Abstract
The bright star
λ
Ser hosts a hot Neptune with a minimum mass of 13.6
M
⊕
and a 15.5 day orbit. It also appears to be a solar analog, with a mean rotation period of 25.8 days and surface ...differential rotation very similar to the Sun. We aim to characterize the fundamental properties of this system and constrain the evolutionary pathway that led to its present configuration. We detect solar-like oscillations in time series photometry from the Transiting Exoplanet Survey Satellite, and we derive precise asteroseismic properties from detailed modeling. We obtain new spectropolarimetric data, and we use them to reconstruct the large-scale magnetic field morphology. We reanalyze the complete time series of chromospheric activity measurements from the Mount Wilson Observatory, and we present new X-ray and ultraviolet observations from the Chandra and Hubble space telescopes. Finally, we use the updated observational constraints to assess the rotational history of the star and estimate the wind braking torque. We conclude that the remaining uncertainty on the stellar age currently prevents an unambiguous interpretation of the properties of
λ
Ser, and that the rate of angular momentum loss appears to be higher than for other stars with a similar Rossby number. Future asteroseismic observations may help to improve the precision of the stellar age.
Abstract
Asteroseismology of bright stars has become increasingly important as a method to determine the fundamental properties (in particular ages) of stars. The Kepler Space Telescope initiated a ...revolution by detecting oscillations in more than 500 main-sequence and subgiant stars. However, most Kepler stars are faint and therefore have limited constraints from independent methods such as long-baseline interferometry. Here we present the discovery of solar-like oscillations in
α
Men A, a naked-eye (
V
= 5.1) G7 dwarf in TESS’s southern continuous viewing zone. Using a combination of astrometry, spectroscopy, and asteroseismology, we precisely characterize the solar analog
α
Men A (
T
eff
= 5569 ± 62 K,
R
⋆
= 0.960 ± 0.016
R
⊙
,
M
⋆
= 0.964 ± 0.045
M
⊙
). To characterize the fully convective M dwarf companion, we derive empirical relations to estimate mass, radius, and temperature given the absolute Gaia magnitude and metallicity, yielding
M
⋆
= 0.169 ± 0.006
M
⊙
,
R
⋆
= 0.19 ± 0.01
R
⊙
, and
T
eff
= 3054 ± 44 K. Our asteroseismic age of 6.2 ± 1.4 (stat) ± 0.6 (sys) Gyr for the primary places
α
Men B within a small population of M dwarfs with precisely measured ages. We combined multiple ground-based spectroscopy surveys to reveal an activity cycle of
P
= 13.1 ± 1.1 yr for
α
Men A, a period similar to that observed in the Sun. We used different gyrochronology models with the asteroseismic age to estimate a rotation period of ∼30 days for the primary. Alpha Men A is now the closest (
d
= 10 pc) solar analog with a precise asteroseismic age from space-based photometry, making it a prime target for next-generation direct-imaging missions searching for true Earth analogs.
The Magnetic Future of the Sun Judge, Philip G.; Egeland, Ricky; Metcalfe, Travis S. ...
The Astrophysical journal,
10/2017, Letnik:
848, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We analyze space- and ground-based data for the old (7.0 0.3 Gyr) solar analogs 16 Cyg A and B. The stars were observed with the Cosmic Origins UV Spectrographs on the Hubble Space Telescope (HST) on ...2015 October 23 and 2016 February 3, respectively, and with the Chandra X-ray Observatory on 2016 February 7. Time-series data in Ca ii data are used to place the UV data in context. The UV spectra of 18 Sco (3.7 0.5 Gyr), the Sun (4.6 0.04 Gyr), and Cen A ( ) appear remarkably similar, pointing to a convergence of magnetic heating rates for G2 main-sequence stars older than 2-4 Gyr. But the B component's X-ray (0.3-2.5 keV) flux lies 20× below a well-known minimum level reported by Schmitt. As reported for Cen A, the coronal temperature probably lies below that detectable in soft X-rays. No solar UV flux spectra of comparable resolution to those of stellar data exist, but they are badly needed for comparison with stellar data. Center-to-limb variations are reevaluated for lines such as Ca ii through X-rays, with important consequences for observing activity cycles in such features. We also call into question work that has mixed solar intensity-intensity statistics with flux-flux relations of stars.
