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G. Handler

Publications and source records attributed to G. Handler.

At least 55 records · Page 3Linked to original sources
arXiv2019

Stellar masses from granulation and oscillations of 23 bright red giants observed by BRITE - Constellation

Context: The study of stellar structure and evolution depends crucially on accurate stellar parameters. The photometry from space telescopes has provided superb data that allowed asteroseismic characterisation of thousands of stars. However, typical targets of space telescopes are rather faint and complementary measurements are difficult to obtain. On the other hand, the brightest, otherwise well-studied stars, are lacking seismic characterization. Aims: Our goal is to use the granulation and/or oscillation time scales measured from photometric time series of bright red giants (1.6$\leq$Vmag$\leq$5.3) observed with BRITE to determine stellar surface gravities and masses. Methods: We use probabilistic methods to characterize the granulation and/or oscillation signal in the power density spectra and the autocorrelation function of the BRITE time series. Results: We detect a clear granulation and/or oscillation signal in 23 red giant stars and extract the corresponding time scales from the power density spectra as well as the autocorrelation function of the BRITE time series. To account for the recently discovered non-linearity of the classical seismic scaling relations, we use parameters from a large sample of Kepler stars to re-calibrate the scalings of the high- and low-frequency components of the granulation signal. We develop a method to identify which component is measured if only one granulation component is statistically significant in the data. We then use the new scalings to determine the surface gravity of our sample stars, finding them to be consistent with those determined from the autocorrelation signal of the time series. We further use radius estimates from the literature to determine the stellar masses of our sample stars from the measured surface gravities. We also define a statistical measure for the evolutionary stage of the stars.

astro-ph.SR↗
arXiv2018

BRITE photometry of the massive post-RLOF system HD149404

HD149404 is an evolved non-eclipsing O-star binary that has previously undergone a Roche lobe overflow interaction. Understanding some key properties of the system requires a determination of the orbital inclination and of the dimensions of the components. The BRITE-Heweliusz satellite was used to collect photometric data of HD149404. Additional photometry was retrieved from the SMEI archive. These data were analysed using a suite of period search tools. The orbital part of the lightcurve was modelled with the nightfall binary star code. The Gaia-DR2 parallax of HD149404 was used to provide additional constraints. The periodograms reveal a clear orbital modulation of the lightcurve with a peak-to-peak amplitude near 0.04 mag. The remaining non-orbital part of the variability is consistent with red noise. The lightcurve folded with the orbital period reveals ellipsoidal variations, but no eclipses. The minimum when the secondary star is in inferior conjunction is deeper than the other minimum due to mutual reflection effects between the stars. Combined with the Gaia-DR2 parallaxes, the photometric data indicate an orbital inclination in the range of 23° to 31° and a Roche lobe filling factor of the secondary larger than or equal to 0.96. The luminosity of the primary star is consistent with its present-day mass, whereas the more evolved secondary appears overluminous for its mass. We confirm that the primary's rotation period is about half the orbital period. Both features most probably stem from the past Roche lobe overflow episode.

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arXiv2018

Short-term variability and mass loss in Be stars IV. Two groups of closely spaced, approximately equidistant frequencies in three decades of space photometry of $ν$ Puppis (B7-8 IIIe)

In early-type Be stars, groups of nonradial pulsation (NRP) modes with numerically related frequencies may be instrumental for the release of excess angular momentum through mass-ejection events. Difference and sum/harmonic frequencies often form additional groups. The goal of this study is to find out whether a similar frequency pattern occurs in the cooler third-magnitude B7-8\,IIIe shell star $ν$ Pup. Time-series analyses are performed of space photometry with BRITE-Constellation (2015, 2016/17, and 2017/18), SMEI (2003--011), and Hipparcos (1989-1993). Two IUE SWP and 27 optical echelle spectra spanning 20 years were retrieved from various archives. The optical spectra exhibit no anomalies or well-defined variabilities. A magnetic field was not detected. All three photometry satellites recorded variability near 0.656 c/d which is resolved into three features separated by ~0.0021 c/d. First harmonics form a second frequency group, also spaced by ~0.0021 c/d. The frequency spacing is very nearly but not exactly equidistant. Variability near 0.0021 c/d was not detected. The long-term frequency stability could be used to derive meaningful constraints on the properties of a putative companion star. The IUE spectra do not reveal the presence of a hot subluminous secondary. $ν$\,Pup is another Be star exhibiting an NRP variability pattern with long-term constancy and underlining the importance of combination frequencies and frequency groups. The star is a good target for efforts to identify an effectively single Be star.

