arXiv subjects

A. A. Miller

Publications and source records attributed to A. A. Miller.

At least 37 records · Page 2Linked to original sources

arXiv2020

A large fraction of hydrogen-rich supernova progenitors experience elevated mass loss shortly prior to explosion

Spectroscopic detection of narrow emission lines traces the presence of circumstellar mass distributions around massive stars exploding as core-collapse supernovae. Transient emission lines disappearing shortly after the supernova explosion suggest that the spatial extent of such material is compact, and hence imply an increased mass loss shortly prior to explosion. Here, we present a systematic survey for such transient emission lines (Flash Spectroscopy) among Type II supernovae detected in the first year of the Zwicky Transient Facility (ZTF) survey. We find that at least six out of ten events for which a spectrum was obtained within two days of estimated explosion time show evidence for such transient flash lines. Our measured flash event fraction ($>30\%$ at $95\%$ confidence level) indicates that elevated mass loss is a common process occurring in massive stars that are about to explode as supernovae.

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arXiv2020

The Spectacular Ultraviolet Flash From the Peculiar Type Ia Supernova 2019yvq

Early observations of Type Ia supernovae (SNe$\,$Ia) provide essential clues for understanding the progenitor system that gave rise to the terminal thermonuclear explosion. We present exquisite observations of SN$\,$2019yvq, the second observed SN$\,$Ia, after iPTF$\,$14atg, to display an early flash of emission in the ultraviolet (UV) and optical. Our analysis finds that SN$\,$2019yvq was unusual, even when ignoring the initial flash, in that it was moderately underluminous for an SN$\,$Ia ($M_g \approx -18.5\,$mag at peak) yet featured very high absorption velocities ($v \approx 15,000\,\mathrm{km\,s}^{-1}$ for Si II $λ$6355 at peak). We find that many of the observational features of SN$\,$2019yvq, aside from the flash, can be explained if the explosive yield of radioactive $^{56}\mathrm{Ni}$ is relatively low (we measure $M_{^{56}\mathrm{Ni}} = 0.31 \pm 0.05\,M_\odot$) and it and other iron-group elements are concentrated in the innermost layers of the ejecta. To explain both the UV/optical flash and peak properties of SN$\,$2019yvq we consider four different models: interaction between the SN ejecta and a nondegenerate companion, extended clumps of $^{56}\mathrm{Ni}$ in the outer ejecta, a double-detonation explosion, and the violent merger of two white dwarfs. Each of these models has shortcomings when compared to the observations; it is clear additional tuning is required to better match SN$\,$2019yvq. In closing, we predict that the nebular spectra of SN$\,$2019yvq will feature either H or He emission, if the ejecta collided with a companion, strong [Ca II] emission, if it was a double detonation, or narrow [O I] emission, if it was due to a violent merger.

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arXiv2020

Discovery of the optical afterglow and host galaxy of short GRB181123B at $z =1.754$: Implications for Delay Time Distributions

We present the discovery of the optical afterglow and host galaxy of the {\it Swift} short-duration gamma-ray burst, GRB\,181123B. Observations with Gemini-North starting at $\approx 9.1$~hr after the burst reveal a faint optical afterglow with $i\approx25.1$~mag, at an angular offset of 0.59 $\pm$ 0.16$''$ from its host galaxy. Using $grizYJHK$ observations, we measure a photometric redshift of the host galaxy of $z = 1.77^{+0.30}_{-0.17}$. From a combination of Gemini and Keck spectroscopy of the host galaxy spanning 4500-18000~Å, we detect a single emission line at 13390~Å, inferred as H$β$ at $z = 1.754 \pm 0.001$ and corroborating the photometric redshift. The host galaxy properties of GRB\,181123B are typical to those of other SGRB hosts, with an inferred stellar mass of $\approx 1.7 \times 10^{10}\,M_{\odot}$, mass-weighted age of $\approx 0.9$~Gyr and optical luminosity of $\approx 0.9L^{*}$. At $z=1.754$, GRB\,181123B is the most distant secure SGRB with an optical afterglow detection, and one of only three at $z>1.5$. Motivated by a growing number of high-$z$ SGRBs, we explore the effects of a missing $z>1.5$ SGRB population among the current {\it Swift} sample on delay time distribution models. We find that log-normal models with mean delay times of $\approx 4-6$~Gyr are consistent with the observed distribution, but can be ruled out to $95\%$ confidence with an additional $\approx1-5$~{\it Swift} SGRBs recovered at $z>1.5$. In contrast, power-law models with $\propto$ $t^{-1}$ are consistent with the redshift distribution and can accommodate up to $\approx30$ SGRBs at these redshifts. Under this model, we predict that $\approx 1/3$ of the current {\it Swift} population of SGRBs is at $z>1$. The future discovery or recovery of existing high-$z$ SGRBs will provide significant discriminating power on their delay time distributions, and thus their formation channels.

