arXiv subjects

Jose L. Prieto

Publications and source records attributed to Jose L. Prieto.

At least 37 records · Page 2Linked to original sources

arXiv2022

Investigating the Nature of the Luminous Ambiguous Nuclear Transient ASASSN-17jz

We present observations of the extremely luminous but ambiguous nuclear transient (ANT) ASASSN-17jz, spanning roughly 1200 days of the object's evolution. ASASSN-17jz was discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN) in the galaxy SDSS J171955.84+414049.4 on UT 2017 July 27 at a redshift of $z=0.1641$. The transient peaked at an absolute $B$-band magnitude of $M_{B,{\rm peak}}=-22.81$, corresponding to a bolometric luminosity of $L_{\rm bol,peak}=8.3\times10^{44}$~erg~s$^{-1}$, and exhibited late-time ultraviolet emission that was still ongoing in our latest observations. Integrating the full light curve gives a total emitted energy of $E_{\rm tot}=(1.36\pm0.08)\times10^{52}$~erg, with $(0.80\pm0.02)\times10^{52}$~erg of this emitted within 200 days of peak light. This late-time ultraviolet emission is accompanied by increasing X-ray emission that becomes softer as it brightens. ASASSN-17jz exhibited a large number of spectral emission lines most commonly seen in active galactic nuclei (AGNs) with little evidence of evolution. It also showed transient Balmer features which became fainter and broader over time, and are still being detected $>1000$ days after peak brightness. We consider various physical scenarios for the origin of the transient, including supernovae (SNe), tidal disruption events (TDEs), AGN outbursts, and ANTs. We find that the most likely explanation is that ASASSN-17jz was an SN~IIn occurring in or near the disk of an existing AGN, and that the late-time emission is caused by the AGN transitioning to a more active state.

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arXiv2022

Unveiling the Nature of SN 2011fh: a Young and Massive Star Gives Rise to a Luminous SN 2009ip-like Event

In recent years, many Type IIn supernovae have been found to share striking similarities with the peculiar SN 2009ip, whose true nature is still under debate. Here, we present 10 years of observations of SN 2011fh, an interacting transient with spectroscopic and photometric similarities to SN 2009ip. SN 2011fh had a M$_r \sim -16$ mag brightening event, followed by a brighter M$_r \sim -18$ mag luminous outburst in August 2011. The spectra of SN 2011fh are dominated by narrow to intermediate Balmer emission lines throughout its evolution, with P Cygni profiles indicating fast-moving material at $\sim 6400 \ \textrm{km s}^{-1}$. HST/WFC3 observations from October of 2016 revealed a bright source with M$_{F814W} \approx -13.3$ mag, indicating that we are seeing the ongoing interaction of the ejecta with the circumstellar material or that the star might be going through an eruptive phase five years after the luminous outburst of 2011. Using HST photometry of the stellar cluster around SN 2011fh, we estimated an age of $\sim 4.5$ Myr for the progenitor, which implies a stellar mass of $\sim 60$ M$_\odot$, using single-star evolution models, or a mass range of $35 - 80 \ \textrm{M}_\odot$, considering a binary system. We also show that the progenitor of SN 2011fh exceeded the classical Eddington limit by a large factor in the months preceding the luminous outburst of 2011, suggesting strong super-Eddington winds as a possible mechanism for the observed mass-loss. These findings favor an energetic outburst in a young and massive star, possibly a luminous blue variable.

