arXiv subjects

Jeff J. Andrews

Publications and source records attributed to Jeff J. Andrews.

71 records · Page 4Linked to original sources

arXiv2019

Double Neutron Star Populations and Formation Channels

In the past five years, the number of known double neutron stars (DNS) in the Milky Way has roughly doubled. We argue that the observed sample can be split into three distinct sub-populations based on their orbital characteristics: (i) short-period, low-eccentricity binaries; (ii) wide binaries; and (iii) short-period, high-eccentricity binaries. These sub-populations also exhibit distinct spin period and spindown rate properties. We focus on sub-population (iii), which contains the Hulse-Taylor binary. Contrary to previous analysis, we demonstrate that, if they are the product of primordial binary evolution, the $P_{\rm orb}$ and $e$ distribution of these systems requires that the second-born NSs must have been formed with small natal kicks ($\lesssim$25 km s$^{-1}$) and have pre-SN masses narrowly distributed around 3.2 M$_{\odot}$. These constraints challenge binary evolution theory and further predict closely aligned spin and orbital axes, inconsistent with the Hulse-Taylor binary's measured spin-orbit misalignment angle of $\approx$20$^{\circ}$. Motivated by the similarity of these DNSs to B2127+11C, a DNS residing in the globular cluster M15, we argue that this sub-population is consistent with being formed in, and then ejected from, globular clusters. This scenario provides a pathway for the formation and merger of DNSs in stellar environments without recent star formation, as observed in the host galaxy population of short gamma ray bursts and the recent detection by LIGO of a merging DNS in an old stellar population.

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arXiv2019

The Complete Evolution of a Neutron-Star Binary through a Common Envelope Phase Using 1D Hydrodynamic Simulations

Over forty years of research suggests that the common envelope phase, in which an evolved star engulfs its companion upon expansion, is the critical evolutionary stage forming short-period, compact-object binary systems, such as coalescing double compact objects, X-ray binaries, and cataclysmic variables. In this work, we adapt the one-dimensional hydrodynamic stellar evolution code, MESA, to model the inspiral of a 1.4M$_{\odot}$ neutron star (NS) inside the envelope of a 12$M_{\odot}$ red supergiant star. We self-consistently calculate the drag force experienced by the NS as well as the back-reaction onto the expanding envelope as the NS spirals in. Nearly all of the hydrogen envelope escapes, expanding to large radii ($\sim$10$^2$ AU) where it forms an optically thick envelope with temperatures low enough that dust formation occurs. We simulate the NS orbit until only 0.8M$_{\odot}$ of the hydrogen envelope remains around the giant star's core. Our results suggest that the inspiral will continue until another $\approx$0.3M$_{\odot}$ are removed, at which point the remaining envelope will retract. Upon separation, a phase of dynamically stable mass transfer onto the NS accretor is likely to ensue, which may be observable as an ultraluminous X-ray source. The resulting binary, comprised of a detached 2.6M$_{\odot}$ helium-star and a NS with a separation of 3.3-5.7R$_{\odot}$, is expected to evolve into a merging double neutron-star, analogous to those recently detected by LIGO/Virgo. For our chosen combination of binary parameters, our estimated final separation (including the phase of stable mass transfer) suggests a very high $α_{\rm CE}$-equivalent efficiency of $\approx$5.

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arXiv2019

Voyage 2050 White Paper: All-Sky Visible and Near Infrared Space Astrometry

A new all-sky visible and Near-InfraRed (NIR) space astrometry mission with a wavelength cutoff in the K-band is not just focused on a single or small number of key science cases. Instead, it is extremely broad, answering key science questions in nearly every branch of astronomy while also providing a dense and accurate visible-NIR reference frame needed for future astronomy facilities. For almost 2 billion common stars the combination of Gaia and a new all-sky NIR astrometry mission would provide much improved proper motions, answering key science questions -- from the solar system and stellar systems, including exoplanet systems, to compact galaxies, quasars, neutron stars, binaries and dark matter substructures. The addition of NIR will result in up to 8 billion newly measured stars in some of the most obscured parts of our Galaxy, and crucially reveal the very heart of the Galactic bulge region. In this white paper we argue that rather than improving on the accuracy, a greater overall science return can be achieved by going deeper than Gaia and by expanding the wavelength range to the NIR.

