Temporal Evolution of the Size and Temperature of Betelgeuse's Extended Atmosphere

We use the Very Large Array (VLA) in the A configuration with the Pie Town (PT) Very Long Baseline Array (VLBA) antenna to spatially resolve the extended atmosphere of Betelgeuse over multiple epochs at 0.7, 1.3, 2.0, 3.5, and 6.1 cm. The extended atmosphere deviates from circular symmetry at all wavelengths while at some epochs we find possible evidence for small pockets of gas significantly cooler than the mean global temperature. We find no evidence for the recently reported e-MERLIN radio hotspots in any of our multi-epoch VLA/PT data, despite having sufficient spatial resolution and sensitivity at short wavelengths, and conclude that these radio hotspots are most likely interferometric artefacts. The mean gas temperature of the extended atmosphere has a typical value of 3000 K at 2 $R_{\star}$ and decreases to 1800 K at 6 $R_{\star}$, in broad agreement with the findings of the single epoch study from Lim et al. (1998). The overall temperature profile of the extended atmosphere between $2 R_{\star} \lesssim r \lesssim 6 R_{\star}$ can be described by a power law of the form $T_{\mathrm{gas}}(r) \propto r^{-0.6}$, with temporal variability of a few 100 K evident at some epochs. Finally, we present over 12 years of V band photometry, part of which overlaps our multi-epoch radio data. We find a correlation between the fractional flux density variability at V band with most radio wavelengths. This correlation is likely due to shock waves induced by stellar pulsations, which heat the inner atmosphere and ionize the more extended atmosphere through radiative means. Stellar pulsations may play an important role in exciting Betelgeuse's extended atmosphere

The Photospheric Temperatures of Betelgeuse during the Great Dimming of 2019/2020: No New Dust Required

The processes that shape the extended atmospheres of red supergiants (RSGs), heat their chromospheres, create molecular reservoirs, drive mass loss, and create dust remain poorly understood. Betelgeuse's V-band "Great Dimming" event of 2019 September /2020 February and its subsequent rapid brightening provides a rare opportunity to study these phenomena. Two different explanations have emerged to explain the dimming; new dust appeared in our line of sight attenuating the photospheric light, or a large portion of the photosphere had cooled. Here we present five years of Wing three-filter (A, B, and C band) TiO and near-IR photometry obtained at the Wasatonic Observatory. These reveal that parts of the photosphere had a mean effective temperature $(T_{\rm eff}$) significantly lower than that found by (Levesque & Massey 2020). Synthetic photometry from MARCS -model photospheres and spectra reveal that the V band, TiO index, and C-band photometry, and previously reported 4000-6800 Angstrom spectra can be quantitatively reproduced if there are multiple photospheric components, as hinted at by VLT-SPHERE images (Montarges et al. 2020). If the cooler component has $\Delta T_{\rm eff} \ge 250$ K cooler than 3650 K, then no new dust is required to explain the available empirical constraints. A coincidence of the dominant short- ($\sim 430$ day) and long-period ($\sim 5.8$ yr) V-band variations occurred near the time of deep minimum (Guinan et al. 2019). This is in tandem with the strong correlation of V mag and photospheric radial velocities, recently reported by Dupree et al. (2020b). These suggest that the cooling of a large fraction of the visible star has a dynamic origin related to the photospheric motions, perhaps arising from pulsation or large-scale convective motions.Comment: Accepted ApJ - 19 pages, 5 figure

The White Dwarfs Within 25 pc of the Sun: Kinematics and Spectroscopic Subtypes

We present the fractional distribution of spectroscopic subtypes, range and distribution of surface temperatures, and kinematical properties of the white dwarfs (WDs) within 25 pc of the Sun. There is no convincing evidence of halo WDs in the total 25 pc sample of 224 WDs. There is also little to suggest the presence of genuine thick disk subcomponent members within 25 pc. It appears that the entire 25 pc sample likely belongs to the thin disk. We also find no significant kinematic differences with respect to spectroscopic subtypes. The total DA to non-DA ratio of the 25 pc sample is 1.8, a manifestation of deepening envelope convection, which transforms DA stars with sufficiently thin H surface layers into non-DAs. We compare this ratio with the results of other studies. We find that at least 11% of the WDs within 25 pc of the Sun (the DAZ and DZ stars) have photospheric metals that likely originate from accretion of circumstellar material (debris disks) around them. If this interpretation is correct, then it suggests the possibility that a similar percentage have planets, asteroid-like bodies, or debris disks orbiting them. Our volume-limited sample reveals a pileup of DC WDs at the well-known cutoff in DQ WDs at Teff ∼ 6000 K. Mindful of small number statistics, we speculate on its possible evolutionary significance. We find that the incidence of magnetic WDs in the 25 pc sample is at least 8% in our volume-limited sample, dominated by cool WDs. We derive approximate formation rates of DB and DQ degenerates and present a preliminary test of the evolutionary scenario that all cooling DB stars become DQ WDs via helium convective dredge-up with the diffusion tail of carbon extending upward from their cores