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arXiv2021

ASASSN-18am/SN 2018gk : An overluminous Type IIb supernova from a massive progenitor

ASASSN-18am/SN 2018gk is a newly discovered member of the rare group of luminous, hydrogen-rich supernovae (SNe) with a peak absolute magnitude of $M_V \approx -20$ mag that is in between normal core-collapse SNe and superluminous SNe. These SNe show no prominent spectroscopic signatures of ejecta interacting with circumstellar material (CSM), and their powering mechanism is debated. ASASSN-18am declines extremely rapidly for a Type II SN, with a photospheric-phase decline rate of $\sim6.0~\rm mag~(100 d)^{-1}$. Owing to the weakening of HI and the appearance of HeI in its later phases, ASASSN-18am is spectroscopically a Type IIb SN with a partially stripped envelope. However, its photometric and spectroscopic evolution show significant differences from typical SNe IIb. Using a radiative diffusion model, we find that the light curve requires a high synthesised $\rm ^{56}Ni$ mass $M_{\rm Ni} \sim0.4~M_\odot$ and ejecta with high kinetic energy $E_{\rm kin} = (7-10) \times10^{51} $ erg. Introducing a magnetar central engine still requires $M_{\rm Ni} \sim0.3~M_\odot$ and $E_{\rm kin}= 3\times10^{51} $ erg. The high $\rm ^{56}Ni$ mass is consistent with strong iron-group nebular lines in its spectra, which are also similar to several SNe Ic-BL with high $\rm ^{56}Ni$ yields. The earliest spectrum shows "flash ionisation" features, from which we estimate a mass-loss rate of $ \dot{M}\approx 2\times10^{-4}~\rm M_\odot~yr^{-1} $. This wind density is too low to power the luminous light curve by ejecta-CSM interaction. We measure expansion velocities as high as $ 17,000 $ km/s for $H_α$, which is remarkably high compared to other SNe II. We estimate an oxygen core mass of $1.8-3.4$ $M_\odot$ using the [OI] luminosity measured from a nebular-phase spectrum, implying a progenitor with a zero-age main sequence mass of $19-26$ $M_\odot$.

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arXiv2021

The ALMA Early Science view of FUor/EXor objects. I. Through the looking-glass of V2775 Ori

As part of an ALMA survey to study the origin of episodic accretion in young eruptive variables, we have observed the circumstellar environment of the star V2775 Ori. This object is a very young, pre-main sequence object which displays a large amplitude outburst characteristic of the FUor class. We present Cycle-2 band 6 observations of V2775 Ori with a continuum and CO (2-1) isotopologue resolution of 0.25\as (103 au). We report the detection of a marginally resolved circumstellar disc in the ALMA continuum with an integrated flux of $106 \pm 2$ mJy, characteristic radius of $\sim$ 30 au, inclination of $14.0^{+7.8}_{-14.5}$ deg, and is oriented nearly face-on with respect to the plane of the sky. The \co~emission is separated into distinct blue and red-shifted regions that appear to be rings or shells of expanding material from quasi-episodic outbursts. The system is oriented in such a way that the disc is seen through the outflow remnant of V2775 Ori, which has an axis along our line-of-sight. The $^{13}$CO emission displays similar structure to that of the \co, while the C$^{18}$O line emission is very weak. We calculated the expansion velocities of the low- and medium-density material with respect to the disc to be of -2.85 km s$^{-1}$ (blue), 4.4 km s$^{-1}$ (red) and -1.35 and 1.15 km s$^{-1}$ (for blue and red) and we derived the mass, momentum and kinetic energy of the expanding gas. The outflow has an hourglass shape where the cavities are not seen. We interpret the shapes that the gas traces as cavities excavated by an ancient outflow. We report a detection of line emission from the circumstellar disc and derive a lower limit of the gas mass of 3 \MJup.

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arXiv2021

Classical Novae Masquerading as Dwarf Novae? Outburst Properties of Cataclysmic Variables with ASAS-SN

