We analyze the K2 light curve of the TRAPPIST-1 system. The Fourier analysis of the data suggests Prot = 3.295 0.003 days. The light curve shows several flares, of which we analyzed 42 events with ...integrated flare energies of 1.26 × 1030-1.24 × 1033 erg. Approximately 12% of the flares were complex, multi-peaked eruptions. The flaring and the possible rotational modulation shows no obvious correlation. The flaring activity of TRAPPIST-1 probably continuously alters the atmospheres of the orbiting exoplanets, which makes these less favorable for hosting life.
ABSTRACT
Coronal mass ejections (CMEs) may have major importance for planetary and stellar evolution. Stellar CME parameters, such as mass and velocity, have yet not been determined statistically. So ...far only a handful of stellar CMEs has been detected mainly on dMe stars using spectroscopic observations. We therefore aim for a statistical determination of CMEs of solar-like stars by using spectroscopic data from the ESO phase 3 and Polarbase archives. To identify stellar CMEs, we use the Doppler signal in optical spectral lines being a signature of erupting filaments that are closely correlated to CMEs. We investigate more than 3700 h of on-source time of in total 425 dF-dK stars. We find no signatures of CMEs and only few flares. To explain this low level of activity, we derive upper limits for the non-detections of CMEs and compare those with empirically modelled CME rates. To explain the low number of detected flares, we adapt a flare power law derived from EUV data to the H α regime, yielding more realistic results for H α observations. In addition, we examine the detectability of flares from the stars by extracting Sun-as-a-star H α light curves. The extrapolated maximum numbers of observable CMEs are below the observationally determined upper limits, which indicates that the on-source times were mostly too short to detect stellar CMEs in H α. We conclude that these non-detections are related to observational biases in conjunction with a low level of activity of the investigated dF-dK stars.
Abstract
We present near-IR (NIR) and optical observations of the Type Ic supernova (SN Ic) SN 2020oi in the galaxy M100 and the broad-lined SN Ic SN 2020bvc in UGC 9379, using Gemini, Las Cumbres ...Observatory, Southern Astrophysical Telescope, and other ground-based telescopes. The NIR spectrum of SN 2020oi at day 63 since the explosion shows strong CO emissions and a rising
K
-band continuum, which is the first unambiguous dust detection from an SN Ic. Non-LTE CO modeling shows that CO is still optically thick and that the lower limit to the CO mass is 10
−3
M
⊙
. The dust temperature is 810 K, and the dust mass is ∼10
−5
M
⊙
. We explore the possibilities that the dust is freshly formed in the ejecta, heated dust in the preexisting circumstellar medium, and an infrared echo. The light curves of SN 2020oi are consistent with a STELLA model with canonical explosion energy, 0.07
M
⊙
Ni mass, and 0.7
M
⊙
ejecta mass. A model of high explosion energy of 10
52
erg, 0.4
M
⊙
Ni mass, and 6.5
M
⊙
ejecta mass with the circumstellar matter reproduces the double-peaked light curves of SN 2020bvc. We observe temporal changes of absorption features of the IR Ca
ii
triplet, S
i
at 1.043
μ
m, and Fe
ii
at 5169 Å. The blueshifted lines indicate high velocities, up to 60,000 km s
−1
for SN 2020bvc and 20,000 km s
−1
for SN 2020oi, and the expansion velocity rapidly declines before the optical maximum. We present modeled spectral signatures and diagnostics of CO and SiO molecular bands between 1.4 and 10
μ
m.
ABSTRACT
We present extensive observations of SN 2018zd covering the first ∼450 d after the explosion. This SN shows a possible shock-breakout signal ∼3.6 h after the explosion in the unfiltered ...light curve, and prominent flash-ionization spectral features within the first week. The unusual photospheric temperature rise (rapidly from ∼12 000 to above 18 000 K) within the earliest few days suggests that the ejecta were continuously heated. Both the significant temperature rise and the flash spectral features can be explained by the interaction of the SN ejecta with the massive stellar wind ($0.18^{+0.05}_{-0.10}\, \rm M_{\odot }$), which accounts for the luminous peak ($L_{\rm max} = 1.36\pm 0.63 \times 10^{43}\, \rm erg\, s^{-1}$) of SN 2018zd. The luminous peak and low expansion velocity (v ≈ 3300 km s−1) make SN 2018zd like a member of the LLEV (luminous SNe II with low expansion velocities) events originating due to circumstellar interaction. The relatively fast post-peak decline allows a classification of SN 2018zd as a transition event morphologically linking SNe IIP and SNe IIL. In the radioactive-decay phase, SN 2018zd experienced a significant flux drop and behaved more like a low-luminosity SN IIP both spectroscopically and photometrically. This contrast indicates that circumstellar interaction plays a vital role in modifying the observed light curves of SNe II. Comparing nebular-phase spectra with model predictions suggests that SN 2018zd arose from a star of $\sim 12\, \rm M_{\odot }$. Given the relatively small amount of 56Ni ($0.013\!-\!0.035 \rm M_{\odot }$), the massive stellar wind, and the faint X-ray radiation, the progenitor of SN 2018zd could be a massive asymptotic giant branch star that collapsed owing to electron capture.
