arXiv subjects

Bradley E. Schaefer

Publications and source records attributed to Bradley E. Schaefer.

At least 37 records · Page 2Linked to original sources

arXiv2022

The Nova KT Eri Is a Recurrent Nova With a Recurrence Time-Scale of 40-50 Years

KT Eridani was a very fast nova in 2009 peaking at V=5.42 mag. We marshal large data sets of photometry to finally work out the nature of KT Eri. From the TESS light curve, as confirmed with our radial velocity curve, we find an orbital period of 2.61595 days. With our 272 spectral energy distributions from simultaneous BVRIJHK measures, the companion star has a temperature of 6200$\pm$500 K. Our century-long average in quiescence has V=14.5. With the Gaia distance (5110$^{+920}_{-430}$ parsecs), the absolute magnitude is +0.7$\pm$0.3. We converted this absolute magnitude (corrected to the disc light alone) to accretion rates, with a full integration of the alpha-disc model. This accretion rate is very high at 3.5x10$^{-7}$ solar masses per year. Our search and analysis of archival photographs shows that no eruption occurred from 1928--1954 or after 1969. With our analysis of the optical light curve, the X-ray light curve, and the radial velocity curve, we derive a white dwarf mass of 1.25$\pm$0.03 solar masses. With the high white dwarf mass and very-high accretion rate, KT Eri must require a short time to accumulate the required mass to trigger the next nova event. Our detailed calculations give a recurrence time-scale of 12 years with a total range of 5--50 years. When combined with the archival constraints, we conclude that the recurrence time-scale must be between 40--50 years. So, KT Eri is certainly a recurrent nova, with the prior eruption remaining undiscovered in a solar gap of coverage from 1959 to 1969.

astro-ph.SR↗

arXiv2022

Comprehensive Catalogue of the Overall Best Distances and Properties of 402 Galactic Novae

I derive the overall best distances for all 402 known galactic novae, and I collect their many properties. The centrepiece is the 74 novae with accurate parallaxes from the new Gaia data release. For the needed priors, I have collected 171 distances based on old methods (including expansion parallaxes and extinction distances). Further, I have collected the V-magnitudes at peak and the extinction measures, so as to produce absolute magnitudes at peak and then derive a crude distance as a prior. Further, I have recognized that 41 per cent of the known novae are concentrated in the bulge, with 68 per cent of these <5.4 degrees from the galactic centre, so the 165 bulge novae must have distances of 8000+-750 parsecs. Putting this all together, I have derived distances to all 402 novae, of which 220 have distances to an accuracy of better than 30 per cent. I find that the disc novae have an exponential scale height of 140+-10 pc. The average peak absolute V-magnitude is -7.45, with an RMS scatter of 1.33 mag. These peak luminosities are significantly correlated with the decline rate (t_3 in days) as M_V,peak = -7.6 + 1.5 * Log[t_3 / 30]. The huge scatter about this relation masks the correlation in many smaller datasets, and makes this relation useless for physical models. The bulge novae are indistinguishable from the disc novae in all properties, except that the novae with red giant companion stars have a strong preference for residing in the bulge population.

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arXiv2022

Comprehensive Listing of 156 Reliable Orbital Periods for Novae, Including 49 New Periods

I report on a large-scale search for the orbital periods (P) of most known nova systems, by looking for significant, coherent, and stable optical photometric modulation in two or more independent light curves taken mostly from the large surveys of TESS, Kepler, AAVSO, SMARTS, OGLE, ASAS, and ZTF. I have discovered 31 new orbital periods. Further, I have measured new periods for 18 novae with evolved companions, to 30 per cent accuracy, as based on their spectral energy distribution. Also, I have confirmed, improved, and rejected prior claims for P in 46 novae. (As part of this effort, I recognize that 5 novae display 1--3 coherent, significant, and transient periodicities 0.12--4.1 days, with these being mysterious as not being the orbital, spin, or superhump periods.) In all, I have compiled a comprehensive list of 156 reliable P values for novae. The histogram of nova periods shows a minimum P at 0.059 hours (85 minutes), and a Period Gap from 0.071--0.111 days (1.70--2.66 hours). The upper edge of the Period Gap is significantly different between novae (0.111 days), nova-like systems (0.131 days), and dwarf novae (0.141 days). A further issue from the histogram is that 31 per cent of nova systems have evolved companions, for which there has been no models or understanding for their current state or evolution. For the novae with red giant companions, 15-out-of-20 are in the bulge population, despite novae with main-sequence and subgiant companions having bulge fractions near 0.11--0.32.