The White Dwarfs within 20 Parsecs of the Sun: Kinematics and Statistics

We present the kinematical properties, distribution of spectroscopic subtypes, stellar population subcomponents of the white dwarfs within 20 pc of the sun. We find no convincing evidence of halo white dwarfs in the total 20 pc sample of 129 white dwarfs nor is there convincing evidence of genuine thick disk subcomponent members within 20 parsecs. Virtually the entire 20 pc sample likely belongs to the thin disk. The total DA to non-DA ratio of the 20 pc sample is 1.6, a manifestation of deepening envelope convection which transforms DA stars with sufficiently thin H surface layers into non-DAs. The addition of 5 new stars to the 20 pc sample yields a revised local space density of white dwarfs of $4.9\pm0.5 \times 10^{-3}$ M_{\sun}/yr and a corresponding mass density of $3.3\pm0.3 \times 10^{-3}$ M_{\sun}/pc$^{3}$. We find that at least 15% of the white dwarfs within 20 parsecs of the sun (the DAZ and DZ stars) have photospheric metals that possibly originate from accretion of circumstellar material (debris disks) around them. If this interpretation is correct, this suggests the possibility that the same percentage have planets or asteroid-like bodies orbiting them.Comment: Accepted for publication in The Astronomical Journa

Spatially Resolved Ultraviolet Spectroscopy of the Great Dimming of Betelgeuse

The bright supergiant, Betelgeuse (Alpha Orionis, HD 39801) experienced a visual dimming during 2019 December and the first quarter of 2020 reaching an historic minimum 2020 February 7$-$13. During 2019 September-November, prior to the optical dimming event, the photosphere was expanding. At the same time, spatially resolved ultraviolet spectra using the Hubble Space Telescope/Space Telescope Imaging Spectrograph revealed a substantial increase in the ultraviolet spectrum and Mg II line emission from the chromosphere over the southern hemisphere of the star. Moreover, the temperature and electron density inferred from the spectrum and C II diagnostics also increased in this hemisphere. These changes happened prior to the Great Dimming Event. Variations in the Mg II k-line profiles suggest material moved outwards in response to the passage of a pulse or acoustic shock from 2019 September through 2019 November. It appears that this extraordinary outflow of material from the star, likely initiated by convective photospheric elements, was enhanced by the coincidence with the outward motions in this phase of the $\sim$400 day pulsation cycle. These ultraviolet observations appear to provide the connecting link between the known large convective cells in the photosphere and the mass ejection event that cooled to form the dust cloud in the southern hemisphere imaged in 2019 December, and led to the exceptional optical dimming of Betelgeuse in 2020 February.Comment: 11 pages, 8 figures, Astrophysical Journal, accepte

Sofia-exes observations of betelgeuse during the great dimming of 2019/2020

In 2019 October Betelgeuse began a decline in V-band brightness that went beyond the minimum expected from its quasi-periodic ∼420 day cycle, becoming the faintest in recorded photometric history. Observations obtained in 2019 December with Very Large Telescope/SPHERE have shown that the southern half of the star has become markedly fainter than in 2019 January, indicating that a major change has occurred in, or near, the photosphere. We present Stratospheric Observatory for Infrared Astronomy (SOFIA) Echelon Cross Echelle Spectrograph (EXES) high spectral-resolution observations of [Fe II] 25.99 μm and [S I] 25.25 μm emission lines from Betelgeuse obtained during the unprecedented 2020 February V-band brightness minimum to investigate potential changes in the circumstellar flow. These spectra are compared to observations obtained in 2015 and 2017 when the V magnitude was typical of brighter phases. We find only very small changes in the gas velocities reflected by either of the line profiles, no significant changes in the flux to continuum ratios, and hence no significant changes in the [Fe ii]/[S i] flux ratios. There is evidence that absorption features have appeared in the 2020 continuum. The Alfvén wave-crossing time from the upper photosphere is sufficiently long that one would not expect a change in the large-scale magnetic field to reach the circumstellar [Fe ii] and [S i] line-forming regions, 3 20R ∗, where significant circumstellar oxygen-rich dust is observed

Spatially Resolved Ultraviolet Spectroscopy of the Great Dimming of Betelgeuse

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SOFIA upGREAT/FIFI-LS Emission-line Observations of Betelgeuse during the Great Dimming of 2019/2020

We report NASA-DLR SOFIA upGREAT circumstellar [O i] 63.2 mu m and [C ii] 157.7 mu m emission profiles and FIFI-LS [O i] 63.2 mu m, [O i] 145.5 mu m, and [C ii] 157.7 mu m fluxes obtained shortly after Betelgeuse's 2019/2020 Great Dimming event. Haas et al. noted a potential correlation between the [O i] 63.2 mu m flux and V magnitude based on three Kuiper Airborne Observatory observations made with the CGS and FIFI instruments. The FIFI observation was obtained when V similar or equal to 0.88 and revealed a 3 sigma non-detection at a quarter of the previous CGS flux measurement made when V similar or equal to 0.35. A potential explanation could be a change in dust-gas drag heating by circumstellar silicates caused by variations in the photospheric radiation field. SOFIA observations provide a unique test of this correlation because the V-band brightness went to its lowest value on record, V similar or equal to 1.61, with the SOFIA observations being made when V (FIFI-LS) similar or equal to 1.51 and V (upGREAT) similar or equal to 1.36. The upGREAT spectra show a [O i] 63.2 mu m flux larger than previous space observatory measurements obtained when V similar or equal to 0.58. The profile is consistent with formation in the slower, more turbulent inner S1 outflow, while the [C ii] 157.7 mu m profile is consistent with formation farther out in the faster S2 outflow. Modeling of dust-gas drag heating, combined with 25 yr of Wing three-filter and V photometry, reveals that it is unlikely that the S1 circumstellar envelope and [O i] 63.2 mu m fluxes are dominated by the dust-gas drag heating and that another heating source is also active. The [O i] 63.2 mu m profile is hard to reconcile with existing outflow velocity models