The ultrafast-rotating (P sub(rot)approximate 0.44 d) fully convective single M4 dwarf V374Peg is a well-known laboratory for studying intense stellar activity in a stable magnetic topology. As an ...observable proxy for the stellar magnetic field, we study the stability of the light curve, hence the spot configuration. We also measure the occurrence rate of flares and coronal mass ejections (CMEs). We have analysed spectroscopic observations, BV(RI) sub(C) photometry covering 5 yrs, and additional R sub(C) photometry that expands the temporal base over 16 yr. The light curve suggests an almost rigid-body rotation and a spot configuration that is stable over about 16 yrs, confirming the previous indications of a very stable magnetic field. We observed small changes on a nightly timescale and frequent flaring, including a possible sympathetic flare. The strongest flares seem to be more concentrated around the phase where the light curve indicates a smaller active region. Spectral data suggest a complex CME with falling-back and re-ejected material with a maximal projected velocity of ~675kms super(-1). We observed a CME rate that is much lower than expected from extrapolations of the solar flare-CME relation to active stars.
Planet Hunters IX. KIC 8462852 – where's the flux? Boyajian, T. S; LaCourse, D. M; Rappaport, S. A ...
Monthly notices of the Royal Astronomical Society,
04/2016, Letnik:
457, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Over the duration of the Kepler mission, KIC 8462852 was observed to undergo irregularly shaped, aperiodic dips in flux of up to ∼20 per cent. The dipping activity can last for between 5 and 80 d. 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 KIC 8462852 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 artefact, and thus are astrophysical in origin. We construct scenario-independent constraints on the size and location of a body in the system that are 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 M⊕, corresponding to an original rocky body of >100 km in diameter. We discuss the necessity of future observations to help interpret the system.
A star under multiple influences Zs. Kővári; Strassmeier, K G; Kriskovics, L ...
Astronomy and astrophysics (Berlin),
04/2024, Letnik:
684
Journal Article
Recenzirano
Context. Close binaries with magnetically active components are astrophysical laboratories for studying the effects of binarity on activity. Of particular interest are binary and multiple star ...systems that contain a solar-type active component with an internal structure similar to the Sun, allowing us to study how the dynamo of a solar-type star would work under different conditions. Aims. We have conducted a comprehensive investigation of V815 Her using photometric and spectroscopic data to understand the origin of the activity and what influences it in the short and long term. Methods. Using space photometry we performed light curve modeling in order to derive astrophysical and orbital parameters for the eclipsing binary subsystem V815 Her B. Using archival photometric data covering a century we carried out a time frequency analysis. Spectral synthesis was applied to determine the basic astrophysical parameters of the rapidly rotating primary using high-resolution STELLA spectra recorded in 2018. Results. Photometric analysis of archived data revealed multiple cycles on timescales between ∼6.5 and ∼26 yr, some of which may be harmonic. From TESS photometry we obtained an orbital solution for the V815 Her B subsystem. By placing the primary component on the Hertzsprung–Russell-diagram, we could deduce an age of ≈30 Myr, in line with the high Li-6707 abundance. The STELLA spectra covering the 200 day-long observing season enabled us to create 19 time-series Doppler images, which revealed a constantly changing spotted surface on a timescale of a few weeks. From the consecutive image pairs we built up the average cross-correlation function map to measure the surface differential rotation of the spotted star, from which we derive a weak solar-type surface shear. Conclusions. We found evidence that the V815 Her B component previously apostrophized as a “third body” is actually an eclipsing close binary subsystem of two M dwarfs with a period of 0.5 d, that is, V815 Her is a 2+2 hierarchical quadruple system. The system is apparently young, only a few times ten million years old, consistent with the spotted primary V815 Her Aa being a zero-age main-sequence star. Spot activity on the primary was found to be vivid. Fast starspot decay suggests that convective-turbulent erosion plays a more significant role in such a rapidly rotating star. The weak surface shear of V815 Her Aa due to differential rotation is presumably confined by tidal forces of the close companion V815 Her Ab. The slowly increasing photometric cycle of about 6.5 yr on average is interpreted as a spot cycle of V815 Her Aa, which is probably modulated by the eccentric wide orbit.