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arXiv2020

The Koala: A Fast Blue Optical Transient with Luminous Radio Emission from a Starburst Dwarf Galaxy at $z=0.27$

We present ZTF18abvkwla (the "Koala"), a fast blue optical transient discovered in the Zwicky Transient Facility (ZTF) One-Day Cadence (1DC) Survey. ZTF18abvkwla has a number of features in common with the groundbreaking transient AT2018cow: blue colors at peak ($g-r\approx-0.5$ mag), a short rise time from half-max of under two days, a decay time to half-max of only three days, a high optical luminosity ($M_{g,\mathrm{peak}}\approx-20.6$mag), a hot ($\gtrsim 40,000$K) featureless spectrum at peak light, and a luminous radio counterpart. At late times ($Δt>80$d) the radio luminosity of ZTF18abvkwla ($νL_ν\gtrsim 10^{40}$erg/s at 10 GHz, observer-frame) is most similar to that of long-duration gamma-ray bursts (GRBs). The host galaxy is a dwarf starburst galaxy ($M\approx5\times10^{8}M_\odot$, $\mathrm{SFR}\approx7 M_\odot$/yr) that is moderately metal-enriched ($\log\mathrm{[O/H]} \approx 8.5$), similar to the hosts of GRBs and superluminous supernovae. As in AT2018cow, the radio and optical emission in ZTF18abvkwla likely arise from two separate components: the radio from fast-moving ejecta ($Γβc >0.38c$) and the optical from shock-interaction with confined dense material ($<0.07M_\odot$ in $\sim 10^{15}$cm). Compiling transients in the literature with $t_\mathrm{rise} <5$d and $M_\mathrm{peak}<-20$mag, we find that a significant number are engine-powered, and suggest that the high peak optical luminosity is directly related to the presence of this engine. From 18 months of the 1DC survey, we find that transients in this rise-luminosity phase space are at least two to three orders of magnitude less common than CC SNe. Finally, we discuss strategies for identifying such events with future facilities like the Large Synoptic Survey Telescope, and prospects for detecting accompanying X-ray and radio emission.

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arXiv2019

The Zwicky Transient Facility Bright Transient Survey I: Spectroscopic Classification and the Redshift Completeness of Local Galaxy Catalogs

The Zwicky Transient Facility (ZTF) is performing a three-day cadence survey of the visible Northern sky (~3$π$). The transient candidates found in this survey are announced via public alerts. As a supplementary product ZTF is also conducting a large spectroscopic campaign: the ZTF Bright Transient Survey (BTS). The goal of the BTS is to spectroscopically classify all extragalactic transients brighter than 18.5 mag at peak brightness and immediately announce those classifications to the public. Extragalactic discoveries from ZTF are predominantly Supernovae (SNe). The BTS is the largest flux-limited SN survey to date. Here we present a catalog of the761 SNe that were classified during the first nine months of the survey (2018 Apr. 1 to 2018 Dec. 31). The BTS SN catalog contains redshifts based on SN template matching and spectroscopic host galaxy redshifts when available. Based on this data we perform an analysis of the redshift completeness of local galaxy catalogs, dubbed as the Redshift Completeness Fraction (RCF; the number of SN host galaxies with known spectroscopic redshift prior to SN discovery divided by the total number of SN hosts). In total, we identify the host galaxies of 512 Type Ia supernovae, 227 of which have known spectroscopic redshifts, yielding an RCF estimate of $44\% \pm1\%$. We find a steady decrease in the RCF with increasing distance in the local universe. For z<0.05, or ~200 Mpc, we find RCF=0.6, which has important ramifications when searching for multimessenger astronomical events. Prospects for dramatically increasing the RCF are limited to new multi-fiber spectroscopic instruments, or wide-field narrowband surveys. We find that existing galaxy redshift catalogs are only $50\%$ complete at $r\approx16.9$ mag. Pushing this limit several magnitudes deeper will pay huge dividends when searching for electromagnetic counterparts to gravitational wave events.

