The role of the Fraunhofer lines in solar brightness variability

The solar brightness varies on timescales from minutes to decades. A clear identification of the physical processes behind such variations is needed for developing and improving physics-based models of solar brightness variability and reconstructing solar brightness in the past. This is, in turn, important for better understanding the solar-terrestrial and solar-stellar connections. We estimate the relative contributions of the continuum, molecular, and atomic lines to the solar brightness variations on different timescales. Our approach is based on the assumption that variability of the solar brightness on timescales greater than a day is driven by the evolution of the solar surface magnetic field. We calculated the solar brightness variations employing the solar disc area coverage of magnetic features deduced from the MDI/SOHO observations. The brightness contrasts of magnetic features relative to the quiet Sun were calculated with a non-LTE radiative transfer code as functions of disc position and wavelength. By consecutive elimination of molecular and atomic lines from the radiative transfer calculations, we assessed the role of these lines in producing solar brightness variability. We show that the variations in Fraunhofer lines define the amplitude of the solar brightness variability on timescales greater than a day and even the phase of the total solar irradiance variability over the 11-year cycle. We also demonstrate that molecular lines make substantial contribution to solar brightness variability on the 11-year activity cycle and centennial timescales. In particular, our model indicates that roughly a quarter of the total solar irradiance variability over the 11-year cycle originates in molecular lines. The maximum of the absolute spectral brightness variability on timescales greater than a day is associated with the CN violet system between 380 and 390 nm.Comment: 9 pages, 4 figures, accepted for publication in Astronomy&Astrophysic

From Solar to Stellar Brightness Variations: The Effect of Metallicity

Context. Comparison studies of Sun-like stars with the Sun suggest an anomalously low photometric variability of the Sun compared to Sun-like stars with similar magnetic activity. Comprehensive understanding of stellar variability is needed, to find a physical reasoning for this observation. Aims. We investigate the effect of metallicity and effective temperature on the photometric brightness change of Sun-like stars seen at different inclinations. The considered range of fundamental stellar parameters is sufficiently small so the stars, investigated here, still count as Sun-like or even as solar twins. Methods. To model the brightness change of stars with solar magnetic activity, we extend a well established model of solar brightness variations, SATIRE (which stands for Spectral And Total Irradiance Reconstruction), which is based on solar spectra, to stars with different fundamental parameters. For that we calculate stellar spectra for different metallicities and effective temperature using the radiative transfer code ATLAS9. Results. We show that even a small change (e.g. within the observational error range) of metallicity or effective temperature significantly affects the photometric brightness change compared to the Sun. We find that for Sun-like stars, the amplitude of the brightness variations obtained for Str\"omgren (b + y)/2 reaches a local minimum for fundamental stellar parameters close to the solar metallicity and effective temperature. Moreover, our results show that the effect of inclination decreases for metallicity values greater than the solar metallicity. Overall, we find that an exact determination of fundamental stellar parameters is crucially important for understanding stellar brightness changes.Comment: 12 pages, 12 figures, accepted in A&

The nature of solar brightness variations

The solar brightness varies on timescales from minutes to decades. Determining the sources of such variations, often referred to as solar noise, is of importance for multiple reasons: a) it is the background that limits the detection of solar oscillations, b) variability in solar brightness is one of the drivers of the Earth's climate system, c) it is a prototype of stellar variability which is an important limiting factor for the detection of extra-solar planets. Here we show that recent progress in simulations and observations of the Sun makes it finally possible to pinpoint the source of the solar noise. We utilise high-cadence observations from the Solar Dynamic Observatory and the SATIRE model to calculate the magnetically-driven variations of solar brightness. The brightness variations caused by the constantly evolving cellular granulation pattern on the solar surface are computed with the MURAM code. We find that surface magnetic field and granulation can together precisely explain solar noise on timescales from minutes to decades, i.e. ranging over more than six orders of magnitude in the period. This accounts for all timescales that have so far been resolved or covered by irradiance measurements. We demonstrate that no other sources of variability are required to explain the data. Recent measurements of Sun-like stars by CoRoT and Kepler uncovered brightness variations similar to that of the Sun but with much wider variety of patterns. Our finding that solar brightness variations can be replicated in detail with just two well-known sources will greatly simplify future modelling of existing CoRoT and Kepler as well as anticipated TESS and PLATO data.Comment: This is the submitted version of the paper published in Nature Astronom

Occurrence, Composition and Formation of Ruppia, Widgeon Grass, balls in Saskatchewan Lakes

Widgeon Grass (Ruppia maritima) is an aquatic vascular plant (Ruppiaceae) which has been the source for rare balls of plant material found at the shores of lakes on four continents. In North America, the lakes involved were in North Dakota, Oregon, and now northern and southern Saskatchewan. The formation of the balls has not been observed in nature, but similar balls have been produced in other studies with Posidonia or Turtle Grass (Hydrocharitaceae) fibers under the wavelike action in a washing machine. Our samples are from a saline lake in southern Saskatchewan (49°N), and an over 40-year-old sample from an unknown lake north of the boreal transition zone (52°N). Comparisons of the plant material with herbarium specimens confirm that the balls are almost entirely comprised of Ruppia maritima, with minor items including invertebrate animal parts, sand pebbles and feathers. The context in which the material was found is consistent with the proposition that they are formed by Ruppia inflorescences breaking apart, drifting to near shore due to wind and being rolled into balls by wave action

