Aims.
Relying on recently available and very precise observational data for the Ai Phe binary system, we explore the robustness of the calibration of stellar models achievable with this system.
...Methods.
We adopt the SCEPtER pipeline with a fitting grid of stellar models computed for different initial chemical compositions and convective core overshooting efficiencies. We investigated the impact of different assumptions about the surface efficiency of microscopic diffusion, whose efficiency is still debated in the mass range of the system. We obtained the fit of this system adopting two alternative scenarios. In the reference scenario, we allowed modification of the surface metallicity due to microscopic diffusion, while in the alternative scenario we assumed that competing mixing from other sources cancels out this effect.
Results.
Due to the fact that the primary star has already experienced the first dredge-up while the secondary has not, the tested scenarios show interesting differences. While the estimated age is quite robust, changing from 4.70
−0.14
+0.13
Gyr to 4.62
−0.06
+0.13
Gyr, the calibration of the convective core overshooting parameter
β
reveals noticeable differences. The reference scenario suggests a wide multi-modal range of possible values of
β
, peaking around 0.10; on the contrary the alternative scenario computations point towards a sharp and lower
β
, peaking around 0.04.
Conclusions.
The impossibility to obtain an unambiguous fit confirms the difficulty in achieving a sensible calibration of the free parameters of stellar models using binary systems, even when very accurate masses and radii are available. The results also suggest that the biases due to the assumptions underlying the stellar track computations may be different from one binary system to another.
Aims.
Relying on recent very precise observations for the CPD-54 810 binary system, we investigate the robustness of the estimated age and convective core overshooting for a system with both stars on ...the main sequence (MS). Our main aim is to assess the variability in the results, accounting for different statistical and systematic sources of uncertainty.
Methods.
We adopt the SCEPtER pipeline, a well-established maximum likelihood technique, based on fine grids of stellar models computed for a different initial chemical composition and convective core overshooting efficiency.
Results.
We performed different fits of the system, under different assumptions. The base fit suggests a common age of 3.02 ± 0.15 Gyr, in agreement with recent literature. This estimated convective core overshooting parameter is
β
= 0.09 ± 0.01, with a corresponding convective core mass $ M_c = 0.059^{+0.017}_{-0.021} $
M
⊙
. The robustness of these estimates were tested assuming a narrow constraint on the helium-to-metal enrichment ratio, which is in agreement with the recently published results on the Hyades cluster. Under this constraint the chemical solution of the system changes, but the age and the overshooting parameter are almost unchanged ($ 3.08^{+0.17}_{-0.14} $ Gyr and 0.09 ± 0.01). In a further test, we halved the uncertainty as to the effective temperature of both stars and again the estimated parameter shows only small variations (3.02 ± 0.12 Gyr and 0.09 ± 0.01).
Conclusions.
This low variability suggests that the age of the system with both stars in the MS can be reliably estimated at a 5% level, but it also indicates that the power of the investigation is probably low, because it is possible to find a satisfactory fit in several different configurations by only varying the initial chemical composition within its uncertainty. Despite the great increase in the observational constraints’ precision, the results support the conclusions of previous theoretical works on the stellar parameter calibration with double MS star binary systems.
We investigate the performance of grid-based techniques in estimating the age of stars in detached eclipsing binary systems. We evaluate the precision of the estimates due to the uncertainty in the ...observational constraints - masses, radii, effective temperatures, and Fe/H - and the systematic bias caused by the uncertainty in convective core overshooting, element diffusion, mixing-length value, and initial helium content. We adopted the SCEPtER grid, which includes stars with mass in the range 0.8; 1.6 M and evolutionary stages from the zero-age main sequence to the central hydrogen depletion. We showed that the typical 1sigma random error in age estimates - due only to the uncertainty affecting the observational constraints - is about + or - 7%, which is nearly independent of the masses of the two stars. We find that random fluctuations within the current observational uncertainties can lead genuine coeval binary components to appear to be non-coeval with a difference in age as high as 60%.
