arXiv subjects

J. L. Tonry

Publications and source records attributed to J. L. Tonry.

At least 91 records · Page 5Linked to original sources

arXiv2013

PS1-12sk is a Peculiar Supernova From a He-rich Progenitor System in a Brightest Cluster Galaxy Environment

We report on our discovery and observations of the Pan-STARRS1 supernova (SN) PS1-12sk, a transient with properties that indicate atypical star formation in its host galaxy cluster or pose a challenge to popular progenitor system models for this class of explosion. The optical spectra of PS1-12sk classify it as a Type Ibn SN (c.f. SN 2006jc), dominated by intermediate-width (3x10^3 km/s) and time variable He I emission. Our multi-wavelength monitoring establishes the rise time dt = 9-23 days and shows an NUV-NIR SED with temperature > 17x10^3 K and a peak rise magnitude of Mz = -18.9 mag. SN Ibn spectroscopic properties are commonly interpreted as the signature of a massive star (17 - 100 M_sun) explosion within a He-enriched circumstellar medium. However, unlike previous Type Ibn supernovae, PS1-12sk is associated with an elliptical brightest cluster galaxy, CGCG 208-042 (z = 0.054) in cluster RXC J0844.9+4258. The expected probability of an event like PS1-12sk in such environments is low given the measured infrequency of core-collapse SNe in red sequence galaxies compounded by the low volumetric rate of SN Ibn. Furthermore, we find no evidence of star formation at the explosion site to sensitive limits (Sigma Halpha < 2x10^-3 M_sun/yr/kpc^2). We therefore discuss white dwarf binary systems as a possible progenitor channel for SNe Ibn. We conclude that PS1-12sk represents either a fortuitous and statistically unlikely discovery, evidence for a top-heavy IMF in galaxy cluster cooling flow filaments, or the first clue suggesting an alternate progenitor channel for Type Ibn SNe.

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arXiv2013

The Pan-STARRS 1 Photometric Reference Ladder, Release 12.0

As of 2012 Jan 21, the Pan-STARRS 1 3πSurvey has observed the 3/4 of the sky visible from Hawaii with a minimum of 2 and mean of 7.6 observations in 5 filters, g_p1, r_p1, i_p1, z_p1, y_p1. Now at the end of the second year of the mission, we are in a position to make an initial public release of a portion of this unprecedented dataset. This article describes the PS1 Photometric Ladder, Release 12.01 This is the first of a series of data releases to be generated as the survey coverage increases and the data analysis improves. The Photometric Ladder has rungs every hour in RA and at 4 intervals in declination. We will release updates with increased area coverage (more rungs) from the latest dataset until the PS1 survey and the final re-reduction are completed. The currently released catalog presents photometry of \approx 1000 objects per square degree in the rungs of the ladder. Saturation occurs at g_p1, r_p1, i_p1 \approx 13.5; z_p1 \approx 13.0; and y_p1 \approx 12.0. Photometry is provided for stars down to g_p1, r_p1, i_p1 \approx 19.1 in the AB system. This data release depends on the rigid `Ubercal' photometric calibration using only the photometric nights, with systematic uncertainties of (8.0, 7.0, 9.0, 10.7, 12.4) millimags in (g_p1, r_p1, i_p1, z_p1, y_p1). Areas covered only with lower quality nights are also included, and have been tied to the Ubercal solution via relative photometry; photometric accuracy of the non-photometric regions is lower and should be used with caution.

astro-ph.IM↗

arXiv2013

PS1-10afx at z=1.388: Pan-STARRS1 Discovery of a New Type of Superluminous Supernova