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arXiv2019

ZTF18aalrxas: A Type IIb Supernova from a very extended low-mass progenitor

We investigate ZTF18aalrxas, a double-peaked Type IIb core-collapse supernova (SN) discovered during science validation of the Zwicky Transient Facility (ZTF). ZTF18aalrxas was discovered while the optical emission was still rising towards the initial cooling peak (0.7 mag over 2 days). Our observations consist of multi-band (UV, optical) light-curves, and optical spectra spanning from $\approx0.7$ d to $\approx180$ d past the explosion. We use a Monte-Carlo based non-local thermodynamic equilibrium (NLTE) model, that simultanously reproduces both the $\rm ^{56}Ni$ powered bolometric light curve and our nebular spectrum. This model is used to constrain the synthesized radioactive nickel mass (0.17 $\mathrm{M}_{\odot}$) and the total ejecta mass (1.7 $\mathrm{M}_{\odot}$) of the SN. The cooling emission is modeled using semi-analytical extended envelope models to constrain the progenitor radius ($790-1050$ $\mathrm{R}_{\odot}$) at the time of explosion. Our nebular spectrum shows signs of interaction with a dense circumstellar medium (CSM), and this spetrum is modeled and analysed to constrain the amount of ejected oxygen ($0.3-0.5$ $\mathrm{M}_{\odot}$) and the total hydrogen mass ($\approx0.15$ $\mathrm{M}_{\odot}$) in the envelope of the progenitor. The oxygen mass of ZTF18aalrxas is consistent with a low ($12-13$ $\mathrm{M}_{\odot}$) Zero Age Main Sequence mass progenitor. The light curves and spectra of ZTF18aalrxas are not consistent with massive single star SN Type IIb progenitor models. The presence of an extended hydrogen envelope of low mass, the presence of a dense CSM, the derived ejecta mass, and the late-time oxygen emission can all be explained in a binary model scenario.

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arXiv2019

A Morphological Classification Model to Identify Unresolved PanSTARRS1 Sources: Application in the ZTF Real-Time Pipeline

In the era of large photometric surveys, the importance of automated and accurate classification is rapidly increasing. Specifically, the separation of resolved and unresolved sources in astronomical imaging is a critical initial step for a wide array of studies, ranging from Galactic science to large scale structure and cosmology. Here, we present our method to construct a large, deep catalog of point sources utilizing Pan-STARRS1 (PS1) 3$π$ survey data, which consists of $\sim$3$\times10^9$ sources with $m\lesssim23.5\,$mag. We develop a supervised machine-learning methodology, using the random forest (RF) algorithm, to construct the PS1 morphology model. We train the model using $\sim$5$\times10^4$ PS1 sources with HST COSMOS morphological classifications and assess its performance using $\sim$4$\times10^6$ sources with Sloan Digital Sky Survey (SDSS) spectra and $\sim$2$\times10^8$ \textit{Gaia} sources. We construct 11 "white flux" features, which combine PS1 flux and shape measurements across 5 filters, to increase the signal-to-noise ratio relative to any individual filter. The RF model is compared to 3 alternative models, including the SDSS and PS1 photometric classification models, and we find that the RF model performs best. By number the PS1 catalog is dominated by faint sources ($m\gtrsim21\,$mag), and in this regime the RF model significantly outperforms the SDSS and PS1 models. For time-domain surveys, identifying unresolved sources is crucial for inferring the Galactic or extragalactic origin of new transients. We have classified $\sim$1.5$\times10^9$ sources using the RF model, and these results are used within the Zwicky Transient Facility real-time pipeline to automatically reject stellar sources from the extragalactic alert stream.

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arXiv2018

iPTF16abc and the population of Type Ia supernovae: Comparing the photospheric, transitional and nebular phases

