arXiv subjects

Maria R. Drout

Publications and source records attributed to Maria R. Drout.

At least 73 records · Page 4Linked to original sources

arXiv2021

An Early-Time Optical and Ultraviolet Excess in the type-Ic SN 2020oi

We present photometric and spectroscopic observations of Supernova 2020oi (SN 2020oi), a nearby ($\sim$17 Mpc) type-Ic supernova (SN Ic) within the grand-design spiral M100. We undertake a comprehensive analysis to characterize the evolution of SN 2020oi and constrain its progenitor system. We detect flux in excess of the fireball rise model $δt \approx 2.5$ days from the date of explosion in multi-band optical and UV photometry from the Las Cumbres Observatory and the Neil Gehrels Swift Observatory, respectively. The derived SN bolometric luminosity is consistent with an explosion with $M_{\rm ej} = 0.81 \pm 0.03 M_{\odot}$, $E_{k}= 0.79 \pm 0.09 \times 10^{51} \rm{erg} \rm{s}^{-1}$, and $M_{\rm Ni56} = 0.08 \pm 0.02 M_{\odot}$. Inspection of the event's decline reveals the highest $Δm_{15,\rm{bol}}$ reported for a stripped-envelope event to date. Modeling of optical spectra near event peak indicates a partially mixed ejecta comparable in composition to the ejecta observed in SN 1994I, while the earliest spectrum shows signatures of a possible interaction with material of a distinct composition surrounding the SN progenitor. Further, Hubble Space Telescope (HST) pre-explosion imaging reveals a stellar cluster coincident with the event. From the cluster photometry, we derive the mass and age of the SN progenitor using stellar evolution models implemented in the BPASS library. Our results indicate that SN 2020oi occurred in a binary system from a progenitor of mass $M_{\rm ZAMS} \approx 9.5 \pm 1.0 M_{\odot}$, corresponding to an age of $27 \pm 7$ Myr. SN 2020oi is the dimmest SN Ic event to date for which an early-time flux excess has been observed, and the first in which an early excess is unlikely to be associated with shock-cooling.

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arXiv2021

Final Targeting Strategy for the SDSS-IV APOGEE-2S Survey

APOGEE is a high-resolution (R sim 22,000), near-infrared, multi-epoch, spectroscopic survey of the Milky Way. The second generation of the APOGEE project, APOGEE-2, includes an expansion of the survey to the Southern Hemisphere called APOGEE-2S. This expansion enabled APOGEE to perform a fully panoramic mapping of all the main regions of the Milky Way; in particular, by operating in the H-band, APOGEE is uniquely able to probe the dust-hidden inner regions of the Milky Way that are best accessed from the Southern Hemisphere. In this paper we present the targeting strategy of APOGEE-2S, with special attention to documenting modifications to the original, previously published plan. The motivation for these changes is explained as well as an assessment of their effectiveness in achieving their intended scientific objective. In anticipation of this being the last paper detailing APOGEE targeting, we present an accounting of all such information complete through the end of the APOGEE-2S project; this includes several main survey programs dedicated to exploration of major stellar populations and regions of the Milky Way, as well as a full list of programs contributing to the APOGEE database through allocations of observing time by the Chilean National Time Allocation Committee (CNTAC) and the Carnegie Institution for Science (CIS). This work was presented along with a companion article, R. Beaton et al. (submitted; AAS29028), presenting the final target selection strategy adopted for APOGEE-2 in the Northern Hemisphere.