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arXiv2022

The Recurrent Nova U Scorpii from the 2010.1 to 2022.4 Eruptions; the Missed Eruption of 2016.78 +- 0.10 and the Critical Complex Period Changes

U Sco is a recurrent nova with eleven observed eruptions, most recently in 2010.1 and 2022.4. I report on my program (running since 1989) of measuring eclipse times and brightnesses of U Sco in quiescence, from 2010 to 2022. The orbital period suddenly increased by +22.4 +- 1.0 parts-per-million across the 2010.1 eruption. This period change is greater than the near-zero period change (+3.9 +- 6.1 parts-per-million) across the 1999.2 eruption. This period change cannot come from any of the usual mechanisms, whereas the one remaining possibility is that the period changes are dominated by the little-known mechanism of the nova ejecting asymmetric shells. From 2010.1 to 2016.78, the O-C curve showed a steady period change that was large, with P-dot equal (-21.0 +- 3.2)X 10^-9. This is greatly higher than the steady period changes in the two previous inter-eruption intervals (-3.2 +- 1.9 and -1.1 +- 1.1 X 10^-9). This large, variable, and negative P-dot apparently comes from magnetic braking of the companion star's rotation. Starting in 2016.9 +- 0.6, the O-C curve showed a strong kink that is a unique characteristic of the sudden period change (+35.4 +- 7.1 parts-per-million) across a nova event. The brightness in quiescence after 2010.4 shows that the white dwarf accreted the trigger mass for the next nova event in the year 2017.1 +- 0.6. Photometric records show the only possible time for the eruption to peak (such that its total duration of 60 days was undetectable by any observation) is during a 75 day interval inside the 2016 solar gap, thus constraining the missed eruption to 2016.78 +- 0.10.

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arXiv2022

Discovery of Extreme, Roughly-Daily Superflares on the Recurrent Nova V2487 Oph

V2487 Oph is a recurrent nova with detected eruptions in 1900 and 1998. Startlingly, V2487 Oph shows flares, called `Superflares', with up to 1.10 mag amplitude, fast rises of under one-minute, always with an initial impulsive spike followed by a roughly-exponential tail, typically one-hour durations, and with random event times averaging once-per-day. The typical flare energy $E$ is over 10$^{38}$ ergs, while the yearly energy budget is 10$^{41}$ ergs. V2487 Oph Superflares obey three relations; the number distribution of flare energies scales as $E^{-2.34\pm0.35}$, the waiting time from one flare to the next is proportional to $E$ of the first event, and flare durations scale as $E^{0.44\pm0.03}$. Scenarios involving gravitational energy and nuclear energy fail to satisfy the three relations. The magnetic energy scenario, however, can explain all three relations. This scenario has magnetic field lines above the disc being twisted and amplified by the motions of their footprints, with magnetic reconnection releasing energy that comes out as Superflare light. This exact mechanism is already well known to occur in white light solar flares, in ordinary M-type flare stars, and in the many Superflare stars observed all across the H-R diagram. Superflares on Superflare stars have rise times, light curve shapes and durations that are very similar to those on V2487 Oph. So we conclude that the V2487 Oph Superflares are caused by large-scale magnetic reconnection. V2487 Oph is now the most extreme Superflare star, exhibiting the largest known flare energy (1.6$\times$10$^{39}$ ergs) and the fastest occurrence rate.

