Yael Hillman - Academia.edu (original) (raw)
Papers by Yael Hillman
Monthly Notices of the Royal Astronomical Society, Nov 14, 2023
The Astrophysical Journal Supplement Series
M87 has been monitored with a cadence of 5 days over a span of 9 months through the near-ultravio... more M87 has been monitored with a cadence of 5 days over a span of 9 months through the near-ultraviolet (NUV; F275W) and optical (F606W) filters of the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope. This unprecedented dataset yields the NUV and optical light and color curves of 94 M87 novae, characterizing the outburst and decline properties of the largest extragalactic nova dataset in the literature (after M31 and M81). We test and confirm nova modelers’ prediction that recurrent novae cannot erupt more frequently than once every 45 days, show that there are zero rapidly recurring novae in the central ∼1/3 of M87 with recurrence times <130 days, demonstrate that novae closely follow the K-band light of M87 to within a few arcsecs of the galaxy nucleus, show that nova NUV light curves are as heterogeneous as their optical counterparts, and usually peak 5–30 days after visible light maximum, determine our observations’ annual detection completeness to be 71%–77%, and measu...
arXiv (Cornell University), Oct 19, 2022
arXiv (Cornell University), Sep 11, 2022
The recently discovered high energy emission from the recurrent nova RS Ophiuchi by Fermi-LAT (> ... more The recently discovered high energy emission from the recurrent nova RS Ophiuchi by Fermi-LAT (> 100 MeV), H.E.S.S. and MAGIC (> 100 GeV), hints towards a possible hadronic origin of this radiation component. From the observed high energy photon flux we derive the expected number of neutrino events that could be detected by present and future neutrino telescopes in the different energy ranges. Preliminary estimates indicate that with the "next-generation" instrument IceCube-Gen2, the expected number of neutrino detections from Galactic novae, admittedly with large uncertainty, should not exceed 1 event per decade.
Monthly Notices of the Royal Astronomical Society
KT Eridani was a very fast nova in 2009 peaking at V = 5.42 mag. We marshal large data sets of p... more 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 d. With our 272 spectral energy distributions from simultaneous BVRIJHK measures, the companion star has a temperature of 6200 ± 500 K. Our century-long average in quiescence has V = 14.5. With the Gaia distance (5110$^{+920}_{-430}$ pc), the absolute magnitude is MVqM_{V_q}MVq = +0.7 ± 0.3. We converted this absolute magnitude (corrected to the disc light alone) to accretion rates, dotM\dot{M}dotM, with a full integration of the α-disc model. This dotM\dot{M}dotM is very high at 3.5 × 10−7 M⊙ yr−1. Our search and analysis of archival photographs shows that no eruption occurred from 1928 to 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 ...
42nd COSPAR Scientific Assembly, Jul 1, 2018
Journal of Cosmology and Astroparticle Physics
The recently discovered high energy emission from the recurrent nova RS Ophiuchi by Fermi-LAT (&g... more The recently discovered high energy emission from the recurrent nova RS Ophiuchi by Fermi-LAT (> 100 MeV), H.E.S.S. and MAGIC (>100 GeV), hints towards a possible hadronic origin of this radiation component. From the observed high energy photon flux we derive the expected number of neutrino events that could be detected by present and future neutrino telescopes in the different energy ranges. Preliminary estimates indicate that with the “next-generation” instrument IceCube-Gen2, the expected number of neutrino detections from Galactic novae, admittedly with large uncertainty, should not exceed 1 event per decade.
Monthly Notices of the Royal Astronomical Society
Fluctuations during a prolonged maximum have been observed in several nova eruptions, although it... more Fluctuations during a prolonged maximum have been observed in several nova eruptions, although it is not clear, and cannot be deduced directly from observations, whether the phenomenon is an actual physical reaction to some mechanism originating in the erupting white dwarf, whether it is occurring in the expanding ejected shell, or whether it is a form of interaction with the red dwarf companion. A handful of erupting nova models are investigated in this work, in order to assess the possibility of this sort of feature being an actual part of the eruption itself. The results explain that the mechanism that may produce these fluctuations is the repeated approach and recession of the convective front from the surface. The efficiency of this mechanism, being dependent on the mass of the WD (white dwarf) envelope and the time-scale of the nova cycle, favours low-mass WDs and long accretion phases.