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arXiv2021

The Nickel Mass Distribution of Stripped-Envelope Supernovae: Implications for Additional Power Sources

We perform a systematic study of the $^{56}$Ni mass ($M_{\rm Ni}$) of 27 stripped envelope supernovae (SESNe) by modeling their light-curve tails, highlighting that use of ``Arnett's rule'' overestimates $M_{\rm Ni}$ for SESN by a factor of $\sim$2. Recently, \citet{Khatami2019} presented a new model relating the peak time ($t_{\rm p}$) and luminosity ($L_{\rm p}$) of a radioactive-powered SN to its $M_{\rm Ni}$ that addresses several limitations of Arnett-like models, but depends on a dimensionless parameter, $β$. Using observed $t_{\rm p}$, $L_{\rm p}$, and tail-measured $M_{\rm Ni}$ values for 27 SESN, we observationally calibrate $β$ for the first time. Despite scatter, we demonstrate that the model of \citet{Khatami2019} with empirically-calibrated $β$ values provides significantly improved measurements of $M_{\rm Ni}$ when only photospheric data is available. However, these observationally-constrained $β$ values are systematically lower than those inferred from numerical simulations, primarily because the observed sample has significantly higher (0.2-0.4 dex) $L_{\rm p}$ for a given $M_{\rm Ni}$. While effects due to composition, mixing, and asymmetry can increase $L_{\rm p}$ current models cannot explain the systematically low $β$ values. However, the discrepancy can be alleviated if $\sim$7--50\% of $L_{\rm p}$ for the observed sample originates from sources other than $^{56}$Ni. Either shock cooling or magnetar spin-down could provide the requisite luminosity. Finally, we find that even with our improved measurements, the $M_{\rm Ni}$ values of SESN are still a factor of $\sim$3 larger than those of hydrogen-rich Type II SN, indicating that these supernovae are inherently different in terms of their progenitor initial mass distributions or explosion mechanisms.

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arXiv2021

Collapsar R-Process Yields Can Reproduce [Eu/Fe] Abundance Scatter in Metal-Poor Stars

It is unclear if neutron star mergers can explain the observed r-process abundances of metal-poor stars. Collapsars, defined here as rotating massive stars whose collapse results in a rapidly accreting disk around a black hole that can launch jets, are a promising alternative. We find that we can produce a self-consistent model in which a population of collapsars with stochastic europium yields synthesizes all of the r-process material in metal-poor ([Fe/H] < -2.5) stars. Our model reproduces the observed scatter and evolution of scatter of [Eu/Fe] abundances. We find that if collapsars are the dominant r-process site for metal-poor stars, r-process synthesis may be linked to supernovae that produce long gamma-ray bursts. Our results also allow for the possibility that core-collapse supernovae beyond those that launch gamma-ray bursts also produce r-process material (e.g., potentially a subset of Type Ic-BL supernovae). Furthermore, we identify collapsar jet properties (isotropic energy, engine luminosity, or engine time) which may trace r-process yield and verify that the amount of r-process yield produced per collapsar in our model (~0.07 Msun) is consistent with other independent estimates. In the future, achieving 0.05 dex precision on distribution scatter or a reliable selection function would further constrain our probe of r-process production. Our model would also hold for another prompt r-process site with a power-law yield, and work is needed to determine if, for example, fast-merging neutron stars can also explain abundance scatter.

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arXiv2021

Rapidly-Declining Hostless Type Ia Supernova KSP-OT-201509b from the KMTNet Supernova Program: Transitional Nature and Constraint on $^{56}$Ni Distribution and Progenitor Type