Key information about the progenitor system and the explosion mechanism of Type Ia supernovae (SNe~Ia) can be obtained from early observations, within a few days from explosion. iPTF16abc was discovered as a young SN~Ia with excellent early time data. Here, we present photometry and spectroscopy of the SN in the nebular phase. A comparison of the early time data with a sample of SNe~Ia shows distinct features, differing from normal SNe~Ia at early phases but similar to normal SNe~Ia at a few weeks after maximum light (i.e. the transitional phase) and well into the nebular phase. The transparency timescales ($t_0$) for this sample of SNe~Ia range between $\sim$ 25 and 41 days indicating a diversity in the ejecta masses. $t_0$ also weakly correlates with the peak bolometric luminosity, consistent with the interpretation that SNe with higher ejecta masses would produce more $^{56}$Ni. Comparing the $t_0$ and the maximum luminosity, L$_{max}$\, distribution of a sample of SNe~Ia to predictions from a wide range of explosion models we find an indication that the sub-Chandrasekhar mass models span the range of observed values. However, the bright end of the distribution can be better explained by Chandrasekhar mass delayed detonation models, hinting at multiple progenitor channels to explain the observed bolometric properties of SNe~Ia. iPTF16abc appears to be consistent with the predictions from the M$_{ch}$ models.

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arXiv2018

Early Observations of the Type Ia Supernova iPTF 16abc: A Case of Interaction with Nearby, Unbound Material and/or Strong Ejecta Mixing

Early observations of Type Ia supernovae (SNe Ia) provide a unique probe of their progenitor systems and explosion physics. Here we report the intermediate Palomar Transient Factory (iPTF) discovery of an extraordinarily young SN Ia, iPTF 16abc. By fitting a power law to our early light curve, we infer that first light for the SN, that is when the SN could have first been detected by our survey, occurred only $0.15\pm_{0.07}^{0.15}$ days before our first detection. In the $\sim$24 hr after discovery, iPTF 16abc rose by $\sim$2 mag, featuring a near-linear rise in flux for $\gtrsim$3 days. Early spectra show strong C II absorption, which disappears after $\sim$7 days. Unlike the extensivelyobserved SN Ia SN 2011fe, the $(B-V)_0$ colors of iPTF 16abc are blue and nearly constant in the days after explosion. We show that our early observations of iPTF 16abc cannot be explained by either SN shock breakout and the associated, subsequent cooling or the SN ejecta colliding with a stellar companion. Instead, we argue that the early characteristics of iPTF 16abc, including (i) the rapid, near-linear rise, (ii) the nonevolving blue colors, and (iii) the strong C II absorption, are the result of either ejecta interaction with nearby, unbound material or vigorous mixing of radioactive $^{56}$Ni in the SN ejecta, or a combination of the two. In the next few years, dozens of very young \textit{normal} SNe Ia will be discovered, and observations similar to those presented here will constrain the white dwarf explosion mechanism.

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arXiv2018

The Redshift Completeness of Local Galaxy Catalogs

There is considerable interest in understanding the demographics of galaxies within the local universe (defined, for our purposes, as the volume within a radius of 200 Mpc or z<0.05). In this pilot paper, using supernovae (SNe) as signposts to galaxies, we investigate the redshift completeness of catalogs of nearby galaxies. In particular, type Ia SNe are bright and are good tracers of the bulk of the galaxy population, since they arise in both old and young stellar populations. Our input sample consists of SNe with redshift < 0.05, discovered by the flux-limited ASAS-SN survey. We define the redshift completeness fraction (RCF) as the number of SN host galaxies with known redshift prior to SN discovery, determined, in this case, via the NASA Extragalactic Database (NED), divided by the total number of newly discovered SNe. Using SNe Ia, we find RCF = 78 (+6, -7, 90% confidence interval) for z < 0.03. We examine the distribution of host galaxies with and without cataloged redshifts as a function of absolute magnitude and redshift, and, unsurprisingly, find that higher-z and fainter hosts are less likely to have a known redshift prior to the detection of the SN. However, surprisingly, some L* galaxies are also missing. We conclude with thoughts on the future improvement of RCF measurements that will be made possible from large SN samples resulting from ongoing and especially upcoming time-domain surveys.

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arXiv2018

iPTF Survey for Cool Transients

We performed a wide-area (2000 deg$^{2}$) g and I band experiment as part of a two month extension to the Intermediate Palomar Transient Factory. We discovered 36 extragalactic transients including iPTF17lf, a highly reddened local SN Ia, iPTF17bkj, a new member of the rare class of transitional Ibn/IIn supernovae, and iPTF17be, a candidate luminous blue variable outburst. We do not detect any luminous red novae and place an upper limit on their rate. We show that adding a slow-cadence I band component to upcoming surveys such as the Zwicky Transient Facility will improve the photometric selection of cool and dusty transients.