Abstract
In the last decade a number of rapidly evolving transients have been discovered that are not easily explained by traditional supernova models. We present optical and UV data on one such ...object, SN 2018gep, that displayed a fast rise with a mostly featureless blue continuum around peak, and evolved to develop broad features typical of an SN Ic-bl while retaining significant amounts of blue flux throughout its observations. This blue excess is most evident in its near-UV flux, which is over 4 mag brighter than other stripped-envelope supernovae, and is still visible in optical
g
–
r
colors. Its fast rise time of
t
rise,
V
= 5.6 ± 0.5 days puts it squarely in the emerging class of Fast Evolving Luminous Transients, or Fast Blue Optical Transients. With a peak absolute magnitude of
M
v
= −19.53 ± 0.23 mag it is on the extreme end of both the rise time and peak magnitude distribution for SNe Ic-bl. These observations are consistent with a simple SN Ic-bl model that has an additional form of energy injection at early times that drives the observed rapid, blue rise. We show that SN 2018gep and the literature SN iPTF16asu have similar photometric and spectroscopic properties and that they overall share many similarities with both SNe Ic-bl and Fast Evolving Transients. Based on our SN 2018gep host galaxy data we derive a number of properties, and we show that the derived host galaxy properties for both SN 2018gep and iPTF16asu are consistent with the SNe Ic-bl and gamma-ray burst/supernova sample while being on the extreme edge of the observed Fast Evolving Transient sample.
Context.
Late-type stars in close binary systems can exhibit strong magnetic activity owing to rapid rotation supported by tidal locking. On the other hand, tidal coupling may suppress the ...differential rotation which is a key ingredient of the magnetic dynamo.
Aims.
We studied the red dwarf component in the eclipsing binary system V471 Tau in order to unravel the relations between the different activity layers, from the stellar surface through the chromosphere up to the corona. Our aim is to study how the magnetic dynamo in the late-type component is affected by the close white dwarf companion.
Methods.
We used space photometry, high-resolution spectroscopy, and X-ray observations from different space instruments to explore the main characteristics of magnetic activity. We applied a light curve synthesis program to extract the eclipsing binary model and to analyze the residual light variations. Photometric periods were obtained using a Fourier-based period search code. We searched for flares by applying an automated flare detection code. Spectral synthesis was used to derive or specify some of the astrophysical parameters. Doppler imaging was used to reconstruct surface temperature maps, which were cross-correlated to derive surface differential rotation. We applied different conversion techniques to make it possible to compare the X-ray emissions obtained from different space instruments.
Results.
From the K2 photometry we found that 5–10 per cent of the apparent surface of the red dwarf is covered by cool starspots. From seasonal photometric period changes we estimated a weak differential rotation. From the flare activity we derived a cumulative flare frequency diagram which suggests that frequent flaring could have a significant role in heating the corona. Using high-resolution spectroscopy we reconstructed four Doppler images for different epochs which reveal an active longitude, that is, a permanent dominant spot facing the white dwarf. From short term changes in the consecutive Doppler images we derived a weak solar-type surface differential rotation with
α
DR
= 0.0026 shear coefficient, similar to that provided by photometry. The long-term evolution of X-ray luminosity reveals a possible activity cycle length of ≈12.7 yr, traces of which were also discovered in the H
α
spectra.
Conclusions.
We conclude that the magnetic activity of the red dwarf component in V471 Tau is strongly influenced by the close white dwarf companion. We confirm the presence of a permanent dominant spot (active longitude) on the red dwarf facing the white dwarf. The weak differential rotation of the red dwarf is very likely the result of tidal confinement by the companion. We find that the periodic appearance of the inter-binary H
α
emission from the vicinity of the inner Lagrangian point is correlated with the activity cycle.
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