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arXiv2019

Constraining Compact Object Formation with 2M0521

We show that the recently discovered binary 2M05215658+4359220 (2M0521), comprised of a giant star (GS) orbiting a suspected black hole (BH) in a ~80 day orbit, may be instrumental in shedding light on uncertain BH-formation physics and can be a test case for studying wind accretion models. Using binary population synthesis with a realistic prescription for the star formation history and metallicity evolution of the Milky Way, we analyze the evolution of binaries containing compact objects (COs) in orbit around GSs with properties similar to 2M0521. We find ~100-1000 CO-GS binaries in the Milky Way observable by Gaia, and 0-12 BH-GS and 0-1 neutron star-GS binaries in the Milky Way with properties similar to 2M0521. We find that all CO-GSs with Porb<5 yr, including 2M0521, go through a common envelope (CE) and hence form a class of higher mass analogs to white dwarf post-CE binaries. We further show how the component masses of 2M0521-like binaries depend strongly on the supernova-engine model we adopt. Thus, an improved measurement of the orbit of 2M0521, imminent with Gaia's third data release, will strongly constrain its component masses and as a result inform supernova-engine models widely used in binary population synthesis studies. These results have widespread implications for the origins and properties of CO binaries, especially those detectable by LIGO and LISA. Finally, we show that the reported X-ray non-detection of 2M0521 is a challenge for wind accretion theory, making 2M0521-like CO-GS binaries a prime target for further study with accretion models.

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arXiv2019

Double Neutron Star Formation: Merger Times, Systemic Velocities, and Travel Distances

The formation and evolution of double neutron stars (DNS) have traditionally been studied using binary population synthesis. In this work, we take an alternative approach by focusing only on the second supernova (SN) forming the DNS and the subsequent orbital decay and merger due to gravitational wave radiation. Using analytic and numerical methods, we explore how different NS natal kick velocity distributions, pre-SN orbital separations, and progenitor He-star masses affect the post-SN orbital periods, eccentricities, merger times, systemic velocities, and distances traveled by the system before merging. Comparison with the set of 17 known DNSs in the Milky Way shows that DNSs have pre-SN orbital separations ranging between 1 and 44 $R_{\odot}$. Those DNSs with pre-SN separations $\sim$1 $R_{\odot}$ have merger time distributions that peak $\sim$10-100 Myr after formation, regardless of the kick velocity received by the NS. These DNSs are typically formed with systemic velocities $\sim$10$^2$ km s$^{-1}$ and may travel $\sim$1-10 kpc before merging. Depending on progenitor mass of the second-born NS, the short merger time can account for the $r$-process enrichment observed in compact stellar systems such as ultra-faint dwarf galaxies. For Milky Way-mass galaxies only DNSs with the tightest pre-SN orbits and large kick velocities ($\gtrsim$10$^2$ km s$^{-1}$) can escape. However, those DNSs that do escape may travel as far as $\sim$Mpc before merging, which as previous studies have pointed out has implications for identifying the host galaxies to short gamma ray bursts and gravitational wave events.

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arXiv2018

Using APOGEE Wide Binaries to Test Chemical Tagging with Dwarf Stars

Stars of a common origin are thought to have similar, if not nearly identical, chemistry. Chemical tagging seeks to exploit this fact to identify Milky Way subpopulations through their unique chemical fingerprints. In this work, we compare the chemical abundances of dwarf stars in wide binaries to test the abundance consistency of stars of a common origin. Our sample of 31 wide binaries is identified from a catalog produced by cross-matching APOGEE stars with UCAC5 astrometry, and we confirm the fidelity of this sample with precision parallaxes from Gaia DR2. For as many as 14 separate elements, we compare the abundances between components of our wide binaries, finding they have very similar chemistry (typically within 0.1 dex). This level of consistency is more similar than can be expected from stars with different origins (which show typical abundance differences of 0.3-0.4 dex within our sample). For the best measured elements, Fe, Si, K, Ca, Mn, and Ni, these differences are reduced to 0.05-0.08 dex when selecting pairs of dwarf stars with similar temperatures. Our results suggest that APOGEE dwarf stars may currently be used for chemical tagging at the level of $\sim$0.1 dex or at the level of $\sim$0.05 dex when restricting for the best-measured elements in stars of similar temperatures. Larger wide binary catalogs may provide calibration sets, in complement to open cluster samples, for on-going spectroscopic surveys.