Fundamental stellar parameters of zeta Pup and gamma^2 Vel from HIPPARCOS data

We report parallax measurements by the HIPPARCOS satellite of zeta Puppis and gamma^2 Velorum. The distance of zeta Pup is d=429 (+120/ -77) pc, in agreement with the commonly adopted value to Vela OB2. However, a significantly smaller distance is found for the gamma^2 Vel system: d=258 (+41/-31) pc. The total mass of gamma^2 Vel derived from its parallax, the angular size of the semi-major axis as measured with intensity interferometry, and the period is M(WR+O)=29.5 (+/-15.9) Msun. This result favors the orbital solution of Pike et al. (1983) over that of Moffat et al. (1986). The stellar parameters for the O star companion derived from line blanketed non-LTE atmosphere models are: Teff=34000 (+/-1500) K, log L/Lsun=5.3 (+/-0.15) from which an evolutionary mass of M=29 (+/-4) Msun and an age of 4.0 (+0.8/-0.5) Myr is obtained from single star evolutionary models. With non-LTE model calculations including He and C we derive a luminosity log L/Lsun~4.7 (+/-0.2) for the WR star. The mass-luminosity relation of hydrogen-free WR stars implies a mass of M(WR)~5 (+/-1.5) Msun. From our data we favor an age of ~10 Myr for the bulk of the Vela OB2 stars. Evolutionary scenarios for zeta Pup and gamma^2 Vel are discussed in the light of our results.Comment: Submitted to ApJ Letters (misprints corrected

The Neon Abundance of Galactic Wolf-Rayet Stars

The fast, dense winds which characterize Wolf-Rayet (WR) stars obscure their underlying cores, and complicate the verification of evolving core and nucleosynthesis models. Core evolution can be probed by measuring abundances of wind-borne nuclear processed elements, partially overcoming this limitation. Using ground-based mid-infrared spectroscopy and the 12.81um [NeII] emission line measured in four Galactic WR stars, we estimate neon abundances and compare to long-standing predictions from evolved-core models. For the WC star WR121, this abundance is found to be >~11x the cosmic value, in good agreement with predictions. For the three less-evolved WN stars, little neon enhancement above cosmic values is measured, as expected. We discuss the impact of clumping in WR winds on this measurement, and the promise of using metal abundance ratios to eliminate sensitivity to wind density and ionization structure.Comment: Accepted for publication in ApJ; 9 pages, 2 color figures, 4 table

XMM-Newton Detection of Hard X-ray Emission in the Nitrogen-Type Wolf-Rayet Star WR110

We have used the excellent sensitivity of XMM-Newton to obtain the first high-quality X-ray spectrum of a Wolf-Rayet (WR) star which is not known to be a member of a binary system. Our target, the nitrogen-type star WR 110 (= HD 165688) was also observed and detected with the VLA at four different frequencies. The radio data are in excellent agreement with that expected for free-free wind emission. and the ionized mass-loss rate is derived. The X-ray emission measure distribution shows a dominant contribution from cool plasma at kT$_{cool}$ = 0.5 keV (6 MK) which is only weakly absorbed. We argue that this cool emission originates at hundreds of radii if the wind is spherical and homogeneous and derive shock velocities and the X-ray filling factor using radiative shock models. A surprising result is the unambiguous detection of a hard X-ray component clearly seen in the hard-band images and the spectra. This hard component accounts for about half of the observed flux and can be acceptably fitted by a hot optically thin thermal plasma or a power-law model. If the emission is thermal, then a temperature kT$_{hot}$ $\geq$ 3 keV is derived. Such high temperatures are not predicted by current instability-driven wind shock models. We examine several alternatives and show that the hard emission could be accounted for by the WR wind shocking onto a close stellar companion which has so far escaped detection. However, until persuasive evidence for binarity is found we are left with the intriguing possibility that the hard X-ray emission is produced entirely by the Wolf-Rayet star.Comment: 2 tables, 7 figure

A Study of Cyg OB2: Pointing the Way Towards Finding Our Galaxy's Super Star Clusters

New optical MK classification spectra have been obtained for 14 OB star candidates identified by Comeron et al. (2002) and presumed to be possible members of the Cyg OB2 cluster. All 14 candidate OB stars observed are indeed early-type stars, strongly suggesting the remaining 31 candidates identified by Comeron et al. are also early-type stars. However, as many as half of the new stars appear to be significantly older than the previously studied optical cluster, making their membership in Cyg OB2 suspect. Despite this, the recognition of Cyg OB2 being a more massive and extensive star cluster than previously realized, along with the recently recognized candidate super star cluster Westerlund 1 only a few kpc away (Clark & Negueruela 2002), reminds us that we are woefully under-informed about the massive cluster population in our Galaxy. Extrapolations of the locally derived cluster luminosity function indicate 10s to perhaps 100 of these very massive open clusters (Mcl ~ 10^4 M_sun, Mv ~ -11) should exist within our galaxy. Radio surveys will not detect these massive clusters if they are more than a few million years old. Our best hope for remedying this shortfall is through deep infrared searches and follow up near-infrared spectroscopic observations, as was used by Comeron et al. to locate candidate members of the Cyg OB2 association.Comment: 30 pages, 12 figures, ApJ in pres

Neon Abundances from a Spitzer/IRS Survey of Wolf-Rayet Stars

We report on neon abundances derived from {\it Spitzer} high resolution spectral data of eight Wolf-Rayet (WR) stars using the forbidden line of [\ion{Ne}{3}] 15.56 microns. Our targets include four WN stars of subtypes 4--7, and four WC stars of subtypes 4--7. We derive ion fraction abundances $\gamma$ of Ne^{2+} for the winds of each star. The ion fraction abundance is a product of the ionization fraction $Q_{\rm i}$ in stage i and the abundance by number ${\cal A}_E$ of element E relative to all nuclei. Values generally consistent with solar are obtained for the WN stars, and values in excess of solar are obtained for the WC stars.Comment: to appear in Astrophysical Journa