We report the early discovery and multi-color ($BVI$) high-cadence light curve analyses of a rapidly-declining sub-Chandrasekhar Type Ia supernova KSP-OT-201509b (= AT2015cx) from the KMTNet Supernova Program. The Phillips parameter and color stretch parameter of KSP-OT-201509b (= AT2015cx) are $Δ$$M_{B,15}$ $\simeq$ 1.62 mag and $s_{BV}$ $\simeq$ 0.54, respectively, at an inferred redshift of 0.072. These, together with other measured parameters (such as the strength of the secondary $I$-band peak, colors and luminosity), identify the source to be a rapidly-declining Type Ia of transitional nature that is closer to Branch Normal than 91bg-like. Its early light curve evolution and bolometric luminosity are consistent with those of homologously expanding ejecta powered by radioactive decay and a Type Ia SN explosion with 0.32 $\pm$ 0.01 $M_{\odot}$ of synthesized $^{56}$Ni mass, 0.84 $\pm$ 0.12 $M_{\odot}$ of ejecta mass and (0.61 $\pm$ 0.14) $\times$ 10$^{51}$ erg of ejecta kinetic energy. While its $B-V$ and $V-I$ colors evolve largely synchronously with the changes in the $I$-band light curve as found in other supernovae, we also find the presence of an early redward evolution in $V-I$ prior to --10 days since peak. The bolometric light curve of the source is compatible with a stratified $^{56}$Ni distribution extended to shallow layers of the exploding progenitor. Comparisons between the observed light curves and those predicted from ejecta-companion interactions clearly disfavor Roche Lobe-filling companion stars at large separation distances, thus supporting a double-degenerate scenario for its origin. The lack of any apparent host galaxy in our deep stack images reaching a sensitivity limit of $\sim$ 28 $\rm mag\;arcsec^{-2}$ makes KSP-OT-201509b a hostless Type Ia supernova and offers new insights into supernova host galaxy environments.

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arXiv2021

FIRST J153350.8+272729: the radio afterglow of a decades-old tidal disruption event

We present the discovery of the fading radio transient FIRST J153350.8+272729. The source had a maximum observed 5-GHz radio luminosity of $8\times10^{39}$ erg s$^{-1}$ in 1986, but by 2019 had faded by a factor of nearly 400. It is located 0.15 arcsec from the center of a galaxy (SDSS J153350.89+272729) at 147 Mpc, which shows weak Type II Seyfert activity. We show that a tidal disruption event (TDE) is the preferred scenario for FIRST J153350.8+272729, although it could plausibly be interpreted as the afterglow of a long-duration gamma-ray burst. This is only the second TDE candidate to be first discovered at radio wavelengths. Its luminosity fills a gap between the radio afterglows of sub-relativistic TDEs in the local universe, and relativistic TDEs at high redshifts. The unusual properties of FIRST J153350.8+272729 (ongoing nuclear activity in the host galaxy, high radio luminosity) motivate more extensive TDE searches in untargeted radio surveys.

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arXiv2021

Gaia EDR3 confirms that Westerlund 1 is closer and older than previously thought

Using {\it Gaia} Early Data Release 3 (EDR3) parallaxes and Bayesian inference, we infer a parallax of the Westerlund 1 (Wd1) cluster. We find a parallax of $0.34\pm{0.05}$ mas corresponding to a distance of $2.8^{+0.7}_{-0.6}$ kpc. The new {\it Gaia} EDR3 distance is consistent with our previous result using {\it Gaia} DR2 parallaxes. This confirms that Wd1 is less massive and older than previously assumed. Compared to DR2, the EDR3 individual parallax uncertainties for each star decreased by 30\%. However, the aggregate parallax uncertainty for the cluster remained the same. This suggests that the uncertainty is dominated by systematics, which is possibly due to crowding, motions within the cluster, or motions due to binary orbits.

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arXiv2021

The Center of Expansion and Age of the Oxygen-rich Supernova Remnant 1E 0102.2-7219

We present new proper motion measurements of optically emitting oxygen-rich knots of supernova remnant 1E 0102.2-7219 (E0102), which are used to estimate the remnant's center of expansion and age. Four epochs of high resolution Hubble Space Telescope images spanning 19 yr were retrieved and analyzed. We found a robust center of expansion of alpha=1:04:02.48 and delta=-72:01:53.92 (J2000) with 1-sigma uncertainty of 1.77 arcseconds using 45 knots from images obtained with the Advanced Camera for Surveys using the F475W filter in 2003 and 2013 having the highest signal-to-noise ratio. We also estimate an upper limit explosion age of 1738 +/- 175 yr by selecting knots with the highest proper motions, that are assumed to be the least decelerated. We find evidence of an asymmetry in the proper motions of the knots as a function of position angle. We conclude that these asymmetries were most likely caused by interaction between E0102's original supernova blast wave and an inhomogeneous surrounding environment, as opposed to intrinsic explosion asymmetry. The observed non-homologous expansion suggests that the use of a free expansion model inaccurately offsets the center of expansion and leads to an overestimated explosion age. We discuss our findings as they compare to previous age and center of expansion estimates of E0102 and their relevance to a recently identified candidate central compact object.