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arXiv2018

A Serendipitous Pulsar Discovery in a Search for a Companion to a Low-Mass White Dwarf

We report the discovery of a previously unidentified pulsar as part of a radio campaign to identify neutron star companions to low-mass white dwarfs (LMWDs) using the Robert C.\ Byrd Green Bank Telescope (GBT). PSR J0802-0955, which is coincident with the position of a WD with a mass of 0.2 solar masses, has a pulse period of 571 ms. Because of its relatively long pulse period, the lack of radial velocity (RV) variations in the radio data, and GBT's large beam size at the observing frequency of 340 MHz, we conclude that PSR J0802-0955 is unassociated with the LMWD at roughly the same position and distance.

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arXiv2018

Validating TGAS wide binaries with Gaia DR2 Radial Velocities and Parallaxes

Featuring greatly improved astrometry and precise radial velocities (RVs), the second data release (DR2) from Gaia affords us a unique opportunity to test the validity of samples of stellar wide binaries. In a previous work, we presented a set of wide binaries identified in the joint Tycho-Gaia Astrometric Solution catalog. Here, we use DR2 to confirm the low contamination rate of this sample, thereby verifying the effectiveness of our algorithm for application to DR2 data.

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arXiv2017

Wide Binaries in Tycho-Gaia II: Metallicities, Abundances, and Prospects for Chemical Tagging

From our recent catalog based on the first Gaia data release (TGAS), we select wide binaries in which both stars have been observed by the Radial Velocity Experiment (RAVE) or the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST). Using RAVE and LAMOST metallicities and RAVE Mg, Al, Si, Ti, and Fe abundances, we find that the differences in the metallicities and elemental abundances of components of wide binaries are consistent with being due to observational uncertainties, in agreement with previous results for smaller and more restricted samples. The metallicity and elemental abundance consistency between wide binary components presented in this work confirms their common origin and bolsters the status of wide binaries as "mini-open clusters." Furthermore, this is evidence that wide binaries are effectively co-eval and co-chemical, supporting their use for e.g., constraining age-activity-rotation relations, the initial-final mass relation for white dwarfs, and M-dwarf metallicity indicators. Additionally, we demonstrate that the common proper motion, common parallax pairs in TGAS with the most extreme separations (s $\gtrsim$ 0.1 pc) typically have inconsistent metallicities, radial velocities or both and are therefore likely to be predominantly comprised of random alignments of unassociated stars with similar astrometry, in agreement with our previous results. Finally, we propose that wide binaries form an ideal data set with which to test chemical tagging as a method to identify stars of common origin, particularly because the stars in wide binaries span a wide range of metallicities, much wider than that spanned by nearby open clusters.

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arXiv2017

Wide Binaries in Tycho-{\it Gaia}: Search Method and the Distribution of Orbital Separations

We mine the Tycho-{\it Gaia} astrometric solution (TGAS) catalog for wide stellar binaries by matching positions, proper motions, and astrometric parallaxes. We separate genuine binaries from unassociated stellar pairs through a Bayesian formulation that includes correlated uncertainties in the proper motions and parallaxes. Rather than relying on assumptions about the structure of the Galaxy, we calculate Bayesian priors and likelihoods based on the nature of Keplerian orbits and the TGAS catalog itself. We calibrate our method using radial velocity measurements and obtain 6196 high-confidence candidate wide binaries with projected separations $s\lesssim1$ pc. The normalization of this distribution suggests that at least 0.6\% of TGAS stars have an associated, distant TGAS companion in a wide binary. We demonstrate that {\it Gaia}'s astrometry is precise enough that it can detect projected orbital velocities in wide binaries with orbital periods as large as 10$^6$ yr. For pairs with $s\ \lesssim\ 4\times10^4$~AU, characterization of random alignments indicate our contamination to be $\approx$5\%. For $s \lesssim 5\times10^3$~AU, our distribution is consistent with Öpik's Law. At larger separations, the distribution is steeper and consistent with a power-law $P(s)\propto s^{-1.6}$; there is no evidence in our data of any bimodality in this distribution for $s \lesssim$ 1 pc. Using radial velocities, we demonstrate that at large separations, i.e., of order $s \sim$ 1 pc and beyond, any potential sample of genuine wide binaries in TGAS cannot be easily distinguished from ionized former wide binaries, moving groups, or contamination from randomly aligned stars.