The unprecedented sky coverage and observing cadence of the All-Sky Automated Survey for SuperNovae (ASAS-SN) has resulted in the discovery and continued monitoring of a large sample of Galactic transients. The vast majority of these are accretion-powered dwarf nova outbursts in cataclysmic variable systems, but a small subset are thermonuclear-powered classical novae. Despite improved monitoring of the Galaxy for novae from ASAS-SN and other surveys, the observed Galactic nova rate is still lower than predictions. One way classical novae could be missed is if they are confused with the much larger population of dwarf novae. Here, we examine the properties of 1617 dwarf nova outbursts detected by ASAS-SN and compare them to classical novae. We find that the mean classical nova brightens by ~11 magnitudes during outburst, while the mean dwarf nova brightens by only ~5 magnitudes, with the outburst amplitude distributions overlapping by roughly 15%. For the first time, we show that the amplitude of an outburst and the time it takes to decline by two magnitudes from maximum are positively correlated for dwarf nova outbursts. For classical novae, we find that these quantities are negatively correlated, but only weakly, compared to the strong anti-correlation of these quantities found in some previous work. We show that, even if located at large distances, only a small number of putative dwarf novae could be mis-classified classical novae suggesting that there is minimal confusion between these populations. Future spectroscopic follow-up of these candidates can show whether any are indeed classical novae.

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arXiv2020

Optical and Near-Infrared Observations of the Nearby Type Ia Supernova 2017cbv

Supernova (SN) 2017cbv in NGC 5643 is one of a handful of type Ia supernovae (SNe~Ia) reported to have excess blue emission at early times. This paper presents extensive $BVRIYJHK_s$-band light curves of SN 2017cbv, covering the phase from $-16$ to $+125$ days relative to $B$-band maximum light. SN 2017cbv reached a $B$-band maximum of 11.710$\pm$0.006~mag, with a post-maximum magnitude decline $Δm_{15}(B)$=0.990$\pm$0.013 mag. The supernova suffered no host reddening based on Phillips intrinsic color, Lira-Phillips relation, and the CMAGIC diagram. By employing the CMAGIC distance modulus $μ=30.58\pm0.05$~mag and assuming $H_0$=72~$\rm km \ s^{-1} \ Mpc^{-1}$, we found that 0.73~\msun $^{56}$Ni was synthesized during the explosion of SN 2017cbv, which is consistent with estimates using reddening-free and distance-free methods via the phases of the secondary maximum of the NIR-band light curves. We also present 14 near-infrared spectra from $-18$ to $+49$~days relative to the $B$-band maximum light, providing constraints on the amount of swept-up hydrogen from the companion star in the context of the single degenerate progenitor scenario. No $Paβ$ emission feature was detected from our post-maximum NIR spectra, placing a hydrogen mass upper limit of 0.1 $M_{\odot}$. The overall optical/NIR photometric and NIR spectral evolution of SN 2017cbv is similar to that of a normal SN~Ia, even though its early evolution is marked by a flux excess no seen in most other well-observed normal SNe~Ia. We also compare the exquisite light curves of SN 2017cbv with some $M_{ch}$ DDT models and sub-$M_{ch}$ double detonation models.

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arXiv2020

ASASSN-14ko is a Periodic Nuclear Transient in ESO 253-G003

We present the discovery that ASASSN-14ko is a periodically flaring AGN at the center of the galaxy ESO 253-G003. At the time of its discovery by the All-Sky Automated Survey for Supernovae (ASAS-SN), it was classified as a supernova close to the nucleus. The subsequent six years of V- and g-band ASAS-SN observations reveal that ASASSN-14ko has nuclear flares occurring at regular intervals. The seventeen observed outbursts show evidence of a decreasing period over time, with a mean period of $P_0 = 114.2 \pm 0.4$ days and a period derivative of $\dot{P} = -0.0017\pm0.0003$. The most recent outburst in May 2020, which took place as predicted, exhibited spectroscopic changes during the rise and a had a UV bright, blackbody spectral energy distribution similar to tidal disruption events (TDEs). The X-ray flux decreased by a factor of 4 at the beginning of the outburst and then returned to its quiescent flux after ~8 days. TESS observed an outburst during Sectors 4-6, revealing a rise time of $5.60 \pm 0.05$ days in the optical and a decline that is best fit with an exponential model. We discuss several possible scenarios to explain ASASSN-14ko's periodic outbursts, but currently favor a repeated partial TDE. The next outbursts should peak in the optical on UT 2020-09-7.4$ \pm $1.1 and UT 2020-12-26.5$ \pm $1.4.