We present the Pan-STARRS1 discovery of PS1-10afx, a unique hydrogen-deficient superluminous supernova (SLSN) at z=1.388. The light curve peaked at z_P1=21.7 mag, making PS1-10afx comparable to the most luminous known SNe, with M_u = -22.3 mag. Our extensive optical and NIR observations indicate that the bolometric light curve of PS1-10afx rose on the unusually fast timescale of ~12 d to the extraordinary peak luminosity of 4.1e44 erg/s (M_bol = -22.8 mag) and subsequently faded rapidly. Equally important, the SED is unusually red for a SLSN, with a color temperature of 6800 K near maximum light, in contrast to previous H-poor SLSNe, which are bright in the UV. The spectra more closely resemble those of a normal SN Ic than any known SLSN, with a photospheric velocity of 11,000 km/s and evidence for line blanketing in the rest-frame UV. Despite the fast rise, these parameters imply a very large emitting radius (>5e15 cm). We demonstrate that no existing theoretical model can satisfactorily explain this combination of properties: (i) a nickel-powered light curve cannot match the combination of high peak luminosity with the fast timescale; (ii) models powered by the spindown energy of a rapidly-rotating magnetar predict significantly hotter and faster ejecta; and (iii) models invoking shock breakout through a dense circumstellar medium cannot explain the observed spectra or color evolution. The host galaxy is well detected in pre-explosion imaging with a luminosity near L*, a star formation rate of 15 M_sun/yr, and is fairly massive (2e10 M_sun), with a stellar population age of 1e8 yr, also in contrast to the dwarf hosts of known H-poor SLSNe. PS1-10afx is distinct from known examples of SLSNe in its spectra, colors, light-curve shape, and host galaxy properties, suggesting that it resulted from a different channel than other hydrogen-poor SLSNe.

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arXiv2013

The GALEX Time Domain Survey I. Selection and Classification of Over a Thousand UV Variable Sources

We present the selection and classification of over a thousand ultraviolet (UV) variable sources discovered in ~ 40 deg^2 of GALEX Time Domain Survey (TDS) NUV images observed with a cadence of 2 days and a baseline of observations of ~ 3 years. The GALEX TDS fields were designed to be in spatial and temporal coordination with the Pan-STARRS1 Medium Deep Survey, which provides deep optical imaging and simultaneous optical transient detections via image differencing. We characterize the GALEX photometric errors empirically as a function of mean magnitude, and select sources that vary at the 5σlevel in at least one epoch. We measure the statistical properties of the UV variability, including the structure function on timescales of days and years. We report classifications for the GALEX TDS sample using a combination of optical host colors and morphology, UV light curve characteristics, and matches to archival X-ray, and spectroscopy catalogs. We classify 62% of the sources as active galaxies (358 quasars and 305 active galactic nuclei), and 10% as variable stars (including 37 RR Lyrae, 53 M dwarf flare stars, and 2 cataclysmic variables). The remaining unclassified sources include UV-bright extragalactic transients, two of which have been spectroscopically confirmed to be a young core-collapse supernova and a flare from the tidal disruption of a star by dormant supermassive black hole. We calculate a surface density for variable sources in the UV with NUV < 23 mag and |Δm| > 0.2 mag of ~ 8.0, 7.7, and 1.8 deg^-2 for quasars, AGNs, and RR Lyrae stars, respectively. We also calculate a surface density rate in the UV for transient sources, using the effective survey time at the cadence appropriate to each class, of ~ 15 and 52 deg^-2 yr^-1 for M dwarfs and extragalactic transients, respectively. (Abridged.)

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arXiv2012

A Pan-STARRS1 View of the Bifurcated Sagittarius Stream

We use data from the Pan-STARRS1 survey to present a panoramic view of the Sagittarius tidal stream in the southern Galactic hemisphere. As a result of the extensive sky coverage of Pan-STARRS1, the southern stream is visible along more than 60 degrees of its orbit, nearly double the length seen by the SDSS. The recently discovered southern bifurcation of the stream is also apparent, with the fainter branch of the stream visible over at least 30 degrees. Using a combination of fitting both the main sequence turn-off and the red clump, we measure the distance to both arms of the stream in the south. We find that the distances to the bright arm of the stream agree very well with the N-body models of Law & Majewski (2010). We also find that the faint arm lies ~5 kpc closer to the Sun than the bright arm, similar to the behavior seen in the northern hemisphere.

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arXiv2012

Photometric Calibration of the First 1.5 Years of the Pan-STARRS1 Survey

We present a precise photometric calibration of the first 1.5 years of science imaging from the Pan-STARRS1 survey (PS1), an ongoing optical survey of the entire sky north of declination -30 degrees in five bands. Building on the techniques employed by Padmanabhan et al. (2008) in the Sloan Digital Sky Survey (SDSS), we use repeat PS1 observations of stars to perform the relative calibration of PS1 in each of its five bands, solving simultaneously for the system throughput, the atmospheric transparency, and the large-scale detector flat field. Both internal consistency tests and comparison against the SDSS indicate that we achieve relative precision of <10 mmag in g, r, and i_P1, and ~10 mmag in z and y_P1. The spatial structure of the differences with the SDSS indicates that errors in both the PS1 and SDSS photometric calibration contribute similarly to the differences. The analysis suggests that both the PS1 system and the Haleakala site will enable <1% photometry over much of the sky.

