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  1. Abstract We present observations of the dwarf galaxies GALFA Dw3 and GALFA Dw4 with the Advanced Camera for Surveys on the Hubble Space Telescope. These galaxies were initially discovered as optical counterparts to compact H i clouds in the GALFA survey. Both objects resolve into stellar populations which display old red giant branch (RGB), younger helium-burning, and massive main sequence stars. We use the tip of the RGB method to determine the distance to each galaxy, finding distances of 7.61 − 0.29 + 0.28 Mpc and 3.10 − 0.17 + 0.16 Mpc, respectively. With these distances we show that bothmore »galaxies are extremely isolated, with no other confirmed objects within ∼1.5 Mpc of either dwarf. GALFA Dw4 is also found to be unusually compact for a galaxy of its luminosity. GALFA Dw3 and Dw4 contain H ii regions with young star clusters and an overall irregular morphology; they show evidence of ongoing star formation through both ultraviolet and H α observations and are therefore classified as dwarf irregulars (dIrrs). The star formation histories of these two dwarfs show distinct differences: Dw3 shows signs of a recently ceased episode of active star formation across the entire dwarf, while Dw4 shows some evidence for current star formation in spatially limited H ii regions. Compact H i sources offer a promising method for identifying isolated field dwarfs in the Local Volume, including GALFA Dw3 and Dw4, with the potential to shed light on the driving mechanisms of dwarf galaxy formation and evolution.« less
    Free, publicly-accessible full text available January 1, 2023
  2. Abstract There is a long-standing discrepancy between the observed Galactic classical nova rate of ∼10 yr −1 and the predicted rate from Galactic models of ∼30–50 yr −1 . One explanation for this discrepancy is that many novae are hidden by interstellar extinction, but the degree to which dust can obscure novae is poorly constrained. We use newly available all-sky three-dimensional dust maps to compare the brightness and spatial distribution of known novae to that predicted from relatively simple models in which novae trace Galactic stellar mass. We find that only half (53%) of the novae are expected to bemore »easily detectable ( g ≲ 15) with current all-sky optical surveys such as the All-Sky Automated Survey for Supernovae (ASAS-SN). This fraction is much lower than previously estimated, showing that dust does substantially affect nova detection in the optical. By comparing complementary survey results from the ASAS-SN, OGLE-IV, and Palomar Gattini IR surveys using our modeling, we find a tentative Galactic nova rate of ∼30 yr −1 , though this could be as high as ∼40 yr −1 , depending on the assumed distribution of novae within the Galaxy. These preliminary estimates will be improved in future work through more sophisticated modeling of nova detection in ASAS-SN and other surveys.« less
    Free, publicly-accessible full text available November 1, 2022
  3. ABSTRACT V445 Puppis is the only helium nova observed to date; its eruption in late 2000 showed high velocities up to 8500 km s−1, and a remarkable bipolar morphology cinched by an equatorial dust disc. Here we present multifrequency radio observations of V445 Pup obtained with the Very Large Array (VLA) spanning 1.5–43.3 GHz, and between 2001 January and 2008 March (days ∼89–2700 after eruption). The radio light curve is dominated by synchrotron emission over these 7 yr, and shows four distinct radio flares. Resolved radio images obtained in the VLA’s A configuration show that the synchrotron emission hugs the equatorial disc, and comparisons tomore »near-IR images of the nova clearly demonstrate that it is the densest ejecta – not the fastest ejecta – that are the sites of the synchrotron emission in V445 Pup. The data are consistent with a model where the synchrotron emission is produced by a wind from the white dwarf impacting the dense equatorial disc, resulting in shocks and particle acceleration. The individual synchrotron flares may be associated with density enhancements in the equatorial disc and/or velocity variations in the wind from the white dwarf. This overall scenario is similar to a common picture of shock production in hydrogen-rich classical novae, but V445 Pup is remarkable in that these shocks persist for almost a decade, much longer than the weeks or months for which shocks are typically observed in classical novae.« less
  4. ABSTRACT We characterize the extreme heartbeat star system MACHO 80.7443.1718 in the Large Magellanic Cloud using Transiting Exoplanet Survey Satellite (TESS) photometry and spectroscopic observations from the Magellan Inamori Kyocera Echelle (MIKE) and SOAR Goodman spectographs. MACHO 80.7443.1718 was first identified as a heartbeat star system in the All-Sky Automated Survey for SuperNovae (ASAS-SN) with $P_{\rm orb}=32.836\pm 0.008\, {\rm d}$. MACHO 80.7443.1718 is a young (∼6 Myr), massive binary, composed of a B0 Iae supergiant with $M_1 \simeq 35\, {\rm M}_\odot$ and an O9.5V secondary with $M_2 \simeq 16\, {\rm M}_\odot$ on an eccentric (e = 0.51 ± 0.03) orbit. In addition tomore »having the largest variability amplitude amongst all known heartbeats stars, MACHO 80.7443.1718 is also one of the most massive heartbeat stars yet discovered. The B[e] supergiant has Balmer emission lines and permitted/forbidden metallic emission lines associated with a circumstellar disc. The disc rapidly dissipates at periastron that could indicate mass transfer to the secondary, but re-emerges immediately following periastron passage. MACHO 80.7443.1718 also shows tidally excited oscillations at the N = 25 and N = 41 orbital harmonics and has a rotational period of 4.4 d.« less
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