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arXiv2021

Discovery of 13 New Orbital Periods for Classical Novae

I report new orbital periods (P) for 13 classical novae, based on light curves from TESS, AAVSO, and other public archives. These new nova periods now constitute nearly one-seventh of all known nova periods. Five of my systems have P>1 day, which doubles the number of such systems that must have evolved companion stars. (This is simply because ground-based time series have neither the coverage nor the stability required to discover these small-amplitude long periods.) V1016 Sgr has a rare P below the period gap, and suddenly becomes useful for current debates on evolution of novae. Five of the novae (FM Cir, V399 Del, V407 Lup, YZ Ret, and V549 Vel) have the orbital modulations in the tail of the eruption after the transition phase. Soon after the transition, YZ Ret shows a unique set of aperiodic diminishing oscillations, plus YZ Ret shows two highly-significant transient periods, 1.1% and 4.5% longer than the orbital period, much like for the superhump phenomenon. I also report an optical 591.27465 second periodicity for V407 Lup, which is coherent and must be tied to the white dwarf spin period. The new orbital periods in days are 0.1883907 +- 0.0000048 for V1405 Cas, 3.4898 +- 0.0072 for FM Cir, 0.162941 +- 0.000060 for V339 Del, 3.513 +- 0.020 for V407 Lup, 1.32379 +- 0.00048 for V2109 Oph, 3.21997 +- 0.00039 for V392 Per, 0.1628714 +- 0.0000110 for V598 Pup, 0.1324539 +- 0.0000098 for YZ Ret, 0.07579635 +- 0.00000017 for V1016 Sgr, 7.101 +- 0.016 for V5583 Sgr, 0.61075 +- 0.00071 for V1534 Sco, 0.40319 +- 0.00005 for V549 Vel, and 0.146501 +- 0.000058 for NQ Vul.

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arXiv2020

A Case Study of the Picture Rocks Sun Dagger, Plus a Review of the Intentionality of Sun Daggers

The Picture Rocks Sun Dagger is a spiral petroglyph on a hillside northwest of Tucson that shows sun dagger events at both the summer solstice and the equinoxes. On each of these dates, a wedge-shaped sunbeam with opening angles 20°-30° touches the center of the spiral, with both of these being confidently intentionally constructed by peoples of the Hohokam culture c. 800--1300 AD. More generally for claimed sun daggers throughout the American Southwest, the critical question is whether the ancient indigenous peoples intentionally placed the petroglyph so as to create a solar marker. The confident starting point for proving the intentionality of sun daggers in general is a histogram measured by the Prestons showing highly significant peaks for indicated declinations within 2° of -23.4°, 0.0°, and +23.4°, with this being not by chance. In a review of solsticial and equinoctal sun daggers, we find that they all have beams of light shaped like a long-thin triangle with an apex opening angle of <40° that touches the center of the petroglyph symbol. The majority of the sun daggers use a spiral petroglyph, with circles and other symbols being used. Where a spiral or circular petroglyph has a coincidental light/shadow display, the false alarm rate is measured to be 20% to 33%. Sun daggers that have indicated declinations other than +-23.4° or 0.0° are false alarms, including claims for alignments to cross-quarter days and lunar standstills, which are certainly wrong. Intentional sun daggers are not related to any form of calendric regulation, astronomical tools, or public ceremony. Rather, abundant ethnographic evidence shows that sun daggers are a part of sites, called Sun Shrines, where a local Sun watcher would have lone vigils, with offerings and prayers to the gods on the solstices and equinoxes.

physics.hist-ph↗

arXiv2020

Startlingly Fast Evolution of the Stingray Planetary Nebula and its Central Star, V839 Arae