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arXiv2012

Ultra-Luminous Supernovae as a New Probe of the Interstellar Medium in Distant Galaxies

We present the Pan-STARRS1 discovery and light curves, and follow-up MMT and Gemini spectroscopy of an ultra-luminous supernova (ULSN; dubbed PS1-11bam) at a redshift of z=1.566 with a peak brightness of M_UV=-22.3 mag. PS1-11bam is one of the highest redshift spectroscopically-confirmed SNe known to date. The spectrum is characterized by broad absorption features typical of previous ULSNe (e.g., CII, SiIII), and by strong and narrow MgII and FeII absorption lines from the interstellar medium (ISM) of the host galaxy, confirmed by an [OII]3727 emission line at the same redshift. The equivalent widths of the FeII2600 and MgII2803 lines are in the top quartile of the quasar intervening absorption system distribution, but are weaker than those of gamma-ray burst intrinsic absorbers (i.e., GRB host galaxies). We also detect the host galaxy in pre-explosion Pan-STARRS1 data and find that its UV spectral energy distribution is best fit with a young stellar population age of tau~15-45 Myr and a stellar mass of M \sim (1.1-2.6)x10^9 M_sun (for Z=0.05-1 Z_sun). The star formation rate inferred from the UV continuum and [OII]3727 emission line is ~10 M_sun/yr, higher than in any previous ULSN host. PS1-11bam provides the first direct demonstration that ULSNe can serve as probes of the interstellar medium in distant galaxies. At the present, the depth and red sensitivity of PS1 are uniquely suited to finding such events at cosmologically interesting redshifts (z~1-2); the future combination of LSST and 30-m class telescopes promises to extend this technique to z~4.

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arXiv2012

HIP 38939B: A New Benchmark T Dwarf in the Galactic Plane Discovered with Pan-STARRS1

We report the discovery of a wide brown dwarf companion to the mildly metal-poor ([Fe/H]=-0.24), low galactic latitude (b = 1.88 deg) K4V star HIP 38939. The companion was discovered by its common proper motion with the primary and its red optical (Pan-STARRS1) and blue infrared (2MASS) colors. It has a projected separation of 1630 AU and a near-infrared spectral type of T4.5. As such it is one of only three known companions to a main sequence star which have early/mid-T spectral types (the others being HN Peg B and eps Indi B). Using chromospheric activity we estimate an age for the primary of 900{+1900,-600} Myr. This value is also in agreement with the age derived from the star's weak ROSAT detection. Comparison with evolutionary models for this age range indicates that HIP 38939B falls in the mass range 38+/-20 Mjup with an effective temperature range of 1090+/-60 K. Fitting our spectrum with atmospheric models gives a best fitting temperature of 1100 K. We include our object in an analysis of the population of benchmark T dwarfs and find that while older atmospheric models appeared to over-predict the temperature of the coolest objects compared to evolutionary models, more recent atmospheric models provide better agreement.

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arXiv2012

An ultraviolet-optical flare from the tidal disruption of a helium-rich stellar core

The flare of radiation from the tidal disruption and accretion of a star can be used as a marker for supermassive black holes that otherwise lie dormant and undetected in the centres of distant galaxies. Previous candidate flares have had declining light curves in good agreement with expectations, but with poor constraints on the time of disruption and the type of star disrupted, because the rising emission was not observed. Recently, two `relativistic' candidate tidal disruption events were discovered, each of whose extreme X-ray luminosity and synchrotron radio emission were interpreted as the onset of emission from a relativistic jet. Here we report the discovery of a luminous ultraviolet-optical flare from the nuclear region of an inactive galaxy at a redshift of 0.1696. The observed continuum is cooler than expected for a simple accreting debris disk, but the well-sampled rise and decline of its light curve follows the predicted mass accretion rate, and can be modelled to determine the time of disruption to an accuracy of two days. The black hole has a mass of about 2 million solar masses, modulo a factor dependent on the mass and radius of the star disrupted. On the basis of the spectroscopic signature of ionized helium from the unbound debris, we determine that the disrupted star was a helium-rich stellar core.