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arXiv2020

Spectral signatures of H-rich material stripped from a non-degenerate companion by a Type Ia supernova

The single-degenerate scenario for Type Ia supernovae (SNe Ia) should yield metal-rich ejecta that enclose some stripped material from the non-degenerate H-rich companion star. We present a large grid of non-local thermodynamic equilibrium steady-state radiative transfer calculations for such hybrid ejecta and provide analytical fits for the Halpha luminosity and equivalent width. Our set of models covers a range of masses for 56Ni and the ejecta, for the stripped material (Mst), and post-explosion epochs from 100 to 300d. The brightness contrast between stripped material and metal-rich ejecta challenges the detection of HI and HeI lines prior to ~100d. Intrinsic and extrinsic optical depth effects also influence the radiation emanating from the stripped material. This inner denser region is marginally thick in the continuum and optically thick in all Balmer lines. The overlying metal-rich ejecta blanket the inner regions, completely below about 5000A, and more sparsely at longer wavelengths. As a consequence, Hbeta should not be observed for all values of Mst through at least 300 days, while Halpha should be observed after ~100d for all Mst >= 0.01Msun. This contrasts with the case of circumstellar (CSM) interaction, not subject to external blanketing, which should produce Halpha and Hbeta lines with a strength dependent primarily on CSM density. We confirm previous analyses that suggest low values of order 0.001Msun for Mst to explain the observations of the two SNe Ia with nebular-phase Halpha detection, in conflict with the much greater stripped mass predicted by hydrodynamical simulations for the single-degenerate scenario. A more likely solution is the double-degenerate scenario, together with CSM interaction, or enclosed material from a tertiary star in a triple system or from a giant planet. [Abridged]

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arXiv2020

The Rise and Fall of ASASSN-18pg: Following a TDE from Early To Late Times

We present nearly 500 days of observations of the tidal disruption event ASASSN-18pg, spanning from 54 days before peak light to 441 days after peak light. Our dataset includes X-ray, UV, and optical photometry, optical spectroscopy, radio observations, and the first published spectropolarimetric observations of a TDE. ASASSN-18pg was discovered on 2018 July 11 by the All-Sky Automated Survey for Supernovae (ASAS-SN) at a distance of $d=78.6$ Mpc, and with a peak UV magnitude of $m\simeq14$ it is both one of the nearest and brightest TDEs discovered to-date. The photometric data allow us to track both the rise to peak and the long-term evolution of the TDE. ASASSN-18pg peaked at a luminosity of $L\simeq2.2\times10^{44}$ erg s$^{-1}$, and its late-time evolution is shallower than a flux $\propto t^{-5/3}$ power-law model, similar to what has been seen in other TDEs. ASASSN-18pg exhibited Balmer lines and spectroscopic features consistent with Bowen fluorescence prior to peak which remained detectable for roughly 225 days after peak. Analysis of the two-component H$α$ profile indicates that, if they are the result of reprocessing of emission from the accretion disk, the different spectroscopic lines may be coming from regions between $\sim10$ and $\sim60$ light-days from the black hole. No X-ray emission is detected from the TDE and there is no evidence of a jet or strong outflow detected in the radio. Our spectropolarimetric observations give no strong evidence for significant asphericity in the emission region, with the emission region having an axis ratio of at least $\sim0.65$.