Over the course of a nova cycle, a white dwarf (WD) quiescently accretes (mostly) hydrogen from i... more Over the course of a nova cycle, a white dwarf (WD) quiescently accretes (mostly) hydrogen from its less evolved companion for a long period of time, until finally igniting a thermonuclear runaway (TNR). This causes the outer layers of the WD to rapidly expand and expel mass. The nature of this phenomenon entails orders of magnitude changes in the surface temperature, seemingly favoring observations in different bands at different epochs of the cycle. However, to better understand the evolution of a nova cycle, it would be extremely informative to have simultaneous observations of the system in multiple bands. To date, such data is sparse. I will present models of multiple nova cycles for various systems, to demonstrate how this feature and others, may assist in identifying the parameters of observed systems and better understanding them.
arXiv: Solar and Stellar Astrophysics, 2020
The hydrogen-rich envelopes accreted by white dwarf stars from their red dwarf companions lead to... more The hydrogen-rich envelopes accreted by white dwarf stars from their red dwarf companions lead to thermonuclear runaways observed as classical nova eruptions peaking at up to 1 Million solar luminosities. Virtually all nova progenitors are novalike binaries exhibiting high rates of mass transfer to their white dwarfs before and after an eruption. It is a puzzle that binaries indistinguishable from novalikes, but with much lower mass transfer rates, and resulting dwarf nova outbursts, co-exist at the same orbital periods. Nova shells surrounding several dwarf novae demonstrate that at least some novae become dwarf novae between successive nova eruptions, though the mechanisms and timescales governing mass transfer rate variations are poorly understood. Here we report simulations of the multiGyr evolution of novae which self-consistently model every eruption's thermonuclear runaway, mass and angular momentum losses, feedback due to irradiation and variable mass transfer, and orbit...
We use a combined binary evolution code including dynamical effects to study nova eruptions in a ... more We use a combined binary evolution code including dynamical effects to study nova eruptions in a symbiotic system. Following the evolution, over sim105\sim10^5sim105 years, of multiple consecutive nova eruptions on the surface of a 1.25Modot1.25 M_\odot1.25Modot white dwarf (WD) accretor, we present a comparison between simulations of two types of systems. The first is the common, well known, cataclysmic variable (CV) system in which a main sequence donor star transfers mass to its WD companion via Roche-lobe overflow. The second is a detached, widely separated, symbiotic system in which an asymptotic giant branch donor star transfers mass to its WD companion via strong winds. For the latter we use the Bondi-Hoyle-Lyttleton prescription along with orbital dynamics to calculate the accretion rate. We use the combined stellar evolution code to follow the nova eruptions of both simulations including changes in mass, accretion rate and orbital features. We find that while the average accretion rate for the CV re...
Monthly Notices of the Royal Astronomical Society, 2021
This study is the direct continuation of a previous work performed by Hillman et al., where they ... more This study is the direct continuation of a previous work performed by Hillman et al., where they used their feedback dominated numerical simulations to model the evolution of four initial models with white dwarf (WD) masses of 0.7 and 1.0,mathrmModot1.0\, \mathrm{M}_\odot1.0,mathrmModot and red dwarf (RD) masses of 0.45 and 0.7,mathrmModot0.7\, \mathrm{M}_\odot0.7,mathrmModot from first Roche lobe contact of the donor RD, over a few times 109 yr, until the RD was eroded down to below 0.1,mathrmModot0.1\, \mathrm{M}_\odot0.1,mathrmModot. This study presents an in-depth analysis of their four models complimented by three models with a higher WD mass of 1.25,mathrmModot1.25\, \mathrm{M}_\odot1.25,mathrmModot, one of which comprises an oxygen-neon (ONe) core. Common features were found for all seven models on a secular time-scale as well as on a cyclic time-scale. On the other hand, certain features were found that are strongly dependent either on the WD or the RD mass but are indifferent to the other of the two. Additionally, a model with a WD composed of an ONe core was compared with its correspo...
