Photospheric Temperature Research Articles

A Long-term Photometric Variability and Spectroscopic Study of Luminous Blue Variable AF And in M31

Abstract We present a photometric and spectroscopic analysis of the Hubble–Sandage variable star AF And in M31. The data have been taken under the Nainital Microlensing Survey during 1998–2002, and follow-up observations were carried out until 2011. During this period, photometric observations in Cousins R and I bands were obtained for 169 nights spanning about 5000 days. AF And showed a prominent outburst around 1999 mid-January, followed by a gradual decrease in brightness of about 1.5 mag in the next 3 yr with a declining rate of ∼0.0015 mag day−1, leading to a quiescent phase at the end of 2001. After lying low for about 9 yr, AF And again went through a secondary outburst phase in late 2010 with an amplitude of 0.44 mag, where it lasted for one year before fading back to its quiescent phase. Spectroscopic observations of AF And show prominent Balmer and He i emission lines along with the comparatively weaker Fe ii and [Fe ii] emissions. Asymmetric emission line profiles in its spectrum imply a mass-loss rate of about 2.2 × 10−4 M ⊙ yr−1 through the stellar winds in the photosphere. Using spectral energy distribution fitting, we find a photospheric temperature of 33,000 ± 3000 K during the visual minimum. Using a weak P Cygni profile of the He i emission line, the wind terminal velocity for AF And is found to be around 280–300 km s−1.

Heating of a Quiet Region of the Solar Chromosphere by Ion and Neutral Acoustic Waves

Using high-resolution numerical simulations we investigate the plasma heating driven by periodic two-fluid acoustic waves that originate at the bottom of the photosphere and propagate into the gravitationally stratified and partially ionized solar atmosphere. We consider ions+electrons and neutrals as separate fluids that interact between themselves via collision forces. The latter play an important role in the chromosphere, leading to significant damping of short-period waves. Long-period waves do not essentially alter the photospheric temperatures, but they exhibit the capability of depositing a part of their energy in the chromosphere. This results in up about a five times increase of ion temperature that takes place there on a time-scale of a few minutes. The most effective heating corresponds to waveperiods within the range of about 30-200 s with a peak value located at 80 s. However, we conclude that for the amplitude of the driver chosen to be equal to 0.1 km s$^{-1}$, this heating is too low to balance the radiative losses in the chromosphere.

Discovering the atmospheres of hot Jupiters

AbstractHot Jupiters are an extraordinary class of exoplanets, orbiting their host stars with periods of hours to a few days. Some of these objects have day-side temperatures approaching photospheric temperatures of late K-type stars. I will give an overview of how we characterize the atmospheres of these fascinating objects and some the more recent exciting results to come from ground and space-based telescopes, as well as what the future holds for detailed characterization of short-period exoplanet atmospheres.

High-resolution Optical Spectropolarimetry of Nova V339 Del: Spatial Distribution of Nova Ejecta during the Early Phase of Explosion

Abstract High-resolution spectropolarimetric observations of V339 Del were performed in the optical wavelength region every night from UT 2013 August 15.49 to 21.23 (corresponding to the pre- and near-maximum phases of the nova) to reveal the asymmetry of the nova ejecta shortly after the explosion. Observed polarization spectra showed intrinsic line polarization for Hα, O i λ777.3 nm, Si ii λλ634.7, 637.1 nm, in addition to intrinsic continuum polarization. The line polarization of Hα on August 15.49 (+0.91 day post-discovery) indicates an expanding nova ejecta, which contains a torus surrounding the nova photosphere. The line polarization of O i λ777.3 nm is also consistent with this hypothesis. The line polarizations of Si ii λλ634.7, 637.1 nm might indicate a variation in the photospheric temperature of the nova on the same date. Based on the day-by-day changes of the intrinsic continuum and line polarization found in the observations, it can be concluded that the effective geometry of V339 Del had changed from a torus-dominant to a bipolar-shaped geometry between +0.91 and +2.15 days post-discovery. The intrinsic continuum polarization became weaker after +2.93 days due to the larger size of the expanding nova ejecta. This is consistent with high-angular observations of V339 Del with a near-infrared interferometer and with a recent nova ejecta model that can explain the γ-ray emission and dust formation found in V339 Del.