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arXiv2021

A Cool and Inflated Progenitor Candidate for the Type Ib Supernova 2019yvr at 2.6 Years Before Explosion

We present Hubble Space Telescope imaging of a pre-explosion counterpart to SN 2019yvr obtained 2.6 years before its explosion as a type Ib supernova (SN Ib). Aligning to a post-explosion Gemini-S/GSAOI image, we demonstrate that there is a single source consistent with being the SN 2019yvr progenitor system, the second SN Ib progenitor candidate after iPTF13bvn. We also analyzed pre-explosion Spitzer/IRAC imaging, but we do not detect any counterparts at the SN location. SN 2019yvr was highly reddened, and comparing its spectra and photometry to those of other, less extinguished SNe Ib we derive $E(B-V)=0.51\substack{+0.27\\-0.16}$ mag for SN 2019yvr. Correcting photometry of the pre-explosion source for dust reddening, we determine that this source is consistent with a $\log(L/L_{\odot}) = 5.3 \pm 0.2$ and $T_{\mathrm{eff}} = 6800\substack{+400\\-200}$ K star. This relatively cool photospheric temperature implies a radius of 320$\substack{+30\\-50} R_{\odot}$, much larger than expectations for SN Ib progenitor stars with trace amounts of hydrogen but in agreement with previously identified SN IIb progenitor systems. The photometry of the system is also consistent with binary star models that undergo common envelope evolution, leading to a primary star hydrogen envelope mass that is mostly depleted but seemingly in conflict with the SN Ib classification of SN 2019yvr. SN 2019yvr had signatures of strong circumstellar interaction in late-time ($>$150 day) spectra and imaging, and so we consider eruptive mass loss and common envelope evolution scenarios that explain the SN Ib spectroscopic class, pre-explosion counterpart, and dense circumstellar material. We also hypothesize that the apparent inflation could be caused by a quasi-photosphere formed in an extended, low-density envelope or circumstellar matter around the primary star.

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arXiv2020

Photometric Classification of 2315 Pan-STARRS1 Supernovae with Superphot

The classification of supernovae (SNe) and its impact on our understanding of the explosion physics and progenitors have traditionally been based on the presence or absence of certain spectral features. However, current and upcoming wide-field time-domain surveys have increased the transient discovery rate far beyond our capacity to obtain even a single spectrum of each new event. We must therefore rely heavily on photometric classification, connecting SN light curves back to their spectroscopically defined classes. Here we present Superphot, an open-source Python implementation of the machine-learning classification algorithm of Villar et al., and apply it to 2315 previously unclassified transients from the Pan-STARRS1 Medium Deep Survey for which we obtained spectroscopic host-galaxy redshifts. Our classifier achieves an overall accuracy of 82%, with completenesses and purities of >80% for the best classes (SNe Ia and superluminous SNe). For the worst performing SN class (SNe Ibc), the completeness and purity fall to 37% and 21%, respectively. Our classifier provides 1257 newly classified SNe Ia, 521 SNe II, 298 SNe Ibc, 181 SNe IIn, and 58 SLSNe. These are among the largest uniformly observed samples of SNe available in the literature and will enable a wide range of statistical studies of each class.