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arXiv2012

SN 2010ay is a Luminous and Broad-lined Type Ic Supernova within a Low-metallicity Host Galaxy

We report on our serendipitous pre-discovery detection and detailed follow-up of the broad-lined Type Ic supernova (SN) 2010ay at z = 0.067 imaged by the Pan-STARRS1 3pi survey just ~4 days after explosion. The SN had a peak luminosity, M_R ~ -20.2 mag, significantly more luminous than known GRB-SNe and one of the most luminous SNe Ib/c ever discovered. The absorption velocity of SN 2010ay is v_Si ~ 19,000 km/s at ~40 days after explosion, 2-5 times higher than other broad-lined SNe and similar to the GRB-SN 2010bh at comparable epochs. Moreover, the velocity declines ~2 times slower than other SNe Ic-BL and GRB-SNe. Assuming that the optical emission is powered by radioactive decay, the peak magnitude implies the synthesis of an unusually large mass of 56 Ni, M_Ni = 0.9 M_solar. Modeling of the light-curve points to a total ejecta mass, M_ej ~ 4.7 M_sol, and total kinetic energy, E_K ~ 11x10^51 ergs. The ratio of M_Ni to M_ej is ~2 times as large for SN 2010ay as typical GRB-SNe and may suggest an additional energy reservoir. The metallicity (log(O/H)_PP04 + 12 = 8.19) of the explosion site within the host galaxy places SN 2010ay in the low-metallicity regime populated by GRB-SNe, and ~0.5(0.2) dex lower than that typically measured for the host environments of normal (broad-lined) Ic supernovae. We constrain any gamma-ray emission with E_gamma < 6x10^{48} erg (25-150 keV) and our deep radio follow-up observations with the Expanded Very Large Array rule out relativistic ejecta with energy, E > 10^48 erg. We therefore rule out the association of a relativistic outflow like those which accompanied SN 1998bw and traditional long-duration GRBs, but place less-stringent constraints on a weak afterglow like that seen from XRF 060218. These observations challenge the importance of progenitor metallicity for the production of a GRB, and suggest that other parameters also play a key role.

astro-ph.HE↗

arXiv2012

PAndromeda - first results from the high-cadence monitoring of M31 with Pan-STARRS 1

The Pan-STARRS 1 (PS1) survey of M31 (PAndromeda) is designed to identify gravitational microlensing events, caused by bulge and disk stars (self-lensing) and by compact matter in the halos of M31 and the Milky Way (halo lensing, or lensing by MACHOs). With the 7 deg2 FOV of PS1, the entire disk of M31 can be imaged with one single pointing. Our aim is to monitor M31 with this wide FOV with daily sampling (20 mins/day). In the 2010 season we acquired in total 91 nights towards M31, with 90 nights in the rP1 and 66 nights in the iP1. The total integration time in rP1 and iP1 are 70740s and 36180s, respectively. As a preliminary analysis, we study a 40'\times40' sub-field in the central region of M31, a 20'\times20' sub-field in the disk of M31 and a 20'\times20' sub-field for the investigation of astrometric precision. We demonstrate that the PSF is good enough to detect microlensing events. We present light curves for 6 candidate microlensing events. This is a competitive rate compared to previous M31 microlensing surveys. We finally also present one example light curve for Cepheids, novae and eclipsing binaries in these sub-fields.

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arXiv2012

The Pan-STARRS1 Photometric System

The Pan-STARRS1 survey is collecting multi-epoch, multi-color observations of the sky north of declination -30 deg to unprecedented depths. These data are being photometrically and astrometrically calibrated and will serve as a reference for many other purposes. In this paper we present our determination of the Pan-STARRS photometric system: gp1, rp1, ip1, zp1, yp1, and wp1. The Pan-STARRS photometric system is fundamentally based on the HST Calspec spectrophotometric observations, which in turn are fundamentally based on models of white dwarf atmospheres. We define the Pan-STARRS magnitude system, and describe in detail our measurement of the system passbands, including both the instrumental sensitivity and atmospheric transmission functions. Byproducts, including transformations to other photometric systems, galactic extinction, and stellar locus are also provided. We close with a discussion of remaining systematic errors.