The planetary nebula (PN) called the Stingray (PN G331.3$-$12.1) suddenly turned on in the 1980s, and its central star (V839 Ara) started a fast evolution with large amplitudes in magnitude, surface temperature, and surface gravity, perhaps as part of a late thermal pulse causing a loop in the Hertzsprung-Russell (HR) diagram. With these fast changes, we have taken images with the Hubble Space Telescope in 2016. We do not see the massive high-velocity mass loss of the 1980s, either close in to the central star, or as clumps or changes in the nebula far from the star, or as localized or general increases in the emission-line flux caused by the shocks of the outflowing ejecta ramming into the slow-moving PN shell. We think that the lack of seeing the outgoing material is because the relatively modest total mass had already suffused the PN before the first resolved imaging in 1992, and it was the shocks from this impact that initially ionized the Stingray starting in the 1980s. We also quantify the complex fast fading of the Stingray, with each emission line and each structure having different fade rates, with half-lives ranging from 3 to 29 years. In a century or two, the PN will fade to invisibility. With this complex fading of different structures, it is impossible to derive any expansion rate for the PN. The central star had its brightness roughly constant from 1996 to 2016, but with substantial erratic variability from 15.50 to 14.18 mag in the $V$ band.

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arXiv2020

A "blue ring nebula" surrounding a thousands of years old stellar merger

Stellar mergers are a brief-lived but common phase in the evolution of binary star systems. Among the many astrophysical implications of these events include the creation of atypical stars (e.g. magnetic stars, blue stragglers, rapid rotators), interpretation of stellar populations, and formation channels of LIGO-detected compact object mergers. Although stellar mergers are thus commonly invoked phenomena, observations of these events and details of their evolution remain elusive. While a handful of stellar mergers have been directly observed in recent years, the central remnants of these events remain shrouded by an opaque shell of dust and molecules, making it impossible to observe their final state (e.g. as a single merged star or a tighter surviving binary). Here we report observations of an unusual, ring-shaped ultraviolet nebula and the star at its center, TYC 2597-735-1. The nebula shows two opposing fronts, suggesting a bipolar outflow from TYC 2597-735-1. TYC 2597-735-1's spectrum and proximity above the Galactic plane suggest it is an old star, yet it shows abnormally low surface gravity and a detectable long-term luminosity decay, uncharacteristic for its evolutionary stage. TYC 2597-735-1 also exhibits H-alpha emission, radial velocity variations, enhanced ultraviolet radiation, and excess infrared emission, common signposts of dusty circumstellar disks, stellar activity, and accretion. The combined observations, paired with stellar evolution models, suggests TYC 2597-735-1 merged with a lower-mass companion several thousand years ago. TYC 2597-735-1 provides a look at an unobstructed stellar merger found at an evolutionary stage between its dynamic onset and the theorized final equilibrium state, directly inferring how two stars merge into a single star.

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arXiv2019

Sudden and Steady Orbital Period Changes Across the Classical Nova Eruptions of DQ Her and BT Mon

I report two new measures of the sudden change in the orbital period ($P$) across the nova eruption ($Δ$P) and the steady period change in quiescence ($\dot{P}$) for classical novae (CNe) DQ Her and BT Mon. The fractional changes ($Δ$P/P) in parts-per-million (ppm) are $-$4.46$\pm$0.03 for DQ Her and +39.6$\pm$0.5 for BT Mon. For BT Mon, the $Δ$P/P value is not large enough (i.e., $>$1580 ppm) to allow for Hibernation in this system. The {\it negative} $Δ$P/P for DQ Her is a confident counterexample of the Hibernation model for the evolution of cataclysmic variables. Further, published models of period changes by nova eruptions do not allow for such a negative value, so some additional mechanism is required, with this perhaps being due to asymmetric ejection of material. My program has also measured the first long-term $\dot{P}$ for CNe, with 0.00$\pm$0.02 for DQ Her and $-$2.3$\pm$0.1 for BT Mon, all with units of $10^{-11}$ days/cycle. These can be directly compared to the predictions of the Magnetic Braking model, where the long-term average $\dot{P}$ is a single universal function of $P$. The predicted values are -0.027 for DQ Her and -0.33 for BT Mon. For both novae, the measured $\dot{P}$ is significantly far from the predictions for Magnetic Braking. Further, the observed $Δ$P for BT Mon imposes an additional {\it positive} period change of +0.60$\times$10$^{-11}$ days/cycle when averaged over the eruption cycle, so this system actually has a long-term {\it rise} in $P$.