Disentangling the Planet from the Star in Late-Type M Dwarfs: A Case Study of TRAPPIST-1g

Abstract The atmospheres of late M stars represent a significant challenge in the characterization of any transiting exoplanets because of the presence of strong molecular features in the stellar atmosphere. TRAPPIST-1 is an ultracool dwarf, host to seven transiting planets, and contains its own molecular signatures that can potentially be imprinted on planetary transit lightcurves as a result of inhomogeneities in the occulted stellar photosphere. We present a case study on TRAPPIST-1g, the largest planet in the system, using a new observation together with previous data, to disentangle the atmospheric transmission of the planet from that of the star. We use the out-of-transit stellar spectra to reconstruct the stellar flux on the basis of one, two, and three temperature components. We find that TRAPPIST-1 is a 0.08 M *, 0.117 R *, M8V star with a photospheric effective temperature of 2400 K, with ∼35% 3000 K spot coverage and a very small fraction, <3%, of ∼5800 K hot spot. We calculate a planetary radius for TRAPPIST-1g to be R p = 1.124 R ⊕with a planetary density of ρ p = 0.8214 ρ ⊕. On the basis of the stellar reconstruction, there are 11 plausible scenarios for the combined stellar photosphere and planet transit geometry; in our analysis, we are able to rule out eight of the 11 scenarios. Using planetary models, we evaluate the remaining scenarios with respect to the transmission spectrum of TRAPPIST-1g. We conclude that the planetary transmission spectrum is likely not contaminated by any stellar spectral features and are able to rule out a clear solar H2/He-dominated atmosphere at greater than 3σ.

Mapping EK Draconis with PEPSI

Aims. We present the first temperature surface map of EK Dra from very-high-resolution spectra obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope. Methods. Changes in spectral line profiles are inverted to a stellar surface temperature map using our iMap code. The long-term photometric record is employed to compare our map with previously published maps. Results. Four cool spots were reconstructed, but no polar spot was seen. The temperature difference to the photosphere of the spots is between 990 and 280 K. Two spots are reconstructed with a typical solar morphology with an umbra and a penumbra. For the one isolated and relatively round spot (spot A), we determine an umbral temperature of 990 K and a penumbral temperature of 180 K below photospheric temperature. The umbra to photosphere intensity ratio of EK Dra is approximately only half of that of a comparison sunspot. A test inversion from degraded line profiles showed that the higher spectral resolution of PEPSI reconstructs the surface with a temperature difference that is on average 10% higher than before and with smaller surface areas by ~10–20%. PEPSI is therefore better suited to detecting and characterising temperature inhomogeneities. With ten more years of photometry, we also refine the spot cycle period of EK Dra to 8.9 ± 0.2 yr with a continuing long-term fading trend. Conclusions. The temperature morphology of spot A so far appears to show the best evidence for the existence of a solar-like penumbra for a starspot. We emphasise that it is more the non-capture of the true umbral contrast rather than the detection of the weak penumbra that is the limiting factor. The relatively small line broadening of EK Dra, together with the only moderately high spectral resolutions previously available, appear to be the main contributors to the lower-than-expected spot contrasts when comparing to the Sun.