Abstract
During the first half of main-sequence lifetimes, the evolution of rotation and magnetic activity in solar-type stars appears to be strongly coupled. Recent observations suggest that ...rotation rates evolve much more slowly beyond middle age, while stellar activity continues to decline. We aim to characterize this midlife transition by combining archival stellar activity data from the Mount Wilson Observatory with asteroseismology from the Transiting Exoplanet Survey Satellite (TESS). For two stars on opposite sides of the transition (88 Leo and
ρ
CrB), we independently assess the mean activity levels and rotation periods previously reported in the literature. For the less active star (
ρ
CrB), we detect solar-like oscillations from TESS photometry, and we obtain precise stellar properties from asteroseismic modeling. We derive updated X-ray luminosities for both stars to estimate their mass-loss rates, and we use previously published constraints on magnetic morphology to model the evolutionary change in magnetic braking torque. We then attempt to match the observations with rotational evolution models, assuming either standard spin-down or weakened magnetic braking. We conclude that the asteroseismic age of
ρ
CrB is consistent with the expected evolution of its mean activity level and that weakened braking models can more readily explain its relatively fast rotation rate. Future spectropolarimetric observations across a range of spectral types promise to further characterize the shift in magnetic morphology that apparently drives this midlife transition in solar-type stars.
Abstract
We analyze space-based time-series photometry of Sun-like stars, mostly in the Pleiades, but also field stars and the Sun itself. We focus on timescales between roughly 1 hr and 1 day. In ...the corresponding frequency band these stars display brightness fluctuations with a decreasing power-law continuous spectrum. K2 and Kepler observations show that the rms flicker due to this mid-frequency continuum (MFC) can reach almost 1%, approaching the modulation amplitude from active regions. The MFC amplitude varies by a factor up to 40 among Pleiades members with similar
T
eff
, depending mainly on the stellar Rossby number Ro. For Ro ≤ 0.04, the mean amplitude is roughly constant at about 0.4%; at larger Ro the amplitude decreases rapidly, shrinking by about two orders of magnitude for Ro ≃ 1. Among stars, the MFC amplitude correlates poorly with that of modulation from rotating active regions. Among field stars observed for 3 yr by Kepler, the quarterly average modulation amplitudes from active regions are much more time variable than the quarterly MFC amplitudes. We argue that the process causing the MFC is largely magnetic in nature and that its power-law spectrum comes from magnetic processes distinct from the star’s global dynamo, with shorter timescales. By analogy with solar phenomena, we hypothesize that the MFC arises from a (sometimes energetic) variant of the solar magnetic network, perhaps combined with rotation-related changes in the morphology of supergranules.
Asteroseismology involves probing the interiors of stars and quantifying their global properties, such as radius and age, through observations of normal modes of oscillation. The technical ...requirements for conducting asteroseismology include ultrahigh precision measured in photometry in parts per million, as well as nearly continuous time series over weeks to years, and cadences rapid enough to sample oscillations with periods as short as a few minutes. We report on results from the first 43 days of observations, in which the unique capabilities ofKeplerin providing a revolutionary advance in asteroseismology are already well in evidence. TheKeplerasteroseismology program holds intrinsic importance in supporting the core planetary search program through greatly enhanced knowledge of host star properties, and extends well beyond this to rich applications in stellar astrophysics.
Stellar models generally use simple parameterizations to treat convection. The most widely used parameterization is the so-called mixing-length theory where the convective eddy sizes are described ...using a single number, alpha , the mixing-length parameter. This is a free parameter, and the general practice is to calibrate alpha using the known properties of the Sun and apply that to all stars. Using data from NASA's Kepler mission we show that using the solar-calibrated alpha is not always appropriate, and that in many cases it would lead to estimates of initial helium abundances that are lower than the primordial helium abundance. Kepler data allow us to calibrate alpha for many other stars and we show that for the sample of stars we have studied, the mixing-length parameter is generally lower than the solar value. We studied the correlation between alpha and stellar properties, and we find that alpha increases with metallicity. We therefore conclude that results obtained by fitting stellar models or by using population-synthesis models constructed with solar values of alpha are likely to have large systematic errors. Our results also confirm theoretical expectations that the mixing-length parameter should vary with stellar properties.
The inner lives of red giants Metcalfe, Travis S.
Nature (London),
03/2011, Letnik:
471, Številka:
7340
Journal Article
Recenzirano
Odprti dostop
For most of their existence, stars shine from the energy released by nuclear reactions that convert hydrogen into helium, but eventually they begin to burn the helium in their cores to synthesize ...heavier elements, such as carbon and oxygen.
Papers that are posted to a digital preprint archive are typically cited twice as often as papers that are not posted. This has been demonstrated for papers published in a wide variety of journals, ...and in many different subfields of astronomy. Most astronomers now use the arXiv.org server (astro-ph) to distribute preprints, but the solar physics community has an independent archive hosted at Montana State University. For several samples of solar physics papers published in 2003, I quantify the boost in citation rates for preprints posted to each of these servers. I show that papers on the MSU archive typically have citation rates 1.7 times higher than the average of similar papers that are not posted as preprints, while those posted to astro-ph get 2.6 times the average. A comparable boost is found for papers published in conference proceedings, suggesting that the higher citation rates are not the result of self-selection of above-average papers.
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