astro-ph.SR↗

arXiv2019

Sudden and Steady Orbital Period Changes Across Six Classical Nova Eruptions; The End of Hibernation and Two Serious Challenges for the Magnetic Braking Model of Cataclysmic Variable Evolution

I report on two new measures of the sudden change in the orbital period ($P$) across the nova eruption ($Δ$P) and the steady period change in quiescence ($\dot{P}$) for classical novae (CNe) RR Pic and HR Del, bringing a total of six such measures for CNe, all in a final report of my large and long observing program. The fractional changes ($Δ$P/P) in parts-per-million (ppm) are -290.71$\pm$0.28 (QZ Aur), -472.1$\pm$4.8 (HR Del), -4.46$\pm$0.03 (DQ Her), +39.6$\pm$0.5 (BT Mon), -2003.7$\pm$0.9 (RR Pic), and -273$\pm$61 (V1017 Sgr). These results are in stark opposition to the Hibernation Model for the evolution of cataclysmic variables (CVs), which requires $Δ$P/P$>$+1000 ppm to get the required drop in the accretion rate to produce hibernation. The Hibernation Model cannot be salvaged in any way. My program has also measured the first long-term $\dot{P}$ for classical novae, with -2.84$\pm$0.22 (QZ Aur), +4.0$\pm$0.9 (HR Del), +0.00$\pm$0.02 (DQ Her), -2.3$\pm$0.1 (BT Mon), and +1.25$\pm$0.01 (RR Pic) in units of $10^{-11}$ days/cycle. These can be directly compared to the predictions of the Magnetic Braking Model, where the long-term average $\dot{P}$ is a single universal function of $P$. The measured values are +5.3, $-$94, 0.00, +6.9, and $-$190 times that predicted by the model, so the predictions are always greatly wrong. Further, the effects of the $Δ$P averaged over the eruption cycle are usually much larger than the magnetic braking effects. To get a realistic model of CV evolution, we must add the physics of the $Δ$P and $\dot{P}$ variations.

astro-ph.SR↗

arXiv2019

Precise Measures of Orbital Period, Before and After Nova Eruption for QZ Aurigae

For the ordinary classical nova QZ Aurigae (which erupted in 1964), we report 1317 magnitudes from 1912--2016, including four eclipses detected on archival photographic plates from long before the eruption. We have accurate and robust measures of the orbital period both pre-eruption and post-eruption, and we find that the orbital period decreased, with a fractional change of -290.71+-0.28 parts-per-million across the eruption, with the orbit necessarily getting smaller. Further, we find that the light curve outside of eclipses and eruption is flat at near B=17.14 from 1912--1981, whereupon the average light curve starts fading down to B=17.49 with large variability. QZ Aur is a robust counter-example against the Hibernation model for the evolution of cataclysmic variables, where the model requires that all novae have their period increase across eruptions. Large period decreases across eruptions can easily arise from mass imbalances in the ejecta, as are commonly seen in asymmetric nova shells.

astro-ph.SR↗

arXiv2018

The Distances to Novae As Seen By Gaia

The Gaia spacecraft has just released a large set of parallaxes, including 41 novae for which the fractional error is <30%. I have used these to evaluate the accuracy and bias of the many prior methods for getting nova-distances. The best of the prior methods is the geometrical parallaxes from HST for just four novae, although the real error bars are 3X larger than stated. The canonical method for prior nova-distances has been the expansion parallaxes from the nova shells, but this method is found to have real 1-sigma uncertainty of 0.95 mag in the distance modulus, and the prior quoted error bars are on average 3.6X worse than advertised. The many variations on the maximum-magnitude-rate-of-decline (MMRD) relation are all found to be poor, too poor to be useable, and even to be non-applicable for 5-out-of-7 samples of nova, so the MMRD should no longer be used. The prior method of using various measures of the extinction from the interstellar medium have been notoriously bad, but now a new version by Ozdonmez and coworkers has improved this to an unbiased method with 1-sigma uncertainty of 1.14 mag in the distance modulus. For the future, I recommend in order (1) using the Gaia parallax, (2) using the catalog of Ozdonmez, (3) using M_max = -7.0 +- 1.4 mag as an empirical method of poor accuracy, and (4) if none of these methods is available, then to not use the nova for purposes where a distance is needed.