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arXiv2016

Today a Duo, But Once a Trio? The Double White Dwarf HS 2220$+$2146 May Be A Post-Blue Straggler Binary

For sufficiently wide orbital separations {\it a}, the two members of a stellar binary evolve independently. This implies that in a wide double white dwarf (DWD), the more massive WD should always be produced first, when its more massive progenitor ends its main-sequence life, and should therefore be older and cooler than its companion. The bound, wide DWD HS 2220$+$2146 ($a\approx500$ AU) does not conform to this picture: the more massive WD is the younger, hotter of the pair. We show that this discrepancy is unlikely to be due to past mass-transfer phases or to the presence of an unresolved companion. Instead, we propose that HS 2220$+$2146 formed through a new wide DWD evolutionary channel involving the merger of the inner binary in a hierarchical triple system. The resulting blue straggler and its wide companion then evolved independently, forming the WD pair seen today. Although we cannot rule out other scenarios, the most likely formation channel has the inner binary merging while both stars are still on the main sequence. This provides us with the tantalizing possibility that Kozai-Lidov oscillations may have played a role in the inner binary's merger. {\it Gaia} may uncover hundreds more wide DWDs, leading to the identification of other systems like HS 2220$+$2146. There are already indications that other WD systems may have been formed through different, but related, hierarchical triple evolutionary scenarios. Characterizing these populations may allow for thorough testing of the efficiency with which KL oscillations induce stellar mergers.

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arXiv2015

Constraints on the Initial-Final Mass Relation from Wide Double White Dwarfs

We present observational constraints on the initial-final mass relation (IFMR) using wide double white dwarfs (DWDs). We identify 65 new candidate wide DWDs within the Sloan Digital Sky Survey, bringing the number of candidate wide DWDs to 142. We then engage in a spectroscopic follow-up campaign and collect existing spectra for these objects; using these spectra, we derive masses and cooling ages for 54 hydrogen (DA) WDs in DWDs. We also identify one new DA/DB pair, four candidate DA/DC pairs, four candidate DA/DAH pairs, and one new candidate triple degenerate system. Because wide DWDs are co-eval and evolve independently, the difference in the pre-WD lifetimes should equal the difference in the WD cooling ages. We use this to develop a Bayesian hierarchical framework and construct a likelihood function to determine the probability that any particular IFMR fits a sample of wide DWDs. We then define a parametric model for the IFMR and find the best parameters indicated by our sample of DWDs. We place robust constraints on the IFMR for initial masses of 2--4 \Msun. The WD masses produced by our model for stars within this mass range differ from those predicted by semi-empirical fits to open cluster WDs. Within this mass range, where there are few constraining open cluster WDs and disagreements in the cluster ages, wide DWDs may provide more reliable constraints on the IFMR. Expanding this method to the many wide DWDs expected to be discovered by Gaia may transform our understanding of the IFMR.

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arXiv2015

Ultracool white dwarfs and the age of the Galactic disc

We present parallax observations and a detailed model atmosphere analysis of 54 cool and ultracool ($T_{\rm eff}$ < 4000 K) white dwarfs (WDs) in the solar neighbourhood. For the first time, a large number of cool and ultracool WDs have distance and tangential velocities measurements available. Our targets have distances ranging from 21 pc to >100 pc, and include five stars within 30 pc. Contrary to expectations, all but two of them have tangential velocities smaller than 150 km s$^{-1}$ thus suggesting Galactic disc membership. The oldest WDs in this sample have WD cooling ages of 10 Gyr, providing a firm lower limit to the age of the thick disc population. Many of our targets have uncharacteristically large radii, indicating that they are low mass WDs. It appears that we have detected the brighter population of cool and ultracool WDs near the Sun. The fainter population of ultracool CO-core WDs remain to be discovered in large numbers. The Large Synoptic Survey Telescope should find these elusive, more massive ultracool WDs in the solar neighbourhood.