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arXiv2018

The search for roAp stars: null results and new candidates from Stroemgren-Crawford photometry

The rapidly oscillating Ap (roAp) stars exhibit pulsational photometric and/or radial velocity variations on time scales of several minutes, which are essential to test current pulsation models as well as our assumptions of atmospheric structure characteristics. In addition, their chemical peculiarity makes them very interesting for probing stellar formation and evolution in the presence of a global magnetic field. To date, a limited number of only 61 roAp stars are known to show photometric variability. On the other hand, a literature survey yields 619 unique stars that have unsuccessfully been searched for variability of this kind. Stroemgren-Crawford uvbybeta photometry of stars from both subgroups were used to investigate whether there is a selection bias of the investigated stars. We also present new photometric measurements (202 hours in 59 different nights) of 55 roAp candidates.We did not detect any new roAp star. Although our detection limits are comparable to other surveys, we also did not find pulsations in the known roAp star HD 12098, which may be a consequence of temporal amplitude changes. On the other hand, we do find some evidence for photometric variability of beta CrB at its spectroscopically derived pulsation period. From the uvbybeta photometry we conclude that the blue border of the roAp instability strip appears observationally well defined, whereas the red border is rather poorly known and studied. Within these boundaries, a total of 4646 candidates were identified which appear worthwhile to be investigated for short-term pulsational variability.

astro-ph.GA↗
arXiv2017

UVSat: a concept of an ultraviolet/optical photometric satellite

Time-series photometry from space in the ultraviolet can be presently done with only a few platforms, none of which is able to provide wide-field long-term high-cadence photometry. We present a concept of UVSat, a twin space telescope which will be capable to perform this kind of photometry, filling an observational niche. The satellite will host two telescopes, one for observations in the ultraviolet, the other for observations in the optical band. We also briefly show what science can be done with UVSat.

astro-ph.IM↗
arXiv2017

BRITEning up the Be Phenomenon

Observations of 25 Ori much expand the picture derived of other early-type Be stars with BRITE and SMEI. Two instead of one difference frequencies rule the variability: (a) The lower one, 0.0129 c/d, is the frequency of events with full amplitudes of 100-200 mmag which may signal mass loss possibly driven by the higher one, 0.1777 c/d. (b) Much of the entire power spectrum is a tightly woven network of combination frequencies: (i) Below 0.25\,c/d, numerous frequencies are difference frequencies. (ii) Many frequencies above 2.5 c/d can be represented as sum frequencies and in a few cases as harmonics. (iii) Many frequencies between 1.1 and 1.75\,c/d can be portrayed as parents of combination frequencies. The number and fraction of combination frequencies increases steeply with decreasing amplitude and and accuracy of the frequency matching.

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arXiv2017

Short-term variability and mass loss in Be stars III. BRITE and SMEI satellite photometry of 28 Cygni

The BRITE Constellation of nanosatellites obtained mmag photometry of 28 Cygni for 11 months in 2014-2016. Observations with the Solar Mass Ejection Imager in 2003-2010 and 118 H$α$ line profiles were added. For decades, 28 Cyg has exhibited four large-amplitude frequencies: two closely spaced frequencies of spectroscopically confirmed $g$ modes near 1.5 c/d, one slightly lower exophotospheric (Stefl) frequency, and at 0.05 c/d the difference frequency between the two g modes. This top-level framework is indistinguishable from eta Cen (Paper I), which is also very similar in spectral type, rotation rate, and viewing angle. The Stefl frequency is the only one that does not seem to be affected by the difference frequency. The amplitude of the latter undergoes large variations; around maximum the amount of near-circumstellar matter is increased, and the amplitude of the Stefl frequency grows by some factor. During such brightenings dozens of transient spikes appear in the frequency spectrum, concentrated in three groups. Only eleven frequencies were common to all years of BRITE observations. Be stars seem to be controlled by several coupled clocks, most of which are not very regular on timescales of weeks to months but function for decades. The combination of g modes to the low difference frequency and/or the atmospheric response to it appears significantly nonlinear. Like in eta Cen, the difference-frequency variability seems the main responsible for the modulation of the star-to-disc mass transfer in 28 Cyg. A hierarchical set of difference frequencies may reach the longest timescales known of the Be phenomenon.