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arXiv2018

Yes, Aboriginal Australians Can and Did Discover the Variability of Betelgeuse

Recently, a widely publicized claim has been made that the Aboriginal Australians discovered the variability of the red star Betelgeuse in the modern Orion, plus the variability of two other prominent red stars: Aldebaran and Antares. This result has excited the usual healthy skepticism, with questions about whether any untrained peoples can discover the variability and whether such a discovery is likely to be placed into lore and transmitted for long periods of time. Here, I am offering an independent evaluation, based on broad experience with naked-eye sky viewing and astro-history. I find that it is easy for inexperienced observers to detect the variability of Betelgeuse over its range in brightness from V = 0.0 to V = 1.3, for example in noticing from season-to-season that the star varies from significantly brighter than Procyon to being greatly fainter than Procyon. Further, indigenous peoples in the Southern Hemisphere inevitably kept watch on the prominent red star, so it is inevitable that the variability of Betelgeuse was discovered many times over during the last 65 millennia. The processes of placing this discovery into a cultural context (in this case, put into morality stories) and the faithful transmission for many millennia is confidently known for the Aboriginal Australians in particular. So this shows that the whole claim for a changing Betelgeuse in the Aboriginal Australian lore is both plausible and likely. Given that the discovery and transmission is easily possible, the real proof is that the Aboriginal lore gives an unambiguous statement that these stars do indeed vary in brightness, as collected by many ethnographers over a century ago from many Aboriginal groups. So I strongly conclude that the Aboriginal Australians could and did discover the variability of Betelgeuse, Aldebaran, and Antares.

physics.hist-ph↗

arXiv2018

The KIC 8462852 Light Curve From 2015.75 to 2018.18 Shows a Variable Secular Decline

The star KIC 8462852 (Boyajian's Star) displays both fast dips of up to 20% on time scales of days, plus long-term secular fading by up to 19% on time scales from a year to a century. We report on CCD photometry of KIC 8462852 from 2015.75 to 2018.18, with 19,176 images making for 1,866 nightly magnitudes in BVRI. Our light curves show a continuing secular decline (by 0.023 +- 0.003 mags in the B-band) with three superposed dips with duration 120-180 days. This demonstrates that there is a continuum of dip durations from a day to a century, so that the secular fading is seen to be by the same physical mechanism as the short-duration Kepler dips. The BVRI light curves all have the same shape, with the slopes and amplitudes for VRI being systematically smaller than in the B-band by factors of 0.77 +- 0.05, 0.50 +- 0.05, and 0.31 +- 0.05. We rule out any hypothesis involving occultation of the primary star by any star, planet, solid body, or optically thick cloud. But these ratios are the same as that expected for ordinary extinction by dust clouds. This chromatic extinction implies dust particle sizes going down to ~0.1 micron, suggesting that this dust will be rapidly blown away by stellar radiation pressure, so the dust clouds must have formed within months. The modern infrared observations were taken at a time when there was at least 12.4% +- 1.3% dust coverage (as part of the secular dimming), and this is consistent with dimming originating in circumstellar dust.

astro-ph.SR↗

arXiv2016

Astronomical and Historical Evaluation of Molnar's Solution for the Star of Bethlehem