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arXiv2019

Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae. X: The Tenth Year (2017)

Continuing the project described by Kato et al. (2009, PASJ, 61, S395, arXiv/0905.1757), we collected times of superhump maxima for 102 SU UMa-type dwarf novae observed mainly during the 2017 season and characterized these objects. WZ Sge-type stars identified in this study are PT And, ASASSN-17ei, ASASSN-17el, ASASSN-17es, ASASSN-17fn, ASASSN-17fz, ASASSN-17hw, ASASSN-17kd, ASASSN-17la, PNV J20205397+2508145 and TCP J00332502-3518565. We obtained new mass ratios for 7 objects using growing superhumps (stage A). ASASSN-17gf is an EI Psc-type object below the period minimum. CRTS J080941.3+171528 and DDE 51 are objects in the period gap and both showed long-lasting phase of stage A superhumps. We also summarized the recent advances in understanding of SU UMa-type and WZ Sge-type dwarf novae.

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arXiv2019

Variable H$α$ Emission in the Nebular Spectra of the Low-Luminosity Type Ia SN2018cqj/ATLAS18qtd

We present optical photometry and spectroscopy of the Type Ia supernova SN2018cqj/ATLAS18qtd. The supernova exploded in an isolated region at $\sim 65$~kpc from the S0 galaxy IC~550 at $z=0.0165$ ($D\approx 74$~Mpc) and has a redshift consistent with a physical association to this galaxy. Multicolor photometry show that SN2018cqj/ATLAS18qtd is a low-luminosity ($M_{B_{max}}\approx -17.9$ mag), fast-declining Type Ia with color stretch $s_{BV} \approx 0.6$ and $B$-band decline rate $Δm_{15}(B) \approx 1.77$ mag. Two nebular-phase spectra obtained as part of the 100IAS survey at +193 and +307 days after peak show the clear detection of a narrow H$α$ line in emission that is resolved in the first spectrum with $\rm FWHM \approx 1200$ km s$^{-1}$ and $L_{Hα} \approx 3.8\times 10^{37}$ erg s$^{-1}$. The detection of a resolved H$α$ line with a declining luminosity is broadly consistent with recent models where hydrogen is stripped from the non-degenerate companion in a single-degenerate progenitor system. However, the amount of hydrogen consistent with the luminosities of the H$α$ line would be $\sim 10^{-3}$ M$_{\odot}$, significantly less than theoretical model predictions in the classical single-degenerate progenitor systems. SN2018cqj/ATLAS18qtd is the second low-luminosity, fast-declining Type Ia SN after SN2018fhw/ASASSN-18tb that shows narrow H$α$ in emission in its nebular-phase spectra.

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arXiv2019

Discovery and Early Evolution of ASASSN-19bt, the First TDE Detected by TESS

We present the discovery and early evolution of ASASSN-19bt, a tidal disruption event (TDE) discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN) at a distance of $d\simeq115$ Mpc and the first TDE to be detected by TESS. As the TDE is located in the TESS Continuous Viewing Zone, our dataset includes 30-minute cadence observations starting on 2018 July 25, and we precisely measure that the TDE begins to brighten $\sim8.3$ days before its discovery. Our dataset also includes 18 epochs of Swift UVOT and XRT observations, 2 epochs of XMM-Newton observations, 13 spectroscopic observations, and ground data from the Las Cumbres Observatory telescope network, spanning from 32 days before peak through 37 days after peak. ASASSN-19bt thus has the most detailed pre-peak dataset for any TDE. The TESS light curve indicates that the transient began to brighten on 2019 January 21.6 and that for the first 15 days its rise was consistent with a flux $\propto t^2$ power-law model. The optical/UV emission is well-fit by a blackbody SED, and ASASSN-19bt exhibits an early spike in its luminosity and temperature roughly 32 rest-frame days before peak and spanning up to 14 days that has not been seen in other TDEs, possibly because UV observations were not triggered early enough to detect it. It peaked on 2019 March 04.9 at a luminosity of $L\simeq1.3\times10^{44}$ ergs s$^{-1}$ and radiated $E\simeq3.2\times10^{50}$ ergs during the 41-day rise to peak. X-ray observations after peak indicate a softening of the hard X-ray emission prior to peak, reminiscent of the hard/soft states in X-ray binaries.