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arXiv2017

Triple system HD 201433 with a SPB star component seen by BRITE-Constellation: Pulsation, differential rotation, and angular momentum transfer

Stellar rotation affects the transport of chemical elements and angular momentum and is therefore a key process during stellar evolution, which is still not fully understood. This is especially true for massive stars, which are important for the chemical enrichment of the universe. It is therefore important to constrain their physical parameters and internal angular momentum distribution to calibrate stellar structure and evolution models. Stellar internal rotation can be probed through asteroseismic studies of rotationally split oscillations but such results are still quite rare, especially for stars more massive than the Sun. The SPB star HD201433 is known to be part of a single-lined spectroscopic triple system, with two low-mass companions orbiting with periods of about 3.3 and 154 d. Our results are based on photometric observations made by BRITE - Constellation and the SMEI on board the Coriolis satellite, high-resolution spectroscopy, and more than 96 years of radial velocity measurements. We identify a sequence of 9 rotationally split dipole modes in the photometric time series and establish that HD201433 is in principle a solid-body rotator with a very slow rotation period of 297+/-76 d. Tidal interaction with the inner companion has, however, significantly accelerated the spin of the surface layers by a factor of approximately one hundred. The angular momentum transfer onto the surface of HD201433 is also reflected by the statistically significant decrease of the orbital period of about 0.9 s during the last 96 years. Combining the asteroseismic inferences with the spectroscopic measurements and the orbital analysis of the inner binary system, we conclude that tidal interactions between the central SPB star and its inner companion have almost circularised the orbit but not yet aligned all spins of the system and have just begun to synchronise rotation.

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arXiv2017

Studying the photometric and spectroscopic variability of the magnetic hot supergiant $ζ$ Orionis Aa

Massive stars play a significant role in the chemical and dynamical evolution of galaxies. However, much of their variability, particularly during their evolved supergiant stage, is poorly understood. To understand the variability of evolved massive stars in more detail, we present a study of the O9.2Ib supergiant $ζ$ Ori Aa, the only currently confirmed supergiant to host a magnetic field. We have obtained two-color space-based BRIght Target Explorer photometry (BRITE) for $ζ$ Ori Aa during two observing campaigns, as well as simultaneous ground-based, high-resolution optical CHIRON spectroscopy. We perform a detailed frequency analysis to detect and characterize the star's periodic variability. We detect two significant, independent frequencies, their higher harmonics, and combination frequencies: the stellar rotation period $P_{\mathrm{rot}} = 6.82\pm0.18$ d, most likely related to the presence of the stable magnetic poles, and a variation with a period of $10.0\pm0.3$ d attributed to circumstellar environment, also detected in the H$α$ and several He I lines, yet absent in the purely photospheric lines. We confirm the variability with $P_{\mathrm{rot}}$/4, likely caused by surface inhomogeneities, being the possible photospheric drivers of the discrete absorption components. No stellar pulsations were detected in the data. The level of circumstellar activity clearly differs between the two BRITE observing campaigns. We demonstrate that $ζ$ Ori Aa is a highly variable star with both periodic and non-periodic variations, as well as episodic events. The rotation period we determined agrees well with the spectropolarimetric value from the literature. The changing activity level observed with BRITE could explain why the rotational modulation of the magnetic measurements was not clearly detected at all epochs.

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arXiv2017

The Most Massive Heartbeat: An In-depth Analysis of ι Orionis

ι Ori is a well studied massive binary consisting of an O9 III + B1 III/IV star. Due to its high eccentricity (e = 0.764) and short orbital period (P orb = 29.13376 d), it has been considered to be a good candidate to show evidence of tidal effects; however, none have previously been identified. Using photometry from the BRITE-Constellation space photometry mission we have confirmed the existence of tidal distortions through the presence of a heartbeat signal at periastron. We combine spectroscopic and light curve analyses to measure the masses and radii of the components, revealing ι Ori to be the most massive heartbeat system known to date. In addition, using a thorough frequency analysis, we also report the unprecedented discovery of multiple tidally induced oscillations in an O star. The amplitudes of the pulsations allow us to empirically estimate the tidal circularization rate, yielding an effective tidal quality factor Q $\approx 4 \times 10^{4}$ .

