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1. The Type II-P Supernova 2019mhm and Constraints on its Progenitor System

   https://doi.org/10.3847/1538-4357/acbd32  

   Vazquez, J. ; Kilpatrick, C. D. ; Dimitriadis, G. ; Foley, R. J. ; Piro, A. L. ; Rest, A. ; Rojas-Bravo, C. ( June 2023 , The Astrophysical Journal)

   We present pre- and postexplosion observations of the Type II-P supernova (SN II-P) 2019mhm located in NGC 6753. Based on optical spectroscopy and photometry, we show that SN 2019mhm exhibits broad lines of hydrogen with a velocity of −8500 ± 200 km s⁻¹and a 111 ± 2 day extended plateau in its luminosity, typical of the Type II-P subclass. We also fit its late-time bolometric light curve and infer that it initially produced a⁵⁶Ni mass of 1.3 × 10⁻²± 5.5 × 10⁻⁴M_⊙. Using imaging from the Wide Field Planetary Camera 2 on the Hubble Space Telescope obtained 19 yr before explosion, we aligned to a postexplosion Wide Field Camera 3 image and demonstrate that there is no detected counterpart to the SN to a limit of >24.53 mag in F814W, corresponding to an absolute magnitude limit ofM_F814W< −7.7 mag. Comparing to massive-star evolutionary tracks, we determine that the progenitor star had a maximum zero-age main-sequence mass <17.5M_⊙, consistent with other SN II-P progenitor stars. SN 2019mhm can be added to the growing population of SNe II-P with both direct constraints on the brightness of their progenitor stars and well-observed SN properties.

    

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2. Hubble Space Telescope Imaging Reveals That SN 2015bh Is Much Fainter than Its Progenitor

   https://doi.org/10.3847/2041-8213/ac867c  

   Jencson, Jacob E. ; Sand, David J. ; Andrews, Jennifer E. ; Smith, Nathan ; Strader, Jay ; Aghakhanloo, Mojgan ; Pearson, Jeniveve ; Valenti, Stefano ( August 2022 , The Astrophysical Journal Letters)

   We present Hubble Space Telescope (HST) imaging of the site of SN 2015bh in the nearby spiral galaxy NGC 2770 taken between 2017 and 2019, nearly four years after the peak of the explosion. In 2017–2018, the transient fades steadily in optical filters before declining more slowly toF814W= −7.1 mag in 2019, ≈4 mag below the level of its eruptive luminous blue variable (LBV) progenitor observed with HST in 2008–2009. The source fades at a constant color ofF555W−F814W= 0.4 mag until 2018, similar to SN 2009ip and consistent with a spectrum dominated by interaction of the ejecta with circumstellar material (CSM). A deep optical spectrum obtained in 2021 lacks signatures of ongoing interaction (L_Hα≲ 10³⁸erg s⁻¹for broadened emission ≲2000 km s⁻¹), but indicates the presence of a nearby Hiiregion (≲300 pc). The color evolution of the fading source makes it unlikely that emission from a scattered-light echo or binary OB companion of the progenitor contributes significantly to the flattening of the late-time light curve. The remaining emission in 2019 may plausibly be attributed an evolved/inflated companion or an unresolved (≲3 pc), young stellar cluster. Importantly, the color evolution of SN 2015bh rules out scenarios in which the surviving progenitor is obscured by nascent dust and does not clearly indicate a transition to a hotter, optically faint state. The simplest explanation is that the massive progenitor did not survive. SN 2015bh likely represents a remarkable example of the terminal explosion of a massive star preceded by decades of end-stage eruptive variability.

    

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3. The Directly Imaged Exoplanet Host Star 51 Eridani is a Gamma Doradus Pulsator

   https://doi.org/10.3847/1538-4357/ac9229  

   Sepulveda, Aldo G. ; Huber, Daniel ; Zhang, Zhoujian ; Li, Gang ; Liu, Michael C. ; Bedding, Timothy R. ( October 2022 , The Astrophysical Journal)

   51 Eri is well known for hosting a directly imaged giant planet and for its membership to theβPictoris moving group. Using 2 minute cadence photometry from the Transiting Exoplanet Survey Satellite (TESS), we detect multiperiodic variability in 51 Eri that is consistent with pulsations of Gamma Doradus (γDor) stars. We identify the most significant pulsation modes (with frequencies between ∼0.5 and 3.9 cycles day⁻¹and amplitudes ranging between ∼1 and 2 mmag) as dipole and quadrupole gravity modes, as well as Rossby modes, as previously observed in KeplerγDor stars. Our results demonstrate that previously reported variability attributed to stellar rotation is instead likely due toγDor pulsations. Using the mean frequency of theℓ= 1 gravity modes, together with empirical trends of the KeplerγDor population, we estimate a plausible stellar core rotation period of${0.9}_{-0.1}^{+0.3}$days for 51 Eri. We find no significant evidence for transiting companions around 51 Eri in the residual light curve. The detection ofγDor pulsations presented here, together with follow-up observations and modeling, may enable the determination of an asteroseismic age for this benchmark system. Future TESS observations would allow a constraint on the stellar core rotation rate, which in turn traces the surface rotation rate, and thus would help clarify whether or not the stellar equatorial plane and orbit of 51 Eri b are coplanar.