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arXiv2020

The Red Supergiant Content of M31 and M33

We identify red supergiants (RSGs) in our spiral neighbors M31 and M33 using near-IR (NIR) photometry complete to a luminosity limit of log L/Lo=4.0. Our archival survey data cover 5 deg^2 of M31, and 3 deg^2 for M33, and are likely spatially complete for these massive stars. Gaia is used to remove foreground stars, after which the RSGs can be separated from asymptotic giant branch (AGB) stars in the color-magnitude diagram. The photometry is used to derive effective temperatures and bolometric luminosities via MARCS stellar atmosphere models. The resulting H-R diagrams show superb agreement with the evolutionary tracks of the Geneva evolutionary group. Our census includes 6400 RSGs in M31 and 2850 RSGs in M33 within their Holmberg radii; by contrast, only a few hundred RSGs are known so far in the Milky Way. Our catalog serves as the basis for a study of the RSG binary frequency being published separately, as well as future studies relating to the evolution of massive stars. Here we use the matches between the NIR-selected RSGs and their optical counterparts to show that the apparent similarity in the reddening of OB stars in M31 and M33 is the result of Malmquist bias; the average extinction in M31 is likely higher than that of M33. As expected, the distribution of RSGs follows that of the spiral arms, while the much older AGB population is more uniformly spread across each galaxy's disk.

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arXiv2020

Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds from ASAS-SN: Implications for Thorne-Żytkow Objects and Super-Asymptotic Giant Branch Stars

Stars with unusual properties can provide a wealth of information about rare stages of stellar evolution and exotic physics. However, determining the true nature of peculiar stars is often difficult. In this work, we conduct a systematic search for cool and luminous stars in the Magellanic Clouds with extreme variability, motivated by the properties of the unusual SMC star and Thorne-Żytkow Object (TŻO) candidate HV2112. Using light curves from ASAS-SN we identify 38 stars with surface temperatures T $<$ 4800K, luminosities $\log$(L/L$_\odot$) $>$ 4.3, variability periods $>$ 400 days, and variability amplitudes $Δ$V $>$ 2.5 mag. Eleven of these stars possess the distinctive double-peaked light curve morphology of HV2112. We use the pulsation properties and derived occurrence rates for these 12 objects to constrain their nature. From comparisons to stellar populations and models, we find that one star may be a red supergiant with large amplitude pulsations. For the other 11 stars we derive current masses of $\sim$5-10 M$_{\odot}$, below the theoretical minimum mass of $\sim$15 M$_{\odot}$ for TŻOs to be stable, casting doubt on this interpretation. Instead, we find that the temperatures, luminosities, mass-loss rates, and periods of these stars are consistent with predictions for super-Asymptotic Giant Branch (s-AGB) stars that have begun carbon burning but have not reached the superwind phase. We infer lifetimes in this phase of $\sim($1$-$7) $\times$ 10$^{4}$ years, also consistent with an s-AGB interpretation. If confirmed, these objects would represent the first identified population of s-AGB stars, illuminating the transition between low- and high-mass stellar evolution.

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arXiv2020

SuperRAENN: A Semi-supervised Supernova Photometric Classification Pipeline Trained on Pan-STARRS1 Medium Deep Survey Supernovae

Automated classification of supernovae (SNe) based on optical photometric light curve information is essential in the upcoming era of wide-field time domain surveys, such as the Legacy Survey of Space and Time (LSST) conducted by the Rubin Observatory. Photometric classification can enable real-time identification of interesting events for extended multi-wavelength follow-up, as well as archival population studies. Here we present the complete sample of 5,243 "SN-like" light curves (in griz) from the Pan-STARRS1 Medium-Deep Survey (PS1-MDS). The PS1-MDS is similar to the planned LSST Wide-Fast-Deep survey in terms of cadence, filters and depth, making this a useful training set for the community. Using this dataset, we train a novel semi-supervised machine learning algorithm to photometrically classify 2,315 new SN-like light curves with host galaxy spectroscopic redshifts. Our algorithm consists of a random forest supervised classification step and a novel unsupervised step in which we introduce a recurrent autoencoder neural network (RAENN). Our final pipeline, dubbed SuperRAENN, has an accuracy of 87% across five SN classes (Type Ia, Ibc, II, IIn, SLSN-I). We find the highest accuracy rates for Type Ia SNe and SLSNe and the lowest for Type Ibc SNe. Our complete spectroscopically- and photometrically-classified samples break down into: 62.0% Type Ia (1839 objects), 19.8% Type II (553 objects), 4.8% Type IIn (136 objects), 11.7% Type Ibc (291 objects), and 1.6% Type I SLSNe (54 objects). Finally, we discuss how this algorithm can be modified for online LSST data streams.