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arXiv2016

The BRITE-Constellation Nanosatellite Space Mission And Its First Scientific Results

The BRIght Target Explorer (BRITE) Constellation is the first nanosatellite mission applied to astrophysical research. Five satellites in low-Earth orbits perform precise optical two-colour photometry of the brightest stars in the night sky. BRITE is naturally well suited for variability studies of hot stars. This contribution describes the basic outline of the mission and some initial problems that needed to be overcome. Some information on BRITE data products, how to access them, and how to join their scientific exploration is provided. Finally, a brief summary of the first scientific results obtained by BRITE is given.

astro-ph.SR↗
arXiv2016

Pulsations and outbursts in Be stars: Small differences - big impacts

New high-cadence observations with BRITE covering many months confirm that coupled pairs of nonradial pulsation modes are widespread among early-type Be stars. With the difference frequency between the parental variations they may form a roughly sinusoidal variability or the amplitude may cyclicly vary. A first - amplified - beat pattern is also found. In all three cases the amplitudes of difference frequencies can exceed the amplitude sum of the base frequencies, and modulations of the star-to-circumstellar-disk mass-transfer rate may be associated with these slow variations. This suggests more strongly than any earlier observations that significant dissipation of pulsational energy in the atmosphere may be a cause of mass ejections from Be stars. A unifying interpretative concept is presented.

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arXiv2016

Combining BRITE and ground-based photometry for the Beta Cephei star Nu Eridani: impact on photometric pulsation mode identification and detection of several g modes

We report a simultaneous ground and space-based photometric study of the Beta Cephei star Nu Eridani. Half a year of observations have been obtained by four of the five satellites constituting BRITE-Constellation, supplemented with ground-based photoelectric photometry. We show that carefully combining the two data sets virtually eliminates the aliasing problem that often hampers time-series analyses. We detect 40 periodic signals intrinsic to the star in the light curves. Despite a lower detection limit we do not recover all the pressure and mixed modes previously reported in the literature, but we newly detect six additional gravity modes. This behaviour is a consequence of temporal changes in the pulsation amplitudes that we also detected for some of the p modes. We point out that the dependence of theoretically predicted pulsation amplitude on wavelength is steeper in visual passbands than those observationally measured, to the extent that the three dominant pulsation modes of Nu Eridani would be incorrectly identified using data in optical filters only. We discuss possible reasons for this discrepancy.

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arXiv2016

The BRITE Constellation nanosatellite mission: Testing, commissioning and operations

BRITE (BRIght Target Explorer) Constellation, the first nanosatellite mission applied to astrophysical research, is a collaboration among Austria, Canada and Poland. The fleet of satellites (6 launched, 5 functioning) performs precise optical photometry of the brightest stars in the night sky. A pioneering mission like BRITE - with optics and instruments restricted to small volume, mass and power in several nanosatellites, whose measurements must be coordinated in orbit - poses many unique challenges. We discuss the technical issues, including problems encountered during on-orbit commissioning (especially higher-than expected sensitivity of the CCDs to particle radiation). We describe in detail how the BRITE team has mitigated these problems, and provide a complete overview of mission operations. This paper serves as a template for how to effectively plan, build and operate future low-cost niche-driven space astronomy missions.

astro-ph.IM↗
arXiv2016

Massive pulsating stars observed by BRITE-Constellation. I. The triple system Beta Centauri (Agena)