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arXiv2017

Illuminating Gravitational Waves: A Concordant Picture of Photons from a Neutron Star Merger

Merging neutron stars offer an exquisite laboratory for simultaneously studying strong-field gravity and matter in extreme environments. We establish the physical association of an electromagnetic counterpart EM170817 to gravitational waves (GW170817) detected from merging neutron stars. By synthesizing a panchromatic dataset, we demonstrate that merging neutron stars are a long-sought production site forging heavy elements by r-process nucleosynthesis. The weak gamma-rays seen in EM170817 are dissimilar to classical short gamma-ray bursts with ultra-relativistic jets. Instead, we suggest that breakout of a wide-angle, mildly-relativistic cocoon engulfing the jet elegantly explains the low-luminosity gamma-rays, the high-luminosity ultraviolet-optical-infrared and the delayed radio/X-ray emission. We posit that all merging neutron stars may lead to a wide-angle cocoon breakout; sometimes accompanied by a successful jet and sometimes a choked jet.

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arXiv2017

iPTF16geu: A multiply imaged, gravitationally lensed type Ia supernova

We report the discovery of a multiply-imaged gravitationally lensed Type Ia supernova, iPTF16geu (SN 2016geu), at redshift $z=0.409$. This phenomenon could be identified because the light from the stellar explosion was magnified more than fifty times by the curvature of space around matter in an intervening galaxy. We used high spatial resolution observations to resolve four images of the lensed supernova, approximately 0.3" from the center of the foreground galaxy. The observations probe a physical scale of $\sim$1 kiloparsec, smaller than what is typical in other studies of extragalactic gravitational lensing. The large magnification and symmetric image configuration implies close alignment between the line-of-sight to the supernova and the lens. The relative magnifications of the four images provide evidence for sub-structures in the lensing galaxy.

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arXiv2017

Preparing for advanced LIGO: A Star-Galaxy Separation Catalog for the Palomar Transient Factory

The search for fast optical transients, such as the expected electromagnetic counterparts to binary neutron star mergers, is riddled with false positives ranging from asteroids to stellar flares. While moving objects are readily rejected via image pairs separated by $\sim$1 hr, stellar flares represent a challenging foreground that significantly outnumber rapidly-evolving explosions. Identifying stellar sources close to and fainter than the transient detection limit can eliminate these false positives. Here, we present a method to reliably identify stars in deep co-adds of Palomar Transient Factory (PTF) imaging. Our machine-learning methodology utilizes the random forest (RF) algorithm, which is trained using $> 3\times{10}^6$ sources with Sloan Digital Sky Survey (SDSS) spectra. When evaluated on an independent test set, the PTF RF model outperforms the SExtractor star classifier by $\sim$4%. For faint sources ($r'\ge{21}$ mag), which dominate the field population, the PTF RF model produces a $\sim$19% improvement over SExtractor. To avoid false negatives in the PTF transient-candidate stream, we adopt a conservative stellar classification threshold, corresponding to a galaxy misclassification rate = 0.005. Ultimately, $\sim$$1.70\times{10}^8$ objects are included in our PTF point-source catalog, of which only $\sim$$10^6$ are expected to be galaxies. We demonstrate that the PTF RF catalog reveals transients that otherwise would have been missed. To leverage its superior image quality, we additionally create an SDSS point-source catalog, which is also tuned to have a galaxy misclassification rate = 0.005. These catalogs have been incorporated into the PTF real-time pipelines to automatically reject stellar sources as non-extragalactic transients.

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arXiv2017

Color Me Intrigued: the Discovery of iPTF 16fnm, a Supernova 2002cx-like Object

Modern wide-field, optical time-domain surveys must solve a basic optimization problem: maximize the number of transient discoveries or minimize the follow-up needed for the new discoveries. Here, we describe the Color Me Intrigued experiment, the first from the intermediate Palomar Transient Factory (iPTF) to search for transients simultaneously in the $g_\mathrm{PTF}$- and $R_\mathrm{PTF}$-bands. During the course of this experiment we discovered iPTF$\,$16fnm, a new member of the 02cx-like subclass of type Ia supernovae (SNe). iPTF$\,$16fnm peaked at $M_{g_\mathrm{PTF}} = -15.09 \pm 0.17 \; \mathrm{mag}$, making it the second least-luminous known type Ia SN. iPTF 16fnm exhibits all the hallmarks of the 02cx-like class: (i) low luminosity at peak, (ii) low ejecta velocities, and (iii) a non-nebular spectra several months after peak. Spectroscopically, iPTF$\,$16fnm exhibits a striking resemblence to 2 other low-luminosity 02cx-like SNe: SNe 2007qd and 2010ae. iPTF$\,$16fnm and SN 2005hk decline at nearly the same rate, despite a 3 mag difference in brightness at peak. When considering the full subclass of 02cx-like SNe, we do not find evidence for a tight correlation between peak luminosity and decline rate in either the $g'$ or $r'$ band. We further examine the $g' - r'$ evolution of 02cx-like SNe and find that their unique color evolution can be used to separate them from 91bg-like and normal type Ia SNe. This selection function will be especially important in the spectroscopically incomplete Zwicky Transient Facility/Large Synoptic Survey Telescope era. We measure the relative rate of 02cx-like SNe to normal SNe Ia and find $r_{N_{02cx}/N_{Ia}} = 25^{+75}_{-18.5}\%$. Finally, we close by recommending that LSST periodically evaluate, and possibly update, its observing cadence to maximize transient science.