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arXiv2011

The Photometric Classification Server for Pan-STARRS1

The Pan-STARRS1 survey is obtaining multi-epoch imaging in 5 bands (gps rps ips zps yps) over the entire sky North of declination -30deg. We describe here the implementation of the Photometric Classification Server (PCS) for Pan-STARRS1. PCS will allow the automatic classification of objects into star/galaxy/quasar classes based on colors, the measurement of photometric redshifts for extragalactic objects, and constrain stellar parameters for stellar objects, working at the catalog level. We present tests of the system based on high signal-to-noise photometry derived from the Medium Deep Fields of Pan-STARRS1, using available spectroscopic surveys as training and/or verification sets. We show that the Pan-STARRS1 photometry delivers classifications and photometric redshifts as good as the Sloan Digital Sky Survey (SDSS) photometry to the same magnitude limits. In particular, our preliminary results, based on this relatively limited dataset down to the SDSS spectroscopic limits and therefore potentially improvable, show that stars are correctly classified as such in 85% of cases, galaxies in 97% and QSOs in 84%. False positives are less than 1% for galaxies, ~19% for stars and ~28% QSOs. Moreover, photometric redshifts for 1000 luminous red galaxies up to redshift 0.5 are determined to 2.4% precision with just 0.4% catastrophic outliers and small (-0.5%) residual bias. PCS will create a value added catalog with classifications and photometric redshifts for eventually many millions sources.

astro-ph.CO↗

arXiv2011

First Results from Pan-STARRS1: Faint, High Proper Motion White Dwarfs in the Medium-Deep Fields

The Pan-STARRS1 survey has obtained multi-epoch imaging in five bands (Pan-STARRS1 gps, rps, ips, zps, and yps) on twelve "Medium Deep Fields", each of which spans a 3.3 degree circle. For the period between Apr 2009 and Apr 2011 these fields were observed 50-200 times. Using a reduced proper motion diagram, we have extracted a list of 47 white dwarf (WD) candidates whose Pan-STARRS1 astrometry indicates a non-zero proper motion at the 6-sigma level, with a typical 1-sigma proper motion uncertainty of 10 mas/yr. We also used astrometry from SDSS (when available) and USNO-B to assess our proper motion fits. None of the WD candidates exhibits evidence of statistically significant parallaxes, with a typical 1-sigma uncertainty of 8 mas. Twelve of these candidates are known WDs, including the high proper motion (1.7"/yr) WD LHS 291. We confirm three more objects as WDs through optical spectroscopy. Based on the Pan-STARRS1 colors, ten of the stars are likely to be cool WDs with 4170 K Teff 5000 K and cooling ages <9 Gyr. We classify these objects as likely thick disk WDs based on their kinematics. Our current sample represents only a small fraction of the Pan-STARRS1 data. With continued coverage from the Medium Deep Field Survey and the 3pi survey, Pan-STARRS1 should find many more high proper motion WDs that are part of the old thick disk and halo.

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arXiv2011

Pan-STARRS1 Discovery of Two Ultra-Luminous Supernovae at z ~ 0.9

We present the discovery of two ultra-luminous supernovae (SNe) at z ~ 0.9 with the Pan-STARRS1 Medium-Deep Survey. These SNe, PS1-10ky and PS1-10awh, are amongst the most luminous SNe ever discovered, comparable to the unusual transients SN 2005ap and SCP 06F6. Like SN 2005ap and SCP 06F6, they show characteristic high luminosities (M_bol ~ -22.5 mag), blue spectra with a few broad absorption lines, and no evidence for H or He. We have constructed a full multi-color light curve sensitive to the peak of the spectral energy distribution in the rest-frame ultraviolet, and we have obtained time-series spectroscopy for these SNe. Given the similarities between the SNe, we combine their light curves to estimate a total radiated energy over the course of explosion of (0.9-1.4) x 10^51 erg. We find photospheric velocities of 12,000-19,000 km/s with no evidence for deceleration measured across ~3 rest-frame weeks around light-curve peak, consistent with the expansion of an optically-thick massive shell of material. We show that, consistent with findings for other ultra-luminous SNe in this class, radioactive decay is not sufficient to power PS1-10ky, and we discuss two plausible origins for these events: the initial spin-down of a newborn magnetar in a core-collapse SN, or SN shock breakout from the dense circumstellar wind surrounding a Wolf-Rayet star.