Advances in Space Research, 2019
Abstract Direct determination of WD masses in CVs is extremely difficult, which is the reason ver... more Abstract Direct determination of WD masses in CVs is extremely difficult, which is the reason very few direct determinations exist. We present a method for calculating an estimate of WD masses in CVs and of the rate at which they accrete matter from their companion, by combining results of nova simulations with cataloged observations of 92 novae. We present observed distributions of the WD mass ( M WD ) and accretion rate ( M ), and corrections of these distributions while taking into account the observation frequency of each nova. We show that WDs in RN systems are typically more massive than WDs in CN systems and their M is higher. If the M is sufficiently high, the WD will eject only a fraction of the accreted mass, resulting in the secular growth of the WD mass. We have followed the evolution of WDs of various initial masses and accretion rates through long series of nova cycles, focusing on the WD mass change, considering both hydrogen and helium accretion and addressing the consequences of helium flashes. We show the circumstances under which WDs can grow up to the Chandrasekhar mass, to become progenitors of type Ia Supernovae.
The Astrophysical Journal, 2019
The supersoft X-ray and optical transient ASASSN-16oh has been interpreted by Maccarone et al. as... more The supersoft X-ray and optical transient ASASSN-16oh has been interpreted by Maccarone et al. as having been induced by an accretion event on a massive white dwarf (WD), resembling a dwarf nova super-outburst. These authors argued that the supersoft X-ray spectrum had a different origin than in an atmosphere heated by shell nuclear burning, because no mass was ejected. We find instead that the event's timescale and other characteristics are typical of non-mass-ejecting thermonuclear runaways, as already predicted by Shara et al. and the extensive grid of nova models by Yaron et al. We suggest that the low X-ray and bolometric luminosity in comparison to the predictions of the models of nuclear burning are due to an optically thick accretion disk, hiding most of the WD surface. If this is the case, we calculated that the optical transient can be explained as a non-ejective thermonuclear event on a WD of ;1.1 M e accreting at the rate of ;3.5-5×10 −7 M e yr −1. We make predictions that should prove whether the nature of the transient event was due to thermonuclear burning or to accretion; observational proof should be obtained in the next few years, because a new outburst should occur within ;10-15 yr of the event.
The Astrophysical Journal, 2018
Models have long predicted that the frequency-averaged masses of white dwarfs (WDs) in Galactic c... more Models have long predicted that the frequency-averaged masses of white dwarfs (WDs) in Galactic classical novae are twice as large as those of field WDs. Only a handful of dynamically well-determined nova WDs masses have been published, leaving the theoretical predictions poorly tested. The recurrence time distributions and mass accretion rate distributions of novae are even more poorly known. To address these deficiencies, we have combined our extensive simulations of nova eruptions with the Strope et al. and Schaefer databases of outburst characteristics of Galactic classical and recurrent novae (RNe) to determine the masses of 92 WDs in novae. We find that the mean mass (frequency-averaged mean mass) of 82 Galactic classical novae is 1.06 (1.13)M e , while the mean mass of 10 RNe is 1.31M e. These masses, and the observed nova outburst amplitude and decline time distributions allow us to determine the long-term mass accretion rate distribution of classical novae. Remarkably, that value is just 1.3×10 −10 M e yr −1 , which is an order of magnitude smaller than that of cataclysmic binaries in the decades before and after classical nova eruptions. This predicts that old novae become low-mass transfer rate systems, and hence dwarf novae, for most of the time between nova eruptions. We determine the mass accretion rates of each of the 10 known Galactic recurrent nova, finding them to be in the range of 10 −7-10 −8 M e yr −1. We are able to predict the recurrence time distribution of novae and compare it with the predictions of population synthesis models.