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arXiv2016

PSR J1024-0719: A Millisecond Pulsar in an Unusual Long-Period Orbit

PSR J1024$-$0719 is a millisecond pulsar that was long thought to be isolated. However, puzzling results concerning its velocity, distance, and low rotational period derivative have led to reexamination of its properties. We present updated radio timing observations along with new and archival optical data that show PSR J1024$-$0719 is most likely in a long period (2$-$20 kyr) binary system with a low-mass ($\approx 0.4\,M_\odot$) low-metallicity ($Z \approx -0.9\,$ dex) main sequence star. Such a system can explain most of the anomalous properties of this pulsar. We suggest that this system formed through a dynamical exchange in a globular cluster that ejected it into a halo orbit, consistent with the low observed metallicity for the stellar companion. Further astrometric and radio timing observations such as measurement of the third period derivative could strongly constrain the range of orbital parameters.

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arXiv2016

iPTF Search for an Optical Counterpart to Gravitational Wave Trigger GW150914

The intermediate Palomar Transient Factory (iPTF) autonomously responded to and promptly tiled the error region of the first gravitational wave event GW150914 to search for an optical counterpart. Only a small fraction of the total localized region was immediately visible in the Northern night sky, due both to sun-angle and elevation constraints. Here, we report on the transient candidates identified and rapid follow-up undertaken to determine the nature of each candidate. Even in the small area imaged of 126 sq deg, after extensive filtering, 8 candidates were deemed worthy of additional follow-up. Within two hours, all 8 were spectroscopically classified by the Keck II telescope. Curiously, even though such events are rare, one of our candidates was a superluminous supernova. We obtained radio data with the Jansky Very Large Array and X-ray follow-up with the Swift satellite for this transient. None of our candidates appear to be associated with the gravitational wave trigger, which is unsurprising given that GW150914 came from the merger of two stellar-mass black holes. This end-to-end discovery and follow-up campaign bodes well for future searches in this post-detection era of gravitational waves.

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arXiv2015

The Synthetic-Oversampling Method: Using Photometric Colors to Discover Extremely Metal-Poor Stars

Extremely metal-poor (EMP) stars ([Fe/H] < -3.0 dex) provide a unique window into understanding the first generation of stars and early chemical enrichment of the Universe. EMP stars are exceptionally rare, however, and the relatively small number of confirmed discoveries limits our ability to exploit these near-field probes of the first ~500 Myr after the Big Bang. Here, a new method to photometrically estimate [Fe/H] from only broadband photometric colors is presented. I show that the method, which utilizes machine-learning algorithms and a training set of ~170,000 stars with spectroscopically measured [Fe/H], produces a typical scatter of ~0.29 dex. This performance is similar to what is achievable via low-resolution spectroscopy, and outperforms other photometric techniques, while also being more general. I further show that a slight alteration to the model, wherein synthetic EMP stars are added to the training set, yields the robust identification of EMP candidates. In particular, this synthetic-oversampling method recovers ~20% of the EMP stars in the training set, at a precision of ~0.05. Furthermore, ~65% of the false positives from the model are very metal-poor stars ([Fe/H] < -2.0 dex). The synthetic-oversampling method is biased towards the discovery of warm (~F-type) stars, a consequence of the targeting bias from the SDSS/SEGUE survey. This EMP selection method represents a significant improvement over alternative broadband optical selection techniques. The models are applied to >12 million stars, with an expected yield of ~600 new EMP stars, which promises to open new avenues for exploring the early universe.

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