astro-ph.HE↗

arXiv2011

A Search for High Proper Motion T Dwarfs with Pan-STARRS1 + 2MASS + WISE

We have searched ~8200 sq. degs for high proper motion (~0.5-2.7"/year) T dwarfs by combining first-epoch data from the Pan-STARRS1 (PS1) 3-Pi Survey, the 2MASS All-Sky Point Source Catalog, and the WISE Preliminary Data Release. We identified two high proper motion objects with the very red (W1-W2) colors characteristic of T dwarfs, one being the known T7.5 dwarf GJ 570D. Near-IR spectroscopy of the other object (PSO J043.5+02 = WISEP J0254+0223) reveals a spectral type of T8, leading to a photometric distance of 7.2+/-0.7 pc. The 2.56"/yr proper motion of PSO J043.5+02 is the second highest among field T dwarfs, corresponding to an tangential velocity of 87+/-8 km/s. According to the Besancon galaxy model, this velocity indicates its galactic membership is probably in the thin disk, with the thick disk an unlikely possibility. Such membership is in accord with the near-IR spectrum, which points to a surface gravity (age) and metallicity typical of the field population. We combine 2MASS, SDSS, WISE, and PS1 astrometry to derive a preliminary parallax of 171+/-45 mas (5.8{+2.0}{-1.2} pc), the first such measurement using PS1 data. The proximity and brightness of PSO J043+02 will facilitate future characterization of its atmosphere, variability, multiplicity, distance, and kinematics. The modest number of candidates from our search suggests that the immediate (~10 pc) solar neighborhood does not contain a large reservoir of undiscovered T dwarfs earlier than about T8.

astro-ph.SR↗

arXiv2011

Four new T dwarfs identified in PanSTARRS 1 commissioning data

A complete well-defined sample of ultracool dwarfs is one of the key science programs of the Pan-STARRS 1 optical survey telescope (PS1). Here we combine PS1 commissioning data with 2MASS to conduct a proper motion search (0.1--2.0\arcsec/yr) for nearby T dwarfs, using optical+near-IR colors to select objects for spectroscopic followup. The addition of sensitive far-red optical imaging from PS1 enables discovery of nearby ultracool dwarfs that cannot be identified from 2MASS data alone. We have searched 3700 sq. deg. of PS1 y-band (0.95--1.03 um) data to y$\approx$19.5 mag (AB) and J$\approx$16.5 mag (Vega) and discovered four previously unknown bright T dwarfs. Three of the objects (with spectral types T1.5, T2 and T3.5) have photometric distances within 25 pc and were missed by previous 2MASS searches due to more restrictive color selection criteria. The fourth object (spectral type T4.5) is more distant than 25 pc and is only a single-band detection in 2MASS. We also examine the potential for completing the census of nearby ultracool objects with the PS1 3$π$ survey.

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arXiv2010

Ultra-bright optical transients are linked with type Ic supernovae

Recent searches by unbiased, wide-field surveys have uncovered a group of extremely luminous optical transients. The initial discoveries of SN 2005ap by the Texas Supernova Search and SCP-06F6 in a deep Hubble pencil beam survey were followed by the Palomar Transient Factory confirmation of host redshifts for other similar transients. The transients share the common properties of high optical luminosities (peak magnitudes ~ -21 to -23), blue colors, and a lack of H or He spectral features. The physical mechanism that produces the luminosity is uncertain, with suggestions ranging from jet-driven explosion to pulsational pair-instability. Here we report the most detailed photometric and spectral coverage of an ultra-bright transient (SN 2010gx) detected in the Pan-STARRS 1 sky survey. In common with other transients in this family, early-time spectra show a blue continuum, and prominent broad absorption lines of O II. However, about 25d after discovery, the spectra developed type Ic supernova features, showing the characteristic broad Fe II and Si II absorption lines. Detailed, post-maximum follow-up may show that all SN 2005ap and SCP-06F6 type transients are linked to supernovae Ic. This poses problems in understanding the physics of the explosions: there is no indication from late-time photometry that the luminosity is powered by 56Ni, the broad lightcurves suggest very large ejected masses, and the slow spectral evolution is quite different from typical Ic timescales. The nature of the progenitor stars and the origin of the luminosity are intriguing and open questions.

astro-ph.SR↗