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arXiv2015

Evolutionary Channels for the Formation of Double Neutron Stars

We analyze binary population models of double-neutron stars and compare results to the accurately measured orbital periods and eccentricities of the eight known such systems in our Galaxy. In contrast to past similar studies, we especially focus on the dominant evolutionary channels (we identify three); for the first time, we use a detailed understanding of the evolutionary history of three double neutron stars as actual constraints on the population models. We find that the evolutionary constraints derived from the double pulsar are particularly tight, and less than half of the examined models survive the full set of constraints. The top-likelihood surviving models yield constraints on the key binary evolution parameters, but most interestingly reveal (i) the need for electron-capture supernovae from relatively low-mass degenerate, progenitor cores, and (ii) the most likely evolutionary paths for the rest of the known double neutron stars. In particular, we find that J1913+16 likely went through a phase of Case BB mass transfer, and J1906+0746 and J1756-2251 are consistent with having been formed in electron-capture supernovae.

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arXiv2014

The Mass Distribution of Companions to Low-Mass White Dwarfs

Measuring the masses of companions to single-line spectroscopic binary stars is (in general) not possible because of the unknown orbital plane inclination. Even when the mass of the visible star can be measured, only a lower limit can be placed on the mass of the unseen companion. However, since these inclination angles should be isotropically distributed, for a large enough, unbiased sample, the companion mass distribution can be deconvolved from the distribution of observables. In this work, we construct a hierarchical probabilistic model to infer properties of unseen companion stars given observations of the orbital period and projected radial velocity of the primary star. We apply this model to three mock samples of low-mass white dwarfs (LMWDs, $M\lesssim0.45~\Msun$) and a sample of post-common-envelope binaries. We use a mixture of two Gaussians to model the WD and neutron star (NS) companion mass distributions. Our model successfully recovers the initial parameters of these test data sets. We then apply our model to 55 WDs in the extremely low-mass (ELM) WD Survey. Our maximum a posteriori model for the WD companion population has a mean mass $μ_{\rm WD} = 0.74~\Msun$, with a standard deviation $σ_{\rm WD} = 0.24~\Msun$. Our model constrains the NS companion fraction $f_{\rm NS}$ to be $<$16\% at 68\% confidence. We make samples from the posterior distribution publicly available so that future observational efforts may compute the NS probability for newly discovered LMWDs.

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arXiv2014

Constraining the Initial-Final Mass Relation with Wide Double White Dwarfs

In wide double white dwarf (DWD) binaries, in which the co-eval WDs evolve independently, the more massive, faster-evolving WD can be used to obtain a main-sequence lifetime for the less-massive WD. By converting this lifetime into an initial mass for the less-massive WD, and combining it with the spectroscopically derived mass for this WD, one can constrain the initial-final mass relation (IFMR). However, the number of known and well-characterized DWDs is small, severely limiting their usefulness for this analysis. To obtain new constraints on the IFMR, we search for wide DWDs in the Sloan Digital Sky Survey (SDSS) Data Release 9. We find 65 new candidate systems, thereby raising the number of known wide DWDs to 142. We then engage in a spectroscopic campaign to characterize these pairs, identifying 32 DA/DA pairs, two DA/DB pairs, four DA/DAH candidate pairs, a previously unidentified candidate triple WD system, and five DA/DC WDs. We present a reanalysis of the constraint on the IFMR placed by Finley & Koester (1997) using the DWD PG 0922+162, and finish by discussing how it could be expanded to a generic set of wide DWDs.

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arXiv2012

Common Proper Motion Wide White Dwarf Binaries Selected From The Sloan Digital Sky Survey

Wide binaries made up of two white dwarfs (WDs) receive far less attention than their tight counterparts. However, our tests using the binary population synthesis code {\tt StarTrack} indicate that, for any set of reasonable initial conditions, there exists a significant observable population of double white dwarfs (WDWDs) with orbital separations of 10$^2$ to 10$^5$ AU. We adapt the technique of Dhital et al.\ to search for candidate common proper motion WD companions separated by $<10\amin$ around the $>$12,000 spectroscopically confirmed hydrogen-atmosphere WDs recently identified in the Sloan Digital Sky Survey. Using two techniques to separate random alignments from high-confidence pairs, we find nine new high-probability wide WDWDs and confirm three previously identified candidate wide WDWDs. This brings the number of known wide WDWDs to 45; our new pairs are a significant addition to the sample, especially at small proper motions ($<$200 mas/yr) and large angular separations ($>$10\asec). Spectroscopic follow-up and an extension of this method to a larger, photometrically selected set of SDSS WDs may eventually produce a large enough dataset for WDWDs to realize their full potential as testbeds for theories of stellar evolution.

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