Monthly Notices of the Royal Astronomical Society, 2017
Classical novae show a rapid rise in optical brightness over a few hours. Until recently the rise... more Classical novae show a rapid rise in optical brightness over a few hours. Until recently the rise phase, particularly the phenomenon of a pre-maximum halt, was observed sporadically. Solar observation satellites observing coronal mass ejections enable us to observe the premaximum phase in unprecedented temporal resolution. We present observations of V5589 Sgr with STEREO HI-1B at a cadence of 40 min, the highest to date. We temporally resolve a pre-maximum halt for the first time, with two examples each rising over 40 min then declining within 80 min. Comparison with a grid of outburst models suggests that this double peak, and the overall rise timescale , is consistent with a white dwarf mass, central temperature and accretion rate close to 1.0 M , 5 × 10 7 K and 10 −10 M yr −1 , respectively. The modelling formally predicts mass-loss onset at JD 245 6038.2391 ± 0.0139, 12 h before the optical maximum. The model assumes a main-sequence donor. Observational evidence is for a subgiant companion, meaning that the accretion rate is underestimated. Post-maximum, we see erratic variations commonly associated with much slower novae. Estimating the decline rate is difficult, but we place the time to decline two magnitudes as 2.1 < t 2 (d) < 3.9, making V5589 Sgr a 'very fast' nova. The brightest point defines 'day 0' as JD 245 6038.8224 ± 0.0139, although at this high cadence the meaning of the observed maximum becomes difficult to define. We suggest that such erratic variability normally goes undetected in faster novae due to the low cadence of typical observations, implying that erratic behaviour is not necessarily related to the rate of decline.
The Astrophysical Journal, 2016
Can a white dwarf (WD), accreting hydrogen-rich matter from a non-degenerate companion star, ever... more Can a white dwarf (WD), accreting hydrogen-rich matter from a non-degenerate companion star, ever exceed the Chandrasekhar mass and explode as a SN Ia? We explore the range of accretion rates that allow a WD to secularly grow in mass, and derive limits on the accretion rate and on the initial mass that will allow it to reach 1.4M e-the Chandrasekhar mass. We follow the evolution through a long series of hydrogen flashes, during which a thick helium shell accumulates. This determines the effective helium mass accretion rate for long-term, self-consistent evolutionary runs with helium flashes. We find that net mass accumulation always occurs despite helium flashes. Although the amount of mass lost during the first few helium shell flashes is a significant fraction of that accumulated prior to the flash, that fraction decreases with repeated helium shell flashes. Eventually no mass is ejected at all during subsequent flashes. This unexpected result occurs because of continual heating of the WD interior by the helium shell flashes near its surface. The effect of heating is to lower the electron degeneracy throughout the WD, especially in the outer layers. This key result yields helium burning that is quasisteady state, instead of explosive. We thus find a remarkably large parameter space within which long-term, selfconsistent simulations show that a WD can grow in mass and reach the Chandrasekhar limit, despite its helium flashes.
Monthly Notices of the Royal Astronomical Society, Nov 14, 2023
The Astrophysical Journal Supplement Series
M87 has been monitored with a cadence of 5 days over a span of 9 months through the near-ultravio... more M87 has been monitored with a cadence of 5 days over a span of 9 months through the near-ultraviolet (NUV; F275W) and optical (F606W) filters of the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope. This unprecedented dataset yields the NUV and optical light and color curves of 94 M87 novae, characterizing the outburst and decline properties of the largest extragalactic nova dataset in the literature (after M31 and M81). We test and confirm nova modelers’ prediction that recurrent novae cannot erupt more frequently than once every 45 days, show that there are zero rapidly recurring novae in the central ∼1/3 of M87 with recurrence times <130 days, demonstrate that novae closely follow the K-band light of M87 to within a few arcsecs of the galaxy nucleus, show that nova NUV light curves are as heterogeneous as their optical counterparts, and usually peak 5–30 days after visible light maximum, determine our observations’ annual detection completeness to be 71%–77%, and measu...
arXiv (Cornell University), Oct 19, 2022
arXiv (Cornell University), Sep 11, 2022
The recently discovered high energy emission from the recurrent nova RS Ophiuchi by Fermi-LAT (> ... more The recently discovered high energy emission from the recurrent nova RS Ophiuchi by Fermi-LAT (> 100 MeV), H.E.S.S. and MAGIC (> 100 GeV), hints towards a possible hadronic origin of this radiation component. From the observed high energy photon flux we derive the expected number of neutrino events that could be detected by present and future neutrino telescopes in the different energy ranges. Preliminary estimates indicate that with the "next-generation" instrument IceCube-Gen2, the expected number of neutrino detections from Galactic novae, admittedly with large uncertainty, should not exceed 1 event per decade.
