks in this mass range do not extend blueward enough to cross the IS at the LMC metallicity. Results from the comparison of our empirical borders with theoretical ones prove that our empirical IS is a useful tool for constraining theoretical models.
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Abstract
We present K-band Multi-Object Spectrograph (KMOS) observations of 18 red supergiant (RSG) stars in the Sculptor Group galaxy NGC 55. Radial velocities are calculated and are shown to be in ...good agreement with previous estimates, confirming the supergiant nature of the targets and providing the first spectroscopically confirmed RSGs in NGC 55. Stellar parameters are estimated for 14 targets using the J-band analysis technique, making use of state-of-the-art stellar model atmospheres. The metallicities estimated confirm the low-metallicity nature of NGC 55, in good agreement with previous studies. This study provides an independent estimate of the metallicity gradient of NGC 55, in excellent agreement with recent results published using hot massive stars. In addition, we calculate luminosities of our targets and compare their distribution of effective temperatures and luminosities to that of other RSGs, in different environments, estimated using the same technique.
Context.
The Baade-Wesselink (BW) method, which combines linear and angular diameter variations, is the most common method to determine the distances to pulsating stars. However, the projection ...factor,
p
-factor, used to convert radial velocities into pulsation velocities, is still poorly calibrated. This parameter is critical on the use of this technique, and often leads to 5–10% uncertainties on the derived distances.
Aims.
We focus on empirically measuring the
p
-factor of a homogeneous sample of 29 LMC and 10 SMC Cepheids for which an accurate average distances were estimated from eclipsing binary systems.
Methods.
We used the SPIPS algorithm, which is an implementation of the BW technique. Unlike other conventional methods, SPIPS combines all observables, i.e. radial velocities, multi-band photometry and interferometry into a consistent physical modelling to estimate the parameters of the stars. The large number and their redundancy insure its robustness and improves the statistical precision.
Results.
We successfully estimated the
p
-factor of several Magellanic Cloud Cepheids. Combined with our previous Galactic results, we find the following
P
−
p
relation: −0.08
± 0.04
(log
P
−1.18) + 1.24
± 0.02
. We find no evidence of a metallicity dependent
p
-factor. We also derive a new calibration of the period-radius relation, log
R
= 0.684
± 0.007
(log
P
−0.517) + 1.489
± 0.002
, with an intrinsic dispersion of 0.020. We detect an infrared excess for all stars at 3.6
μ
m and 4.5
μ
m, which might be the signature of circumstellar dust. We measure a mean offset of Δ
m
3.6
= 0.057 ± 0.006 mag and Δ
m
4.5
= 0.065 ± 0.008 mag.
Conclusions.
We provide a new
P
−
p
relation based on a multi-wavelength fit that can be used for the distance scale calibration from the BW method. The dispersion is due to the LMC and SMC width we took into account because individual Cepheids distances are unknown. The new
P
−
R
relation has a small intrinsic dispersion: 4.5% in radius. This precision will allow us to accurately apply the BW method to nearby galaxies. Finally, the infrared excesses we detect again raise the issue of using mid-IR wavelengths to derive period-luminosity relation and to calibrate the Hubble constant. These IR excesses might be the signature of circumstellar dust, and are never taken into account when applying the BW method at those wavelengths. Our measured offsets may give an average bias of ~ 2.8% on the distances derived through mid-IR
P
−
L
relations.
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The availability of updated model atmospheres for red supergiants and improvements in single and binary stellar evolution models, together with previously unpublished data, prompted us to revisit the ...progenitor of the low-luminosity type II-plateau supernova (type IIP SN), SN 2008bk. Using mid-infrared (mid-IR) data in combination with dust models, we find that high-temperature (4250−4500 K), high extinction (
E
(
B
−
V
)> 0.7) solutions are incompatible with the data. We therefore favour a cool (∼3500−3700 K) progenitor with a luminosity of log(
L
/
L
⊙
) ∼ 4.53. Comparing with evolutionary tracks, we infer progenitor masses in the 8–10
M
⊙
range in agreement with some previous studies. This mass is consistent with the observed pattern of low-luminosity type IIP SNe coming from the explosion of red supergiant stars (RSGs) at the lower extremum for core collapse. We also present multi-epoch data for the progenitor, but do not find clear evidence of variability.
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Aims.
The surface brightness–colour relation (SBCR) is a basic tool for establishing precise and accurate distances within the Local Group. Detached eclipsing binary stars with accurately determined ...radii and trigonometric parallaxes allow calibration of the SBCRs with unprecedented accuracy.
Methods.
We analysed four nearby eclipsing binary stars containing late F-type main sequence components: AL Ari, AL Dor, FM Leo, and BN Scl. We determined very precise spectroscopic orbits and combined them with high-precision ground- and space-based photometry. We derived the astrophysical parameters of their components with mean errors of 0.1% for mass and 0.4% for radius. We combined those four systems with another 24 nearby eclipsing binaries with accurately known radii from the literature for which
Gaia
EDR3 parallaxes are available in order to derive the SBCRs.
Results.
The resulting SBCRs cover stellar spectral types from B9 V to G7 V. For calibrations, we used Johnson optical
B
and
V
,
Gaia
G
BP
and
G
, and 2MASS
J
H
K
bands. The most precise relations are calibrated using the infrared
K
band and allow angular diameters of A-, F-, and G-type dwarf and subgiant stars to be predicted with a precision of 1%.
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