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arXiv2020

The Red Supergiant Binary Fraction of the Large Magellanic Cloud

The binary fraction of unevolved massive stars is thought to be 70-100% but there are few observational constraints on the binary fraction of the evolved version of a subset of these stars, the red supergiants (RSGs). Here we identify a complete sample of RSGs in the Large Magellanic Cloud (LMC) using new spectroscopic observations and archival UV, IR and broadband optical photometry. We find 4090 RSGs with log L/Lo > 3.5 with 1820 of them having log L/Lo > 4, which we believe is our completeness limit. We additionally spectroscopically confirmed 38 new RSG+B star binaries in the LMC, bringing the total known up to 55. We then estimated the binary fraction using a k-nearest neighbors algorithm that classifies stars as single or binary based on photometry with a spectroscopic sample as a training set. We take into account observational biases such as line-of-sight stars and binaries in eclipse while also calculating model-dependent corrections for RSGs with companions that our observations were not designed to detect. Based on our data, we find an initial result of 13.5 +7.56/-6.67% for RSGs with O or B-type companions. Using the Binary Population and Spectral Synthesis (BPASS) models to correct for unobserved systems, this corresponds to a total RSG binary fraction of 19.5 +7.6/-6.7%. This number is in broad agreement with what we would expect given an initial OB binary distribution of 70%, a predicted merger fraction of 20-30% and a binary interaction fraction of 40-50%.

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arXiv2020

The $R$-Process Alliance: Fourth Data Release from the Search for $r$-Process-Enhanced Stars in the Galactic Halo

This compilation is the fourth data release from the $R$-Process Alliance (RPA) search for $r$-process-enhanced stars, and the second release based on "snapshot" high-resolution ($R \sim 30,000$) spectra collected with the du Pont 2.5m Telescope. In this data release, we propose a new delineation between the $r$-I and $r$-II stellar classes at $\mathrm{[Eu/Fe]} = +0.7$, instead of the empirically chosen $\mathrm{[Eu/Fe]} = +1.0$ level previously in use, based on statistical tests of the complete set of RPA data released to date. We also statistically justify the minimum level of [Eu/Fe] for definition of the $r$-I stars, [Eu/Fe] $> +0.3$. Redefining the separation between $r$-I and $r$-II stars will aid in analysis of the possible progenitors of these two classes of stars and whether these signatures arise from separate astrophysical sources at all. Applying this redefinition to previous RPA data, the number of identified $r$-II and $r$-I stars changes to 51 and 121, respectively, from the initial set of data releases published thus far. In this data release, we identify 21 new $r$-II, 111 new $r$-I (plus three re-identified), and 7 new (plus one re-identified) limited-$r$ stars out of a total of 232 target stars, resulting in a total sample of 72 new $r$-II stars, 232 new $r$-I stars, and 42 new limited-$r$ stars identified by the RPA to date.

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arXiv2020

A Demonstration of Extremely Low Latency $γ$-ray, X-Ray & UV Follow-Up of a Millisecond Radio Transient