Previously, the most prominent explanation for the Star of Bethlehem was to identify one of many astronomical events in the sky as being the inspiration for the trip of the Magi. However, all the astronomical answers have detailed refutations, and general disproofs, so all the astronomical answers for the Star are dead. In 1999, Michael Molnar put forth a completely new solution, where the Star originated as a report of a natal horoscope for 17 April 6 BC. This natal horoscope shows very impressive regal portents and points to Judea. It is very improbable that such a very rare planet configuration (averaging only once per millennium or longer) would coincide with the very restricted day of Jesus' birth (springtime in a year shortly before Herod's death in 4 BC), unless there is some causal connection. The Magi (as labeled by the gospel author) were astrologers, so they were only interested in horoscopes. The astrological solution is further supported by the presence of an astrological technical term ('in the east' meaning what astronomers call the 'heliacal rise') in the Nativity narrative of Matthew. Importantly, Molnar has only identified the Star as originating from a particular natal horoscope, while making no statement about the nature or historicity of any of the other elements of the story in Matthew. So for example, the Magi might be astrologers as idealized by the Greeks of the time, or they might have been non-existent (invented by a latter-day Greek seeking omens for the birth of a great king), and they might have arrived on 17 April 6 BC or months later, and they might or might not have been three in numbers with given names. But for the Star of Bethlehem alone, Molnar's astrological solution is convincing.

physics.hist-ph↗

arXiv2016

Accurate Pre-Eruption and Post-Eruption Orbital Periods for the Dwarf/Classical Nova V1017 Sgr

V1017 Sgr is a classical nova (in 1919) that displayed an earlier dwarf nova eruption (in 1901), and two more dwarf nova events (in 1973 and 1991). Previous work on this bright system in quiescence (V=13.5) has only been a few isolated magnitudes, a few spectra, and an ambiguous claim for an orbital period of 5.714 days as based on nine radial velocities. To test this period, we have collected 2896 magnitudes (plus 53 in the literature) in the UBVRIJHKL bands from 1897 to 2016, making an essentially complete photometric history of this unique cataclysmic variable. We find that the light curve in all bands is dominated by the ellipsoidal modulations of a G giant companion star, with a post-eruption (after the 1919 nova event) orbital period of 5.786290 +- 0.000032 days. This is the longest period for any classical nova, the accretion must be powered by the nuclear evolution of the companion star, and the dwarf nova events occur only because the outer parts of the large disk are cool enough to be unstable. Furthermore, we measure the pre-eruption orbital period (from 1907 to 1916), and there is a small steady period change in quiescence. The orbital period has decreased by 273 +- 61 parts-per-million across the 1919 eruption, with the significance of the period change being at the 5.7-sigma confidence level. This is startling and mystifying for nova-theory, because the three known period change effects cannot account for a period decrease in V1017 Sgr, much less one of such a large size.

astro-ph.SR↗

arXiv2016

All Known Hot RCB Stars Are Fading Fast Over the Last Century

The R Coronae Borealis (RCB) stars are cool supergiants that display irregular and deep dips in their light curves, caused by dust formation. There are four known hot RCB stars (DY Cen, MV Sgr, V348 Sgr, and HV 2671), with surface temperatures of 15,000--25,000 K, and prior work has suggested that three of these have secular fading in brightness. I have tested this result by measuring century-long light curves in the Johnson B-band with modern comparison star magnitudes, and I have extended this by measuring many magnitudes over a wide time range as well as for the fourth hot RCB star. In all four cases, the B-band magnitude of the maximum light is now fast fading. The fading rates (in units of magnitudes per century) are 2.5 for DY Cen after 1960, 1.3 for MV Sgr, 1.3 for V348 Sgr, and 0.7 for HV 2671. This secular fading is caused by the expected evolution of the star across the top of the HR diagram at constant luminosity, as the temperature rises and the bolometric correction changes. For DY Cen, the brightness at maximum light is rising from 1906 to 1932, and this is caused by the temperature increase from near 5,800 to 7,500 K. Before 1934 DY Cen had frequent dust dips, while after 1934 there are zero dust dips, so there is some apparent connection between the rising temperature and the formation of the dust. Thus, we have watched DY Cen evolve from an ordinary RCB star up to a hot RCB star and now appearing as an extreme helium star, all in under one century.

astro-ph.SR↗