The Effect of Spots on the Luminosity Spread of the Pleiades

Abstract Cool spots on the surface of magnetically active stars modulate their observed brightnesses and temperatures, thereby affecting the stellar locus on the H-R diagram. Recent high-precision space-based photometric surveys reveal the rotational modulation from spots on stars in young clusters, including K2 monitoring of the 125 Myr old Pleiades cluster. However, light curves reveal only the asymmetries in the visible spot distributions rather than the total sizes of spots on stellar surfaces, which causes a discrepancy between the spot coverage measured by photometric and spectroscopic observations. In this paper, we simulate photometric variability introduced by randomly distributed starspots on a 125 Myr old coeval cluster. Our simulation results show that randomly distributed small spots on the stellar surface would explain the discrepancy that the photometric observations only reveal 10%–40% of the spot coverage measured by spectra. The colors and luminosities of photospheres are modeled for a range of photospheric temperature, spot coverage, and spot temperature. The colors and luminosities of a simulated population are then compared to the luminosity spread of Pleiades members, excluding the 25% of candidates that are identified as non-members with Gaia DR2 astrometry. The observed luminosities of Pleiades members have a standard deviation of 0.05 dex, which could be entirely explained by spots with a star-to-star standard deviation of spot coverage of 10%, but with an average coverage area that is not well constrained.

SN 2014J in M82: new insights on the spectral diversity of Type Ia supernovae

ABSTRACT We present extensive spectroscopic observations for one of the closest Type Ia supernovae (SNe Ia), SN 2014J discovered in M82, ranging from 10.4 d before to 473.2 d after B-band maximum light. The diffuse interstellar band features detected in a high-resolution spectrum allow an estimate of line-of-sight extinction as Av ∼ 1.9 ± 0.6 mag. Spectroscopically, SN 2014J can be put into the high-velocity (HV) subgroup in Wang’s classification with a velocity of Si ii λ 6355 at maximum light of $v$0 = 1.22 ± 0.01 × 104 km s−1 but has a low velocity gradient (LVG, following Benetti’s classification) of $\dot{v}=41\pm 2$ km s−1 d−1, which is inconsistent with the trend that HV SNe Ia generally have larger velocity gradients. We find that the HV SNe Ia with LVGs tend to have relatively stronger Si iii (at ∼4400 Å) absorptions in early spectra, larger ratios of S ii λ 5468 to S ii λ 5640, and weaker Si ii 5972 absorptions compared to their counterparts with similar velocities but high velocity gradients. This shows that the HV+LVG subgroup of SNe Ia may have intrinsically higher photospheric temperature, which indicates that their progenitors may experience more complete burning in the explosions relative to the typical HV SNe Ia.

Temperature gradient in the solar photosphere. Test of a new spectroscopic method and study of its feasibility for ground-based telescopes

Context. The contribution of quiet-Sun regions to the solar irradiance variability is currently unclear. Some solar-cycle variations of the quiet-Sun physical structure, such as the temperature gradient, might affect the irradiance. The synoptic measurement of this quantity along the activity cycle would improve our understanding of long-term irradiance variations. Aims. We intend to test a method previously introduced for measuring the photospheric temperature gradient from high-resolution spectroscopic observation and to study its feasibility with ground-based instruments with and without adaptative optics. Methods. We used synthetic profiles of the FeI 630.15 nm obtained from realistic three-dimensional hydrodynamical simulations of the photospheric granulation and line radiative transfer computations under local thermodynamical equilibrium conditions. Synthetic granulation images at different levels in the line are obtained by convolution with the instrumental point spread function (PSF) under various conditions of atmospheric turbulence, with and without correction by an adaptative optics (AO) system. The PSF are obtained with the PAOLA software, and the AO performances are inspired by the system that will be operating on the Daniel K. Inouye Solar Telescope. Results. We consider two different conditions of atmospheric turbulence, with Fried parameters of 7 cm and 5 cm, respectively. We show that the degraded images lead to both a bias and a loss of precision in the temperature-gradient measurement, and that the correction with the AO system allows us to drastically improve the measurement quality. Conclusions. Long-term synoptic observations of the temperature gradient in the solar photosphere can be undertaken by implementing this method on ground-based solar telescopes that are equipped with an AO correction system.