Monthly Notices of the Royal Astronomical Society
KT Eridani was a very fast nova in 2009 peaking at V = 5.42 mag. We marshal large data sets of p... more 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 d. With our 272 spectral energy distributions from simultaneous BVRIJHK measures, the companion star has a temperature of 6200 ± 500 K. Our century-long average in quiescence has V = 14.5. With the Gaia distance (5110$^{+920}_{-430}$ pc), the absolute magnitude is MVqM_{V_q}MVq = +0.7 ± 0.3. We converted this absolute magnitude (corrected to the disc light alone) to accretion rates, dotM\dot{M}dotM, with a full integration of the α-disc model. This dotM\dot{M}dotM is very high at 3.5 × 10−7 M⊙ yr−1. Our search and analysis of archival photographs shows that no eruption occurred from 1928 to 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 ...
42nd COSPAR Scientific Assembly, Jul 1, 2018
Journal of Cosmology and Astroparticle Physics
The recently discovered high energy emission from the recurrent nova RS Ophiuchi by Fermi-LAT (&g... more The recently discovered high energy emission from the recurrent nova RS Ophiuchi by Fermi-LAT (> 100 MeV), H.E.S.S. and MAGIC (>100 GeV), hints towards a possible hadronic origin of this radiation component. From the observed high energy photon flux we derive the expected number of neutrino events that could be detected by present and future neutrino telescopes in the different energy ranges. Preliminary estimates indicate that with the “next-generation” instrument IceCube-Gen2, the expected number of neutrino detections from Galactic novae, admittedly with large uncertainty, should not exceed 1 event per decade.
Monthly Notices of the Royal Astronomical Society
Fluctuations during a prolonged maximum have been observed in several nova eruptions, although it... more Fluctuations during a prolonged maximum have been observed in several nova eruptions, although it is not clear, and cannot be deduced directly from observations, whether the phenomenon is an actual physical reaction to some mechanism originating in the erupting white dwarf, whether it is occurring in the expanding ejected shell, or whether it is a form of interaction with the red dwarf companion. A handful of erupting nova models are investigated in this work, in order to assess the possibility of this sort of feature being an actual part of the eruption itself. The results explain that the mechanism that may produce these fluctuations is the repeated approach and recession of the convective front from the surface. The efficiency of this mechanism, being dependent on the mass of the WD (white dwarf) envelope and the time-scale of the nova cycle, favours low-mass WDs and long accretion phases.
Over the course of a nova cycle, a white dwarf (WD) quiescently accretes (mostly) hydrogen from i... more Over the course of a nova cycle, a white dwarf (WD) quiescently accretes (mostly) hydrogen from its less evolved companion for a long period of time, until finally igniting a thermonuclear runaway (TNR). This causes the outer layers of the WD to rapidly expand and expel mass. The nature of this phenomenon entails orders of magnitude changes in the surface temperature, seemingly favoring observations in different bands at different epochs of the cycle. However, to better understand the evolution of a nova cycle, it would be extremely informative to have simultaneous observations of the system in multiple bands. To date, such data is sparse. I will present models of multiple nova cycles for various systems, to demonstrate how this feature and others, may assist in identifying the parameters of observed systems and better understanding them.
arXiv: Solar and Stellar Astrophysics, 2020
The hydrogen-rich envelopes accreted by white dwarf stars from their red dwarf companions lead to... more The hydrogen-rich envelopes accreted by white dwarf stars from their red dwarf companions lead to thermonuclear runaways observed as classical nova eruptions peaking at up to 1 Million solar luminosities. Virtually all nova progenitors are novalike binaries exhibiting high rates of mass transfer to their white dwarfs before and after an eruption. It is a puzzle that binaries indistinguishable from novalikes, but with much lower mass transfer rates, and resulting dwarf nova outbursts, co-exist at the same orbital periods. Nova shells surrounding several dwarf novae demonstrate that at least some novae become dwarf novae between successive nova eruptions, though the mechanisms and timescales governing mass transfer rate variations are poorly understood. Here we report simulations of the multiGyr evolution of novae which self-consistently model every eruption's thermonuclear runaway, mass and angular momentum losses, feedback due to irradiation and variable mass transfer, and orbit...