    

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4. On the Unusual Variability of 2MASS J06195260–2903592: A Long-lived Disk around a Young Ultracool Dwarf

   https://doi.org/10.3847/1538-3881/ac8cee  

   Liu, Michael C. ; Magnier, Eugene A. ; Zhang, Zhoujian ; Gaidos, Eric ; Dupuy, Trent J. ; Liu, Pengyu ; Biller, Beth A. ; Vos, Johanna M. ; Allers, Katelyn N. ; Hinkle, Jason T. ; et al ( September 2022 , The Astronomical Journal)

   We present the characterization of the low-gravity M6 dwarf 2MASS J06195260-2903592, previously identified as an unusual field object based on its strong IR excess and variable near-IR spectrum. Multiple epochs of low-resolution (R≈ 150) near-IR spectra show large-amplitude (≈0.1–0.5 mag) continuum variations on timescales of days to 12 yr, unlike the small-amplitude variability typical for field ultracool dwarfs. The variations between epochs are well-modeled as changes in the relative extinction (ΔA_V≈ 2 mag). Similarly, Panoramic Survey Telescope and Rapid Response System 1 optical photometry varies on timescales as long as 11 yr (and possibly as short as an hour) and implies comparableA_Vchanges. Near Earth Object Wide-field Infrared Survey Explorer mid-IR light curves also suggest changes on 6 month timescales, with amplitudes consistent with the optical/near-IR extinction variations. However, near-IR spectra, near-IR photometry, and optical photometry obtained in the past year indicate that the source can also be stable on hourly and monthly timescales. From comparison to objects of similar spectral type, the total extinction of 2MASS J0619-2903 seems to beA_V≈ 4–6 mag, with perhaps epochs of lower extinction. Gaia Early Data Release 3 (EDR3) finds that 2MASS J0619-2903 has a wide-separation (1.′2 = 10,450 au) stellar companion, with an isochronal age of${31}_{-10}^{+22}$Myr and a mass of${0.30}_{-0.03}^{+0.04}$M_☉. Adopting this companion’s age and EDR3 distance (145.2 ± 0.6 pc), we estimate a mass of 0.11–0.17M_☉for 2MASS J0619-2903. Altogether, 2MASS J0619-2903 appears to possess an unusually long-lived primordial circumstellar disk, perhaps making it a more obscured analog to the “Peter Pan” disks found around a few M dwarfs in nearby young moving groups.

    

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5. Significant luminosity differences of two twin Type Ia supernovae

   https://doi.org/10.1093/mnras/stz3324  

   Foley, Ryan J. ; Hoffmann, Samantha L. ; Macri, Lucas M. ; Riess, Adam G. ; Brown, Peter J. ; Filippenko, Alexei V. ; Graham, Melissa L. ; Milne, Peter A. ( November 2019 , Monthly Notices of the Royal Astronomical Society)

   The Type Ia supernovae (SNe Ia) 2011by, hosted in NGC 3972, and 2011fe, hosted in M101, are optical ‘twins,’ having almost identical optical light-curve shapes, colours, and near-maximum-brightness spectra. However, SN 2011fe had significantly more ultraviolet (UV; 1600 < λ < 2500 Å) flux than SN 2011by before and at peak luminosity. Several theoretical models predict that SNe Ia with higher progenitor metallicity should (1) have additional UV opacity and thus lower UV flux; (2) have an essentially unchanged optical spectral-energy distribution; (3) have a similar optical light-curve shape; and (4) because of the excess neutrons, produce more stable Fe-group elements at the expense of radioactive 56Ni and thus have a lower peak luminosity. Following these predictions, Foley and Kirshner suggested that the difference in UV flux between SNe 2011by and 2011fe was the result of their progenitors having significantly different metallicities. They also measured a large, but insignificant, difference between the peak absolute magnitudes of the SNe (ΔMV, peak = 0.60 ± 0.36 mag), with SN 2011fe being more luminous. We present a new Cepheid-based distance to NGC 3972, substantially improving the precision of the distance measurement for SN 2011by. With these new data, we determine that the SNe have significantly different peak luminosities (ΔMV, peak = 0.335 ± 0.069 mag). Consequently, SN 2011fe produced 38 per cent more 56Ni than SN 2011by, consistent with predictions for progenitor metallicity differences for these SNe, although alternative models may also explain this difference. We discuss how progenitor metallicity differences can contribute to the intrinsic scatter for light-curve-shape-corrected SN luminosities, the use of ‘twin’ SNe for measuring distances, and implications for using SNe Ia for constraining cosmological parameters.

    

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