Aims. We aim to estimate the impact on the stellar evolution of the uncertainties in the 3α and the 12C(α, γ)16O reaction rates, taking into account the recent improvements in their precision. ...Methods. We calculated models of low- and intermediate-mass stars for different values of 3α and 12C(α, γ)16O reaction rates. The 3α reaction rate was varied up to ±24% around the reference value, while the 12C(α, γ)16O reaction rate was varied by up to ±35%, taking into account different recent values for these quantities available in the literature. The models were calculated with the FRANEC evolutionary code for two different initial chemical compositions, namely, Y = 0.246, Z = 0.0001, and Y = 0.28, Z = 0.015 to represent different stellar populations. A M = 0.67 M⊙ model was chosen as representative of the first class (halo ancient stars), while for the second composition (disk stars), the M = 1.5 M⊙ and M = 2.5 M⊙ models were considered. The impact of 3α and 12C(α, γ)16O reaction rates on the central He-burning lifetime and the asymptotic giant branch (AGB) lifetime, as well as the mass of the C/O core at the central He exhaustion and the internal C and O abundances, was investigated. Results. A variation of the 12C(α, γ)16O reaction rates within its nominal error resulted in marginal differences in the analysed features among the three considered stellar masses, except for the C/O abundances. The central He-burning lifetime changed by less than 4%, while the AGB lifetime was affected only at the 1% level. The internal C and O abundances showed greater variation, with a change of about 15%. The uncertainty in the 3α reaction rate mainly influences the C and O central abundances (up to 10%) for all the models considered, and the AGB lifetime for intermediate mass stars (up to 5%). Most of the investigated features were affected by less than 2%. Conclusions. The current uncertainty in the explored reaction rates has a negligible effect on the predicted evolutionary time scale with respect to other uncertainty sources. On the other hand, the variability in the chemical profile left at the end of the shell He-burning phase is still relevant. We also checked that there is no interaction between the effects of the two reaction rates, as would be expected in the case of small perturbations.
Abstract
Stellar age assignment still represents a difficult task in Astrophysics. This unobservable fundamental parameter can be estimated only through indirect methods, as well as generally the ...mass. Bayesian analysis is a statistical approach largely used to derive stellar properties by taking into account the available information about the quantities we are looking for. In this paper, we propose to apply the method to the double-lined spectroscopic binaries (SB2), for which the only available information about masses is the observed mass ratio of the two components. We validated the method on a synthetic sample of pre-main-sequence (PMS) SB2 systems showing the capability of the technique to recover the simulated age and masses. Then, we applied our procedure to the PMS eclipsing binaries Parenago 1802 and RX J0529.4+0041 A, whose masses of both components are known, by treating them as SB2 systems. The estimated masses are in agreement with those dynamically measured. We conclude that the method, if based on high resolution and high signal-to-noise spectroscopy, represents a robust way to infer the masses of the very numerous SB2 systems together with their age, allowing to date the hosting astrophysical environments.
Mixing-length calibration from field stars Valle, G.; Dell’Omodarme, M.; Prada Moroni, P. G. ...
Astronomy and astrophysics (Berlin),
03/2019, Letnik:
623
Journal Article
Recenzirano
Odprti dostop
Aims
. We critically analysed the theoretical foundation and statistical reliability of the mixing-length calibration by means of standard (
T
eff
, Fe/H) and global asteroseismic observables (Δ
ν
,
...ν
max
) of field stars. We also discussed the soundness of inferring a possible metallicity dependence of the mixing-length parameter from field stars.
Methods
. We followed a theoretical approach based on mock datasets of artificial stars sampled from a grid of stellar models with a fixed mixing-length parameter
α
ml
. We then recovered the mixing-length parameter of the mock stars by means of SCEPtER maximum-likelihood algorithm. We finally analysed the differences between the true and recovered mixing-length values quantifying the random errors due to the observational uncertainties and the biases due to possible discrepancies in the chemical composition and input physics between artificial stars and the models adopted in the recovery.