We use a combined binary evolution code including dynamical effects to study nova eruptions in a ... more We use a combined binary evolution code including dynamical effects to study nova eruptions in a symbiotic system. Following the evolution, over sim105\sim10^5sim105 years, of multiple consecutive nova eruptions on the surface of a 1.25Modot1.25 M_\odot1.25Modot white dwarf (WD) accretor, we present a comparison between simulations of two types of systems. The first is the common, well known, cataclysmic variable (CV) system in which a main sequence donor star transfers mass to its WD companion via Roche-lobe overflow. The second is a detached, widely separated, symbiotic system in which an asymptotic giant branch donor star transfers mass to its WD companion via strong winds. For the latter we use the Bondi-Hoyle-Lyttleton prescription along with orbital dynamics to calculate the accretion rate. We use the combined stellar evolution code to follow the nova eruptions of both simulations including changes in mass, accretion rate and orbital features. We find that while the average accretion rate for the CV re...
Monthly Notices of the Royal Astronomical Society, 2021
This study is the direct continuation of a previous work performed by Hillman et al., where they ... more This study is the direct continuation of a previous work performed by Hillman et al., where they used their feedback dominated numerical simulations to model the evolution of four initial models with white dwarf (WD) masses of 0.7 and 1.0,mathrmModot1.0\, \mathrm{M}_\odot1.0,mathrmModot and red dwarf (RD) masses of 0.45 and 0.7,mathrmModot0.7\, \mathrm{M}_\odot0.7,mathrmModot from first Roche lobe contact of the donor RD, over a few times 109 yr, until the RD was eroded down to below 0.1,mathrmModot0.1\, \mathrm{M}_\odot0.1,mathrmModot. This study presents an in-depth analysis of their four models complimented by three models with a higher WD mass of 1.25,mathrmModot1.25\, \mathrm{M}_\odot1.25,mathrmModot, one of which comprises an oxygen-neon (ONe) core. Common features were found for all seven models on a secular time-scale as well as on a cyclic time-scale. On the other hand, certain features were found that are strongly dependent either on the WD or the RD mass but are indifferent to the other of the two. Additionally, a model with a WD composed of an ONe core was compared with its correspo...
Advances in Space Research, 2019
Abstract Direct determination of WD masses in CVs is extremely difficult, which is the reason ver... more Abstract Direct determination of WD masses in CVs is extremely difficult, which is the reason very few direct determinations exist. We present a method for calculating an estimate of WD masses in CVs and of the rate at which they accrete matter from their companion, by combining results of nova simulations with cataloged observations of 92 novae. We present observed distributions of the WD mass ( M WD ) and accretion rate ( M ), and corrections of these distributions while taking into account the observation frequency of each nova. We show that WDs in RN systems are typically more massive than WDs in CN systems and their M is higher. If the M is sufficiently high, the WD will eject only a fraction of the accreted mass, resulting in the secular growth of the WD mass. We have followed the evolution of WDs of various initial masses and accretion rates through long series of nova cycles, focusing on the WD mass change, considering both hydrogen and helium accretion and addressing the consequences of helium flashes. We show the circumstances under which WDs can grow up to the Chandrasekhar mass, to become progenitors of type Ia Supernovae.
The Astrophysical Journal, 2019
The supersoft X-ray and optical transient ASASSN-16oh has been interpreted by Maccarone et al. as... more The supersoft X-ray and optical transient ASASSN-16oh has been interpreted by Maccarone et al. as having been induced by an accretion event on a massive white dwarf (WD), resembling a dwarf nova super-outburst. These authors argued that the supersoft X-ray spectrum had a different origin than in an atmosphere heated by shell nuclear burning, because no mass was ejected. We find instead that the event's timescale and other characteristics are typical of non-mass-ejecting thermonuclear runaways, as already predicted by Shara et al. and the extensive grid of nova models by Yaron et al. We suggest that the low X-ray and bolometric luminosity in comparison to the predictions of the models of nuclear burning are due to an optically thick accretion disk, hiding most of the WD surface. If this is the case, we calculated that the optical transient can be explained as a non-ejective thermonuclear event on a WD of ;1.1 M e accreting at the rate of ;3.5-5×10 −7 M e yr −1. We make predictions that should prove whether the nature of the transient event was due to thermonuclear burning or to accretion; observational proof should be obtained in the next few years, because a new outburst should occur within ;10-15 yr of the event.