Multicolor Photometry and Spectroscopy of the Yellow Supergiant with Dust Envelope HD 179821 = V1427 Aquilae

We present the results of multicolor (UBVJHKLM) photometry (2009-2017) and low-resolution spectroscopy (2016-2017) of the semi-regular variable V1427 Aql=HD 179821, a yellow supergiant with gas-dust envelope. The star displays low-amplitude ({\Delta}V<0.2 mag) semi-periodic brightness variation superimposed on a long-term trend. The light curve shape and timescale change from cycle to cycle. There are temperature variations characteristic for pulsations, and brightness oscillations with no significant change of color are also observed. The UBV data for the 2009-2011 interval are well reproduced by a superposition of two periodic components with P=170 d and P=141 d (or P=217 d - the one year alias of P=141 d). The variation became less regular after 2011, the timescale increased and exceeded 250 days. In 2009-2016 the annual average brightness monotonically increased in V, J, K, whereas it decreased in U and B. The annual average U-B, B-V, and J-K colors grew, the star was getting redder. The cooling and expanding of the star photosphere along with the increasing of luminosity may explain the long-term trend in brightness and colors. Based on our photometric data we suppose that the photosphere temperature decreased by ~400 K in the 2008-2016 interval, the radius increased by ~24 %, and the luminosity grew by ~19 %. Low-resolution spectra in the {\lambda}4000-9000 {\AA} wavelength range obtained in 2016-2017 indicate significant changes in the spectrum of V1427 Aql as compared with the 1994-2008 interval, i.e. the BaII and near-infrared CaII triplet absorptions have turned stronger while the OI {\lambda}7771-4 triplet blend has weakened that points out the decrease of temperature in the region where the absorptions are formed. The evolutionary stage of the star is discussed. We also compare V1427 Aql with post-AGB stars and yellow hypergiants.

The Productivity of Oxygenic Photosynthesis around Cool, M Dwarf Stars

Abstract In the search for life around cool stars, the presence of atmospheric oxygen is a prominent biosignature, as it may indicate oxygenic photosynthesis (OP) on the planetary surface. On Earth, most oxygenic photosynthesizing organisms (OPOs) use photons between 400 and 750 nm, which have sufficient energy to drive the photosynthetic reaction that generates O2 from H2O and CO2. OPOs around cool stars may evolve similar biological machinery capable of producing oxygen from water. However, in the habitable zones (HZs) of the coolest M dwarf stars, the flux of 400–750 nm photons may be just a few percent that of Earth’s. We show that the reduced flux of 400–750 nm photons around M dwarf stars could result in Earth-like planets being growth limited by light, unlike the terrestrial biosphere, which is limited by nutrient availability. We consider stars with photospheric temperatures between 2300 and 4200 K and show that such light-limited worlds could occur at the outer edge of the HZ around TRAPPIST-1-like stars. We find that even if OP can use photons longer than 750 nm, there would still be insufficient energy to sustain the Earth’s extant biosphere throughout the HZ of the coolest stars. This is because such stars emit largely in the infrared and near-infrared, which provide sufficient energy to make the planet habitable, but limits the energy available for OP. TRAPPIST-1f and g may fall into this category. Biospheres on such planets, potentially limited by photon availability, may generate small biogenic signals, which could be difficult for future observations to detect.

An Empirical Fitting Method to Type Ia Supernova Light Curves. III. A Three-parameter Relationship: Peak Magnitude, Rise Time, and Photospheric Velocity

Abstract We examine the relationship between three parameters of Type Ia supernovae (SNe Ia): peak magnitude, rise time, and photospheric velocity at the time of peak brightness. The peak magnitude is corrected for extinction using an estimate determined from MLCS2k2 fitting. The rise time is measured from the well-observed B-band light curve with the first detection at least 1 mag fainter than the peak magnitude, and the photospheric velocity is measured from the strong absorption feature of Si ii λ6355 at the time of peak brightness. We model the relationship among these three parameters using an expanding fireball with two assumptions: (a) the optical emission is approximately that of a blackbody, and (b) the photospheric temperatures of all SNe Ia are the same at the time of peak brightness. We compare the precision of the distance residuals inferred using this physically motivated model against those from the empirical Phillips relation and the MLCS2k2 method for 47 low-redshift SNe Ia (0.005 &lt; z &lt; 0.04) and find comparable scatter. However, SNe Ia in our sample with higher velocities are inferred to be intrinsically fainter. Eliminating the high-velocity SNe and applying a more stringent extinction cut to obtain a “low-v golden sample” of 22 SNe, we obtain significantly reduced scatter of 0.108 ± 0.018 mag in the new relation, better than those of the Phillips relation and the MLCS2k2 method. For 250 km s−1 of residual peculiar motions, we find 68% and 95% upper limits on the intrinsic scatter of 0.07 and 0.10 mag, respectively.