This paper aims to precisely determine the masses and detect pulsation modes in the two massive components of Beta Cen with BRITE-Constellation photometry. In addition, seismic models for the components are considered and the effects of fast rotation are discussed. This is done to test the limitations of seismic modeling for this very difficult case. A simultaneous fit of visual and spectroscopic orbits is used to self-consistently derive the orbital parameters, and subsequently the masses, of the components. The derived masses are equal to 12.02 +/- 0.13 and 10.58 +/- 0.18 M_Sun. The parameters of the wider, A - B system, presently approaching periastron passage, are constrained. Analysis of the combined blue- and red-filter BRITE-Constellation photometric data of the system revealed the presence of 19 periodic terms, of which eight are likely g modes, nine are p modes, and the remaining two are combination terms. It cannot be excluded that one or two low-frequency terms are rotational frequencies. It is possible that both components of Beta Cen are Beta Cep/SPB hybrids. An attempt to use the apparent changes of frequency to distinguish which modes originate in which component did not succeed, but there is potential for using this method when more BRITE data become available. Agena seems to be one of very few rapidly rotating massive objects with rich p- and g-mode spectra, and precisely known masses. It can therefore be used to gain a better understanding of the excitation of pulsations in relatively rapidly rotating stars and their seismic modeling. Finally, this case illustrates the potential of BRITE-Constellation data for the detection of rich-frequency spectra of small-amplitude modes in massive pulsating stars.

astro-ph.SR↗
arXiv2016

Short-term variability and mass loss in Be stars I. BRITE satellite photometry of $η$ and $μ$ Centauri

Empirical evidence for the involvement of nonradial pulsations (NRP's) in the mass loss from Be stars ranges from (i) a singular case (\object{$μ$ Cen}) of repetitive mass ejections triggered by multi-mode beating to (ii) several photometric reports about enormous numbers of pulsation modes popping up during outbursts and on to (iii) effective single-mode pulsators. The BRITE Constellation of nanosatellites was used to obtain mmag photometry of the Be stars $η$ and \object{$μ$ Cen}. In the low-inclination star \object{$μ$ Cen}, light pollution by variable amounts of near-stellar matter prevented any new insights into the variability and other properties of the central star. In the equator-on star \object{$η$ Cen}, BRITE photometry and {\sc Heros} echelle spectroscopy from the 1990s reveal an intricate clockwork of star-disk interactions. The mass transfer is modulated with the frequency difference of two NRP modes and an amplitude three times as large as the amplitude sum of the two NRP modes. This process feeds a high-amplitude circumstellar activity running with the incoherent and slightly lower so-called Štefl frequency. The mass loss-modulation cycles are tightly coupled to variations in the value of the Štefl frequency and in its amplitude, albeit with strongly drifting phase differences. The observations are well described by the decomposition of the mass loss into a pulsation-related engine in the star and a viscosity-dominated engine in the circumstellar disk. Arguments are developed that large-scale gas-circulation flows occur at the interface. The propagation rates of these eddies manifest themselves as Štefl frequencies. Bursts in power spectra during mass-loss events can be understood as the noise inherent to these gas flows.

astro-ph.SR↗
arXiv2016

Planet Hunters X. KIC 8462852 - Where's the Flux?

Over the duration of the Kepler mission, KIC8462852 was observed to undergo irregularly shaped, aperiodic dips in flux of up to $\sim 20$\%. The dipping activity can last for between 5 and 80 days. We characterize the object with high-resolution spectroscopy, spectral energy distribution fitting, radial velocity measurements, high-resolution imaging, and Fourier analyses of the Kepler light curve. We determine that KIC8462852 is a typical main-sequence F3 V star that exhibits no significant IR excess, and has no very close interacting companions. In this paper, we describe various scenarios to explain the dipping events observed in the Kepler light curve. We confirm that the dipping signals in the data are not caused by any instrumental or data processing artifact, and thus are astrophysical in origin. We construct scenario-independent constraints on the size and location of a body in the system that is needed to reproduce the observations. We deliberate over several assorted stellar and circumstellar astrophysical scenarios, most of which have problems explaining the data in hand. By considering the observational constraints on dust clumps in orbit around a normal main-sequence star, we conclude that the scenario most consistent with the data in hand is the passage of a family of exocomet or planetesimal fragments, all of which are associated with a single previous break-up event, possibly caused by tidal disruption or thermal processing. The minimum total mass associated with these fragments likely exceeds $10^{-6}$~\mearth, corresponding to an original rocky body of $>100$~km in diameter. We discuss the necessity of future observations to help interpret the system.

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