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arXiv2019

1ES 1927+654: An AGN Caught Changing Look on a Timescale of Months

We study the sudden optical and ultraviolet (UV) brightening of 1ES 1927+654, which until now was known as a narrow-line active galactic nucleus (AGN). 1ES 1927+654 was part of the small and peculiar class of "true Type-2" AGN, which lack broad emission lines and line-of-sight obscuration. Our high-cadence spectroscopic monitoring captures the appearance of a blue, featureless continuum, followed several weeks later by the appearance of broad Balmer emission lines. This timescale is generally consistent with the expected light travel time between the central engine and the broad-line emission region in (persistent) broad-line AGN. Hubble Space Telescope spectroscopy reveals no evidence for broad UV emission lines (e.g., CIV1549, CIII]1909, MgII2798), probably owing to dust in the broad-line emission region. To the best of our knowledge, this is the first case where the lag between the change in continuum and in broad-line emission of a "changing-look" AGN has been temporally resolved. The nature and timescales of the photometric and spectral evolution disfavor both a change in line-of-sight obscuration and a change of the overall rate of gas inflow as driving the drastic spectral transformations seen in this AGN. Although the peak luminosity and timescales are consistent with those of tidal disruption events seen in inactive galaxies, the spectral properties are not. The X-ray emission displays a markedly different behavior, with frequent flares on timescales of hours to days, and will be presented in a companion publication.

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arXiv2018

The largest M dwarfs flares from ASAS-SN

The All-Sky Automated Survey for Supernovae (ASAS-SN) is the only project in existence to scan the entire sky in optical light every $\sim$day, reaching a depth of $g\sim18$ mag. Over the course of its first four years of transient alerts (2013-2016), ASAS-SN observed 53 events classified as likely M dwarf flares. We present follow-up photometry and spectroscopy of all 53 candidates, confirming flare events on 47 M dwarfs, one K dwarf, and one L dwarf. The remaining four objects include a previously identified TT Tauri star, a young star with outbursts, and two objects too faint to confirm. A detailed examination of the 49 flare star light curves revealed an additional six flares on five stars, resulting in a total of 55 flares on 49 objects ranging in $V$-band contrast from $ΔV = -1$ to $-10.2$ mags. Using an empirical flare model to estimate the unobserved portions of the flare light curve, we obtain lower limits on the $V$-band energy emitted during each flare, spanning $\log(E_V/{\rm ergs})=32$ to $35$, which are among the most energetic flares detected on M dwarfs. The ASAS-SN M-dwarf flare stars show a higher fraction of H$α$ emission as well as stronger H$α$ emission compared to M dwarfs selected without reference to activity, consistent with belonging to a population of more magnetically active stars. We also examined the distribution of tangential velocities, finding that the ASAS-SN flaring M dwarfs are likely to be members of the thin disk and are neither particularly young nor old.

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arXiv2018

Exceptionally fast ejecta seen in light echoes of Eta Carinae's Great Eruption

In our ongoing study of eta Carinae's light echoes, there is a relatively bright echo that has been fading slowly, reflecting the 1845-1858 plateau of the eruption. A separate paper discusses its detailed evolution, but here we highlight one important result: the H-alpha line shows extremely broad emission wings that reach -10,000km/s to the blue and +20,000km/s to the red. The line profile shape is inconsistent with electron scattering wings, indicating high-velocity outflowing material. These are the fastest outflow speeds ever seen in a non-terminal massive star eruption. The broad wings are absent in early phases of the eruption, but strengthen in the 1850s. These speeds are two orders of magnitude faster than the escape speed from a warm supergiant, and 5-10 times faster than winds from O-type or Wolf-Rayet stars. Instead, they are reminiscent of fast supernova ejecta or outflows from accreting compact objects, profoundly impacting our understanding of eta Car and related transients. This echo views eta Car from latitudes near the equator, so the high speed does not trace a collimated polar jet aligned with the Homunculus. Combined with fast material in the Outer Ejecta, it indicates a wide-angle explosive outflow. The fast material may constitute a small fraction of the total outflowing mass, most of which expands at 600 km/s. This is reminiscent of fast material revealed by broad absorption during the presupernova eruptions of SN2009ip.