Super-luminous Type II supernovae powered by magnetars

Magnetar power is believed to be at the origin of numerous super-luminous supernovae (SNe) of Type Ic, arising from compact, hydrogen-deficient, Wolf-Rayet type stars. Here, we investigate the properties that magnetar power would have on standard-energy SNe associated with 15–20 M⊙ supergiant stars, either red (RSG; extended) or blue (BSG; more compact). We have used a combination of Eulerian gray radiation-hydrodynamics and non-LTE steady-state radiative transfer to study their dynamical, photometric, and spectroscopic properties. Adopting magnetar fields of 1, 3.5, 7 × 1014 G and rotational energies of 0.4, 1, and 3 × 1051 erg, we produce bolometric light curves with a broad maximum covering 50–150 d and a magnitude of 1043–1044 erg s−1. The spectra at maximum light are analogous to those of standard SNe II-P but bluer. Although the magnetar energy is channelled in equal proportion between SN kinetic energy and SN luminosity, the latter may be boosted by a factor of 10–100 compared to a standard SN II. This influence breaks the observed relation between brightness and ejecta expansion rate of standard Type II SNe. Magnetar energy injection also delays recombination and may even cause re-ionization, with a reversal in photospheric temperature and velocity. Depositing the magnetar energy in a narrow mass shell at the ejecta base leads to the formation of a dense shell at a few 1000 km s−1, which causes a light-curve bump at the end of the photospheric phase. Depositing this energy over a broad range of mass in the inner ejecta, to mimic the effect of multi-dimensional fluid instabilities, prevents the formation of a dense shell and produces an earlier-rising and smoother light curve. The magnetar influence on the SN radiation is generally not visible prior to 20–30 d, during which one may discern a BSG from a RSG progenitor. We propose a magnetar model for the super-luminous Type II SN OGLE-SN14-073.

Observed Type II supernova colours from the Carnegie Supernova Project-I

We present a study of observed Type II supernova (SN~II) colours using optical/near-infrared photometric data from the \textit{Carnegie Supernovae Project-I}. We analyse four colours ($B-V$, $u-g$, $g-r$, and $g-Y$) and find that SN~II colour curves can be described by two linear regimes during the photospheric phase. The first ($s_{\rm 1,colour}$) is steeper and has a median duration of $\sim 40$ days. The second, shallower slope ($s_{\rm 2,colour}$) lasts until the end of the "plateau" ($\sim 80$ days). The two slopes correlate in the sense that steeper initial colour curves also imply steeper colour curves at later phases. As suggested by recent studies, SNe~II form a continuous population of objects from the colour point of view as well. We investigate correlations between the observed colours and a range of photometric and spectroscopic parameters including the absolute magnitude, the $V$-band light-curve slopes, and metal-line strengths. We find that less luminous SNe~II appear redder, a trend that we argue is not driven by uncorrected host-galaxy reddening. While there is significant dispersion, we find evidence that redder SNe~II (mainly at early epochs) display stronger metal-line equivalent widths. Host-galaxy reddening does not appear to be a dominant parameter, neither driving observed trends nor dominating the dispersion in observed colours. Intrinsic SN~II colours are most probably dominated by photospheric temperature differences, with progenitor metallicity possibly playing a minor role. Such temperature differences could be related to differences in progenitor radius, together with the presence or absence of circumstellar material close to the progenitor stars.