Results
. We verified that the
α
ml
estimates are affected by a huge spread, even in the ideal configuration of perfect agreement between the mock data and the recovery grid of models. While the artificial stars were computed at fixed solar-calibrated
α
ml
= 2.10, the recovered values had a mean of 2.20 and a standard deviation of 0.52. Then we explored the case in which the solar heavy-element mixture used to compute the models is different from that adopted in the artificial stars. We found an estimated mixing-length mean of 2.24 ± 0.48 and, more interestingly, a metallicity relationship in which
α
ml
increases by 0.4 for an increase of 1 dex in Fe/H. Thus, a simple heavy-element mixture mismatch induced a spurious, but statistically robust, dependence of the estimated mixing-length on metallicity. The origin of this trend was further investigated considering the differences in the initial helium abundance
Y
– Fe/H – initial metallicity
Z
relation assumed in the models and data. We found that a discrepancy between the adopted helium-to-metal enrichment ratio Δ
Y
/Δ
Z
caused the appearance of spurious trends in the estimated mixing-length values. An underestimation of its value from Δ
Y
/Δ
Z
= 2.0 in the mock data to Δ
Y
/Δ
Z
= 1.0 in the recovery grid resulted in an increasing trend, while the opposite behaviour occurred for an equivalent overestimation. A similar effect was caused by an offset in the Fe/H to global metallicity
Z
conversion. A systematic overestimation of Fe/H by 0.1 dex in the recovery grid of models forced an increasing trend of
α
ml
versus Fe/H of about 0.2 per dex. We also explored the impact of some possible discrepancies between the adopted input physics in the recovery grid of models and mock data. We observed an induced trend with metallicity of about Δ
α
ml
= 0.3 per dex when the effect of the microscopic diffusion is neglected in the recovery grid, while no trends originated from a wrong assumption on the effective temperature scale by ±100 K. Finally, we proved that the impact of different assumptions on the outer boundary conditions was apparent only in the RGB phase.
Conclusions
. We showed that the mixing-length estimates of field stars are affected by a huge spread even in an ideal case in which the stellar models used to estimate
α
ml
are exactly the same models as used to build the mock dataset. Moreover, we proved that there are many assumptions adopted in the stellar models used in the calibration that can induce spurious trend of the estimated
α
ml
with Fe/H. Therefore, any attempt to calibrate the mixing-length parameter by means of
T
eff
, Fe/H, Δ
ν
, and
ν
max
of field stars seems to be statistically poorly reliable. As such, any claim about the possible dependence of the mixing-length on the metallicity for field stars should be considered cautiously and critically.
The dust cloud TGU H645 P2 and embedded in it the young open cluster NGC 7129 are investigated using the results of medium-band photometry of 159 stars in the Vilnius seven-colour system down to ...V = 18.8 mag. The photometric data were used to classify about 50 per cent of the measured stars in spectral and luminosity classes. The extinction A
V
versus distance diagram for the 20 arcmin × 20 arcmin area is plotted for 155 stars with two-dimensional classification from the present and the previous catalogues. The extinction values are found in the range between 0.6 and 3.4 mag. However, some red giants, located in the direction of the dense parts of the cloud, exhibit the infrared extinction equivalent up to A
V
= 13 mag. The distance to the cloud (and the cluster) is found to be 1.15 kpc (the true distance modulus 10.30 mag). For determining the age of NGC 7129, a luminosity versus temperature diagram for six cluster members of spectral classes B3–A1 was compared with the Pisa pre-main-sequence evolution tracks and the Palla birthlines. The cluster can be as old as about 3 Myr, but star formation continues till now as witnessed by the presence in the cloud of many younger pre-main-sequence objects identified with photometry from 2MASS, Spitzer and WISE infrared surveys.
Context. The lower limit for the mass of white dwarfs (WDs) with a C-O core is commonly assumed to be roughly 0.5 $M_{\odot}$. As a consequence, WDs of lower masses are usually identified as He-core ...remnants. Aims. When the initial mass of the progenitor star is between 1.8 and 3 $M_\odot$, which corresponds to the so-called red giant (RGB) phase transition, the mass of the H-exhausted core at the tip of the RGB is 0.3 < $M_{\rm H}/M_{\odot}$ < 0.5. Prompted by this well known result of stellar evolution theory, we investigate the possibility to form C-O WDs with mass M < 0.5 $M_{\odot}$. Methods. The pre-WD evolution of stars was computed with initial mass of about 2.3 $M_\odot$, undergoing anomalous mass-loss episodes during the RGB phase and leading to the formation of WDs with He-rich or CO-rich cores. The cooling sequences of the resulting WDs are also described. Results. We show that the minimum mass for a C-O WD is about 0.33 $M_{\odot}$, so that both He and C-O core WDs can exist in the mass range 0.33-0.5 $M_{\odot}$. The models computed for the present paper provide the theoretical tools for indentifying the observational counterpart of very low-mass remnants with a C-O core among those commonly ascribed to the He-core WD population in the progressively growing sample of observed WDs of low mass. Moreover, we show that the central He-burning phase of the stripped progeny of the 2.3 $M_\odot$ star lasts longer and longer as the total mass decreases. In particular, the M = 0.33 $M_{\odot}$ model takes about 800 Myr to exhaust its central helium, which is more than three times longer than the value for the standard 2.3 $M_{\odot}$ star: it is, by far, the longest core-He burning lifetime. Finally, we find the occurrence of gravonuclear instabilities during the He-burning shell phase.