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arXiv2018

Light-echoes from the plateau in Eta Carinae's Great Eruption reveal a two-stage shock-powered event

We present multi-epoch photometry and spectroscopy of a light echo from eta Carinae's 19th century Great Eruption. This echo shows a steady decline over a decade, sampling the 1850s plateau of the eruption. Spectra show the bulk outflow speed increasing from 150 km/s at early times, up to 600 km/s in the plateau. Later phases also develop remarkably broad emission wings indicating mass accelerated to more than 10,000 km/s. Together with other clues, this provides direct evidence for an explosive ejection. This is accompanied by a transition from narrow absorption lines to emission lines, often with broad P Cygni profiles. These changes imply that the pre-1845 luminosity spikes are distinct from the 1850s plateau. The key reason for this change may be that shock interaction dominates the plateau. The spectral evolution of eta Car closely resembles that of UGC2773-OT, which had clear signatures of shock interaction. We propose a 2-stage scenario for eta Car's eruption: (1) a slow outflow in the decades before the eruption, driven by binary interaction that produced a dense equatorial outflow, followed by (2) explosive energy injection that drove CSM interaction, powering the plateau and sweeping slower CSM into a fast shell that became the Homunculus. We discuss how this sequence could arise from a stellar merger in a triple system, leaving the eccentric binary seen today. This gives a self-consistent scenario that may explain interacting transients across a wide range of initial mass.

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arXiv2018

ASASSN-18di: discovery of a $ΔV \sim 10$ flare on a mid-M dwarf

We report and characterize a white-light superflare on a previously undiscovered M dwarf detected by the ASAS-SN survey. Employing various color-magnitude and color-spectral type relationships, we estimate several stellar parameters, including the quiescent V-band magnitude, from which we derive a flare amplitude of $ΔV \sim 10$. We determine an r-band absolute magnitude of $M_{r} = 11.4$, consistent with a mid-M dwarf, and an approximate distance to the source of $2.2$ kpc. Using classical-flare models, we infer a flare energy of $E_{V} \simeq (4.1\pm 2.2)\times 10^{36}$ ergs, making this one of the strongest flares documented on an M dwarf.

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arXiv2017

Metal-Rich RRc stars in the Carnegie RR Lyrae Survey

We describe and employ a stacking procedure to investigate abundances derived from the low S/N spectra obtained in the Carnegie RR Lyrae Survey (CARRS; Kollmeier et al. 2013). We find iron metallicities that extend from [Fe/H] ~ -2.5 to values at least as large as [Fe/H] ~ -0.5 in the 274-spectrum CARRS RRc data set. We consider RRc sample contamination by high amplitude solar metallicity delta Scuti stars (HADS) at periods less than 0.3 days, where photometric discrimination between RRc and delta Scuti stars has proven to be problematic. We offer a spectroscopic discriminant, the well-marked overabundance of heavy elements, principally [Ba/H], that is a common, if not universal, characteristic of HADS of all periods and axial rotations. No bona fide RRc stars known to us have verified heavy-element overabundances. Three out of 34 stars in our sample with [Fe/H] > -0.7 exhibit anomalously strong features of Sr, Y, Zr, Ba, and many rare earths. However, carbon is not enhanced in these three stars, and we conclude that their elevated n-capture abundances have not been generated in interior neutron-capture nucleosynthesis. Contamination by HADS appears to be unimportant, and metal-rich RRc stars occur in approximately the same proportion in the Galactic field as do metal-rich RRab stars. An apparent dearth of metal-rich RRc is probably a statistical fluke. Finally we show that RRc stars have a similar inverse period-metallicity relationship as has been found for RRab stars.

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