SN 2016X: a type II-P supernova with a signature of shock breakout from explosion of a massive red supergiant

We present extensive ultraviolet (UV) and optical photometry, as well as dense optical spectroscopy for type II Plateau (IIP) supernova SN 2016X that exploded in the nearby ($\sim$ 15 Mpc) spiral galaxy UGC 08041. The observations span the period from 2 to 180 days after the explosion; in particular, the Swift UV data probably captured the signature of shock breakout associated with the explosion of SN 2016X. It shows very strong UV emission during the first week after explosion, with contribution of $\sim$ 20 -- 30% to the bolometric luminosity (versus $\lesssim$ 15% for normal SNe IIP). Moreover, we found that this supernova has an unusually long rise time of about 12.6 $\pm$ 0.5 days in the $R$ band (versus $\sim$ 7.0 days for typical SNe IIP). The optical light curves and spectral evolution are quite similar to the fast-declining type IIP object SN 2013ej, except that SN 2016X has a relatively brighter tail. Based on the evolution of photospheric temperature as inferred from the $Swift$ data in the early phase, we derive that the progenitor of SN 2016X has a radius of about 930 $\pm$ 70 R$_{\odot}$. This large-size star is expected to be a red supergiant star with an initial mass of $\gtrsim$ 19 -- 20 M$_{\odot}$ based on the mass $--$ radius relation of the Galactic red supergiants, and it represents one of the most largest and massive progenitors found for SNe IIP.

Helium Star Models with Optically Thick Winds: Implications for the Internal Structures and Mass-loss Rates of Wolf–Rayet Stars

Abstract We construct helium (He) star models with optically thick winds and compare them with the properties of Galactic Wolf–Rayet (WR) stars. Hydrostatic He-core solutions are connected smoothly to trans-sonic wind solutions that satisfy the regularity conditions at the sonic point. Velocity structures in the supersonic parts are assumed by a simple β-type law. By constructing a center-to-surface structure, a mass-loss rate can be obtained as an eigenvalue of the equations. Sonic points appear at temperatures below the Fe-group opacity peak, where the radiation force becomes comparable to the local gravity. Photospheres are located at radii 3–10 times larger than sonic points. The obtained mass-loss rates are comparable to those of WR stars. Our –luminosity relation agrees well with the relation recently obtained by Gräfener et al. Photospheric temperatures of WR stars tend to be cooler than our predictions. We discuss the effects of stellar evolution, detailed radiation transfer, and wind clumping, which are ignored in this paper.

An infrared photometric and spectroscopic study of post-AGB stars

We present here $Spitzer$ mid-infrared (IR) spectra and modeling of the spectral energy distribution (SED) of a selection of post-Asymptotic Giant Branch (PAGB) stars. The mid-IR spectra of majority of these sources showed spectral features such as polycyclic aromatic hydrocarbons (PAHs) and silicates in emission. Our results from SED modeling showed interesting trends of dependency between the photospheric and circumstellar parameters. A trend of dependency is also noticed between the ratios of equivalent widths (EWs) of various vibrational modes of PAHs and the photospheric temperature T$_{*}$ and model-derived stellar parameters for the sample stars. The PAGB mass loss rates derived from the SED models are found to be higher than those for the AGB stars. In a few objects, low and high excitation fine structure emission lines were identified, indicating their advanced stage of evolution. Further, IR vibration modes of fullerene (C$_{60}$) were detected for the first time in the PAGB star IRAS 21546+4721.

Age Spreads and the Temperature Dependence of Age Estimates in Upper Sco

Abstract Past estimates for the age of the Upper Sco Association are typically 11–13 Myr for intermediate-mass stars and 4–5 Myr for low-mass stars. In this study, we simulate populations of young stars to investigate whether this apparent dependence of estimated age on spectral type may be explained by the star formation history of the association. Solar and intermediate mass stars begin their pre-main sequence evolution on the Hayashi track, with fully convective interiors and cool photospheres. Intermediate-mass stars quickly heat up and transition onto the radiative Henyey track. As a consequence, for clusters in which star formation occurs on a timescale similar to that of the transition from a convective to a radiative interior, discrepancies in ages will arise when ages are calculated as a function of temperature instead of mass. Simple simulations of a cluster with constant star formation over several Myr may explain about half of the difference in inferred ages versus photospheric temperature; speculative constructions that consist of a constant star formation followed by a large supernova-driven burst could fully explain the differences, including those between F and G stars where evolutionary tracks may be more accurate. The age spreads of low-mass stars predicted from these prescriptions for star formation are consistent with the observed luminosity spread of Upper Sco. The conclusion that a lengthy star formation history will yield a temperature dependence in ages is expected from the basic physics of pre-main sequence evolution, and is qualitatively robust to the large uncertainties in pre-main sequence evolutionary models.