The Astrophysical Journal, 2018
Models have long predicted that the frequency-averaged masses of white dwarfs (WDs) in Galactic c... more Models have long predicted that the frequency-averaged masses of white dwarfs (WDs) in Galactic classical novae are twice as large as those of field WDs. Only a handful of dynamically well-determined nova WDs masses have been published, leaving the theoretical predictions poorly tested. The recurrence time distributions and mass accretion rate distributions of novae are even more poorly known. To address these deficiencies, we have combined our extensive simulations of nova eruptions with the Strope et al. and Schaefer databases of outburst characteristics of Galactic classical and recurrent novae (RNe) to determine the masses of 92 WDs in novae. We find that the mean mass (frequency-averaged mean mass) of 82 Galactic classical novae is 1.06 (1.13)M e , while the mean mass of 10 RNe is 1.31M e. These masses, and the observed nova outburst amplitude and decline time distributions allow us to determine the long-term mass accretion rate distribution of classical novae. Remarkably, that value is just 1.3×10 −10 M e yr −1 , which is an order of magnitude smaller than that of cataclysmic binaries in the decades before and after classical nova eruptions. This predicts that old novae become low-mass transfer rate systems, and hence dwarf novae, for most of the time between nova eruptions. We determine the mass accretion rates of each of the 10 known Galactic recurrent nova, finding them to be in the range of 10 −7-10 −8 M e yr −1. We are able to predict the recurrence time distribution of novae and compare it with the predictions of population synthesis models.
Monthly Notices of the Royal Astronomical Society, 2017
Classical novae show a rapid rise in optical brightness over a few hours. Until recently the rise... more Classical novae show a rapid rise in optical brightness over a few hours. Until recently the rise phase, particularly the phenomenon of a pre-maximum halt, was observed sporadically. Solar observation satellites observing coronal mass ejections enable us to observe the premaximum phase in unprecedented temporal resolution. We present observations of V5589 Sgr with STEREO HI-1B at a cadence of 40 min, the highest to date. We temporally resolve a pre-maximum halt for the first time, with two examples each rising over 40 min then declining within 80 min. Comparison with a grid of outburst models suggests that this double peak, and the overall rise timescale , is consistent with a white dwarf mass, central temperature and accretion rate close to 1.0 M , 5 × 10 7 K and 10 −10 M yr −1 , respectively. The modelling formally predicts mass-loss onset at JD 245 6038.2391 ± 0.0139, 12 h before the optical maximum. The model assumes a main-sequence donor. Observational evidence is for a subgiant companion, meaning that the accretion rate is underestimated. Post-maximum, we see erratic variations commonly associated with much slower novae. Estimating the decline rate is difficult, but we place the time to decline two magnitudes as 2.1 < t 2 (d) < 3.9, making V5589 Sgr a 'very fast' nova. The brightest point defines 'day 0' as JD 245 6038.8224 ± 0.0139, although at this high cadence the meaning of the observed maximum becomes difficult to define. We suggest that such erratic variability normally goes undetected in faster novae due to the low cadence of typical observations, implying that erratic behaviour is not necessarily related to the rate of decline.
The Astrophysical Journal, 2016
Can a white dwarf (WD), accreting hydrogen-rich matter from a non-degenerate companion star, ever... more Can a white dwarf (WD), accreting hydrogen-rich matter from a non-degenerate companion star, ever exceed the Chandrasekhar mass and explode as a SN Ia? We explore the range of accretion rates that allow a WD to secularly grow in mass, and derive limits on the accretion rate and on the initial mass that will allow it to reach 1.4M e-the Chandrasekhar mass. We follow the evolution through a long series of hydrogen flashes, during which a thick helium shell accumulates. This determines the effective helium mass accretion rate for long-term, self-consistent evolutionary runs with helium flashes. We find that net mass accumulation always occurs despite helium flashes. Although the amount of mass lost during the first few helium shell flashes is a significant fraction of that accumulated prior to the flash, that fraction decreases with repeated helium shell flashes. Eventually no mass is ejected at all during subsequent flashes. This unexpected result occurs because of continual heating of the WD interior by the helium shell flashes near its surface. The effect of heating is to lower the electron degeneracy throughout the WD, especially in the outer layers. This key result yields helium burning that is quasisteady state, instead of explosive. We thus find a remarkably large parameter space within which long-term, selfconsistent simulations show that a WD can grow in mass and reach the Chandrasekhar limit, despite its helium flashes.