ΔY/ΔZ from the analysis of local K dwarfs Gennaro, M.; Prada Moroni, P. G.; Degl'Innocenti, S.
Astronomy and astrophysics (Berlin),
07/2010, Letnik:
518, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Context. The stellar helium-to-metal enrichment ratio, ΔY/ΔZ, is a widely studied astrophysical quantity. However, its value has still not been precisely constrained. Aims. This paper is focused on ...studying the main sources of uncertainty that affect the ΔY/ΔZ ratio derived from the analysis of the low-main sequence (MS) stars in the solar neighborhood. Methods. The possibility of inferring the value of the helium-to-metal enrichment ratio from the study of low-MS stars relies on the dependence of the stellar luminosity and effective temperature on the initial helium and metal abundances. The ΔY/ΔZ ratio is obtained by comparing the magnitude difference between the observed stars and a reference, theoretical zero age main sequence (ZAMS) with the related theoretical magnitude differences computed from a new set of stellar models with up-to-date input physics and a fine grid of chemical compositions. A Monte Carlo approach has been used to evaluate the impact of different sources of uncertainty, i.e. observational errors, evolutionary effects, systematic uncertainties of the models. As a check of the procedure, the method was applied to a different data set, namely the low-MS of the Hyades. Results. Once a set of ZAMS and atmosphere models had been chosen, we found that the inferred value of ΔY/ΔZ is sensitive to the age of the stellar sample, even if we restricted the data set to low-luminosity stars. The lack of an accurate age estimate of low-mass field stars leads to underestimating the inferred ΔY/ΔZ of ~2 units. On the contrary, the method firmly recovers the ΔY/ΔZ value for unevolved samples of stars like the Hyades low-MS. Adopting a solar-calibrated mixing-length parameter and the PHOENIX GAIA v2.6.1 atmospheric models, we find ΔY/ΔZ = 5.3 ± 1.4 once the age correction has been applied. The Hyades sample provided a perfectly consistent value. Conclusions. We have demonstrated that assuming that low-mass stars in the solar neighborhood can be considered as unevolved does not necessarily hold, and it may indeed lead to a bias in the inferred ΔY/ΔZ. The effect of the still poorly constrained efficiency of the superadiabatic convection and of different atmosphere models adopted to transform luminosities and effective temperature into colors and magnitudes are also discussed.
We report the discovery of Kepler-77b (alias KOI-127.01), a Saturn-mass transiting planet in a 3.6-day orbit around a metal-rich solar-like star. We combined the publicly available Kepler photometry ...(quarters 1−13) with high-resolution spectroscopy from the Sandiford at McDonald and FIES at NOT spectrographs. We derived the system parameters via a simultaneous joint fit to the photometric and radial velocity measurements. Our analysis is based on the Bayesian approach and is carried out by sampling the parameter posterior distributions using a Markov chain Monte Carlo simulation. Kepler-77b is a moderately inflated planet with a mass of Mp = 0.430 ± 0.032 MJup, a radius of Rp = 0.960 ± 0.016 RJup, and a bulk density of ρp = 0.603 ± 0.055 g cm-3. It orbits a slowly rotating (Prot = 36 ± 6 days) G5 V star with M⋆ = 0.95 ± 0.04 M⊙, R⋆ = 0.99 ± 0.02 R⊙, Teff = 5520 ± 60 K, M/H = 0.20 ± 0.05 dex, that has an age of 7.5 ± 2.0 Gyr. The lack of detectable planetary occultation with a depth higher than ~10 ppm implies a planet geometric and Bond albedo of Ag ≤ 0.087 ± 0.008 and AB ≤ 0.058 ± 0.006, respectively, placing Kepler-77b among the gas-giant planets with the lowest albedo known so far. We found neither additional planetary transit signals nor transit-timing variations at a level of ~0.5 min, in accordance with the trend that close-in gas giant planets seem to belong to single-planet systems. The 106 transitsobserved in short-cadence mode by Kepler for nearly 1.2 years show no detectable signatures of the planet’s passage in front of starspots. We explored the implications of the absence of detectable spot-crossing events for the inclination of the stellar spin-axis, the sky-projected spin-orbit obliquity, and the latitude of magnetically active regions.
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