The inhomogeneous submillimeter atmosphere of Betelgeuse

The mechanisms responsible for heating the extended atmospheres of early-M spectral-type supergiants are poorly understood. So too is the subsequent role these mechanisms play in driving the large mass-loss rates of these stars. Here we present ALMA long (i.e., $\sim$16 km) baseline 338 GHz (0.89 mm) continuum observations of the free-free emission in the extended atmosphere of the M2 spectral-type supergiant Betelgeuse. The spatial resolution of 14 mas exquisitely resolves the atmosphere, revealing it to have a mean temperature of 2760 K at $\sim$1.3 R$_{\star}$, which is below both the photospheric effective temperature ($T_{\textrm{eff}} = 3690$ K) and the temperatures at $\sim$$2 $R$_{\star}$. This is unambiguous proof for the existence of an inversion of the mean temperature in the atmosphere of a red supergiant. The emission is clearly not spherically symmetric with two notable deviations from a uniform disk detected in both the images and visibilities. The most prominent asymmetry is located in the north-east quadrant of the disk and is spatially resolved showing it to be highly elongated with an axis-ratio of 2.4 and occupying $\sim$$5\%$ of the disk projected area. Its temperature is approximately 1000 K above the measured mean temperature at 1.3 R$_{\star}$. The other main asymmetry is located on the disk limb almost due east of the disk center and occupies $\sim$$3\%$ of the disk projected area. Both emission asymmetries are clear evidence for localized heating taking place in the atmosphere of Betelgeuse. We suggest that the detected localized heating is related to magnetic activity generated by large-scale photospheric convection.

Far-ultraviolet to Near-infrared Spectroscopy of a Nearby Hydrogen-poor Superluminous Supernova Gaia16apd

Abstract We report the first maximum-light far-ultraviolet (FUV) to near-infrared (NIR) spectra (1000 Å − 1.62 μm, rest) of a hydrogen-poor superluminous supernova, Gaia16apd. At z = 0.1018, it is the second closest and the UV brightest SLSN-I, with 17.4 mag in Swift UVW2 band at −11 days pre-maximum. The coordinated observations with HST, Palomar, and Keck were taken at −2 to +25 days. Assuming an exponential (or t 2) form, we derived the rise time of 33 days and the peak bolometric luminosity of 3 × 1044 erg s−1. At the maximum, the photospheric temperature and velocity are 17,000 K and 14,000 km s−1, respectively. The inferred radiative and kinetic energy are roughly 1 × 1051 and 2 × 1052 erg. Gaia16apd is extremely UV luminous, and emits 50% of its total luminosity at 1000–2500 Å. Compared to the UV spectra (normalized at 3100 Å) of well studied SN1992A (Ia), SN2011fe (Ia), SN1999em (IIP), and SN1993J (IIb), it has orders of magnitude more FUV emission. This excess is interpreted primarily as a result of weaker metal-line blanketing due to a much lower abundance of iron group elements in the outer ejecta. Because these elements originate either from the natal metallicity of the star, or have been newly produced, our observation provides direct evidence that little of these freshly synthesized material, including 56Ni, were mixed into the outer ejecta, and the progenitor metallicity is likely sub-solar. This disfavors Pair-instability Supernova models with helium core masses , where substantial 56Ni material is produced. A higher photospheric temperature definitely contributes to the FUV excess from Gaia16apd. Compared with Gaia16apd, we find PS1-11bam is also UV luminous.