We report results of a novel high-energy follow-up observation of a potential Fast Radio Burst. The radio burst was detected by VLA/realfast and followed-up by the Neil Gehrels Swift Observatory in very low latency utilizing new operational capabilities of Swift (arXiv:2005.01751), with pointed soft X-ray and UV observations beginning at T0+32 minutes, and hard X-ray/gamma-ray event data saved around T0. These observations are $>10$x faster than previous X-ray/UV follow-up of any radio transient to date. No emission is seen coincident with the FRB candidate at T0, with a 0.2s fluence $5σ$ upper limit of $1.35\times10^{-8}$ erg cm$^{-2}$ (14-195 keV) for a SGR 1935+2154-like flare, nor at T0+32 minutes down to $3σ$ upper limits of 22.18 AB mag in UVOT u band, and $3.33\times10^{-13}$ erg cm$^{-2}$ s$^{-1}$ from 0.3-10 keV for the 2 ks observation. The candidate FRB alone is not significant enough to be considered astrophysical, so this note serves as a technical demonstration. These new Swift operational capabilities will allow future FRB detections to be followed up with Swift at even lower latencies than demonstrated here: 15-20 minutes should be regularly achievable, and 5-10 minutes occasionally achievable. We encourage FRB detecting facilities to release alerts in low latency to enable this science.

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arXiv2020

A Deep CFHT Optical Search for a Counterpart to the Possible Neutron Star -- Black Hole Merger GW190814

We present a wide-field optical imaging search for electromagnetic counterparts to the likely neutron star - black hole (NS-BH) merger GW190814/S190814bv. This compact binary merger was detected through gravitational waves by the LIGO/Virgo interferometers, with masses suggestive of a NS-BH merger. We imaged the LIGO/Virgo localization region using the MegaCam instrument on the Canada-France-Hawaii Telescope. We describe our hybrid observing strategy of both tiling and galaxy-targeted observations, as well as our image differencing and transient detection pipeline. Our observing campaign produced some of the deepest multi-band images of the region between 1.7 and 8.7 days post-merger, reaching a 5sigma depth of g > 22.8 (AB mag) at 1.7 days and i > 23.1 and i > 23.9 at 3.7 and 8.7 days, respectively. These observations cover a mean total integrated probability of 67.0% of the localization region. We find no compelling candidate transient counterparts to this merger in our images, which suggests that either the lighter object was tidally disrupted inside of the BH's innermost stable circular orbit, the transient lies outside of the observed sky footprint, or the lighter object is a low-mass BH. We use 5sigma source detection upper limits from our images in the NS-BH interpretation of this merger to constrain the mass of the kilonova ejecta to be Mej < 0.015Msun for a 'blue' (kappa = 0.5 cm^2 g^-1) kilonova, and Mej < 0.04Msun for a 'red' (kappa = 5-10 cm^2 g^-1) kilonova. Our observations emphasize the key role of large-aperture telescopes and wide-field imagers such as CFHT MegaCam in enabling deep searches for electromagnetic counterparts to gravitational wave events.

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arXiv2020

Inferring the parallax of Westerlund 1 from Gaia DR2

Westerlund 1 (Wd1) is potentially the largest star cluster in the Galaxy. That designation critically depends upon the distance to the cluster, yet the cluster is highly obscured, making luminosity-based distance estimates difficult. Using {\it Gaia} Data Release 2 (DR2) parallaxes and Bayesian inference, we infer a parallax of $0.35^{+0.07}_{-0.06}$ mas corresponding to a distance of $2.6^{+0.6}_{-0.4}$ kpc. To leverage the combined statistics of all stars in the direction of Wd1, we derive the Bayesian model for a cluster of stars hidden among Galactic field stars; this model includes the parallax zero-point. Previous estimates for the distance to Wd1 ranged from 1.0 to 5.5 kpc, although values around 5 kpc have usually been adopted. The {\it Gaia} DR2 parallaxes reduce the uncertainty from a factor of 3 to 18\% and rules out the most often quoted value of 5 kpc with 99\% confidence. This new distance allows for more accurate mass and age determinations for the stars in Wd1. For example, the previously inferred initial mass at the main-sequence turn-off was around 40 M$_{\odot}$; the new {\it Gaia} DR2 distance shifts this down to about 22 M$_{\odot}$. This has important implications for our understanding of the late stages of stellar evolution, including the initial mass of the magnetar and the LBV in Wd1. Similarly, the new distance suggests that the total cluster mass is about four times lower than previously calculated.

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