arXiv subjects

Robert Williams

Publications and source records attributed to Robert Williams.

At least 37 records · Page 2Linked to original sources

arXiv2020

Chandra High Energy Transmission Gratings Spectra of V3890 Sgr

The recurrent nova (RN) V3890 Sgr was observed during the 7th day after the onset of its most recent outburst, with the Chandra ACIS-S camera and High Energy Transmission Gratings (HETG). A rich emission line spectrum was detected, due to transitions of Fe-L and K-shell ions ranging from neon to iron. The measured absorbed flux is $\approx 10^{-10}$ erg cm$^{-2}$ s$^{-1}$ in the 1.4-15 Angstrom range (0.77-8.86 keV). The line profiles are asymmetric, blue-shifted and skewed towards the blue side, as if the ejecta moving towards us are less absorbed than the receding ones. The full width at half maximum of most emission lines is 1000-1200 km s$^{-1}$, with some extended blue wings. The spectrum is thermal and consistent with a plasma in collisional ionization equilibrium with column density 1.3 $\times 10^{22}$ cm$^{-2}$ and at least two components at temperatures of about 1 keV and 4 keV, possibly a forward and a reverse shock, or regions with differently mixed ejecta and red giant wind. The spectrum is remarkably similar to the symbiotic RNe V745 Sco and RS Oph, but we cannot distinguish whether the shocks occurred at a distance of few AU from the red giant, or near the giant's photosphere, in a high density medium containing only a small mass. The ratios of the flux in lines of aluminum, magnesium and neon relative to the flux in lines of silicon and iron probably indicate a carbon-oxygen white dwarf (CO WD).

astro-ph.HE↗

arXiv2020

Hubble Telescope 30 Years in Orbit: Personal Reflections

With an initial requirement to make observations a minimum of 5-10 years, Hubble Space Telescope (HST) has continued to operate well for 30 years. It has relied upon five servicing missions to repair and replace essential components. Since the final Space Shuttle mission 10 years ago it has avoided major breaks in its operation, with the only serious effects of ageing in space being a progressive deterioration in the performance of the gyroscopes and sensitivity of the instrument detectors. A number of factors were important in making HST a scientific landmark. Ground-breaking discoveries have been made with HST----the most important being the discovery of cosmic acceleration. When HST operation ceases future observations in space should be assured with successful operation of major missions now planned by NASA, ESA, and the China and Japanese Space Agencies.

astro-ph.IM↗

arXiv2019

Discovery of a Rare Late-Type, Low-Mass Wolf-Rayet Star in the LMC

We report the serendipitous discovery of an object, UVQS J060819.93-715737.4, with a spectrum dominated by extremely intense, narrow C II emission lines. The spectrum is similar to those of the very rare, late-type [WC11] low-mass Wolf-Rayet stars. Despite the recognition of these stars as a distinct class decades ago, there remains barely a handful of Galactic members, all of which are also planetary-nebula central stars. Although no obvious surrounding nebulosity is present in J0608, [O II], [N II], and [S II] emission suggest the presence of an inconspicuous, low-excitation nebula. There is low-amplitude incoherent photometric variability on timescales of days to years, as well as numerous prominent P Cygni profiles, implying mass loss. There are indications of a binary companion. The star is located on the outskirts of the LMC, and the observed radial velocity (~250 km/s) and proper motion strongly suggest membership. If indeed an LMC member, this is the first extragalactic late [WC] star, and the first with an accurately determined luminosity, as the Galactic examples are too distant for precise parallax determinations. A high-quality, broad-coverage spectrum of the prototype of the late [WC] class, CPD -56 8032, is also presented. We discuss different excitation mechanisms capable of producing the great strength of the C II emission. Numerous autoionizing levels of C II are definitely populated by processes other than dielectronic recombination. Despite the spectacular emission spectra, observational selection makes objects such as these difficult to discover. Members of the [WC11] class may in fact be considerably more common than the handful of previously known late [WC] stars.

astro-ph.SR↗

arXiv2018

The Long-Term Secular Mass Accretion Rate of the Recurrent Nova T Pyxidis

We present Hubble Space Telescope ultraviolet spectroscopy of the recurrent nova T Pyxidis obtained more than 5 years after its 2011 outburst indicating that the system might not have yet reached its deep quiescent state. The ultraviolet data exhibit a 20% decline in the continuum flux from the pre-outburst deep quiescence state to the post-outburst near quiescent state. We suggest that a decline across each recurring nova eruption might help explain the proposed 2mag steady decline of the system since 1866. Using an improved version of our accretion disk model as well as International Ultraviolet Explorer ultraviolet and optical data, and the 4.8 kpc distance, we corroborate our previous findings that the quiescent mass accretion rate in T Pyx is of the order of 1e-6 Solar mass per year. Such a large mass accretion rate would imply that the mass of the white dwarf is increasing with time. However, with the just-release Gaia DR 2 distance of 3.3 kpc (after submission of the first version of this manuscript), we find a mass accretion of the order of 1e-7 Solar mass per year. Our results predict powerful soft X-ray or extreme ultraviolet emission from the hot inner region of the high accretion rate disk. Using constraining X-ray observations and assuming the accretion disk doesn't depart too much from the standard model, we are left with two possible scenarios. The disk either emits mainly extreme ultraviolet radiation which, at a distance of a few kpc, is completely absorbed by the interstellar medium, or the hot inner disk, emitting soft X-rays, is masked by the bulging disk seen at a higher inclination.

astro-ph.SR↗

arXiv2017

Restrictions on Galois groups of Schubert problems

The Galois group of a Schubert problem encodes some structure of its set of solutions. Galois groups are known for a few infinite families and some special problems, but what permutation groups may appear as a Galois group of a Schubert problem is still unknown. We expand the list of Schubert problems with known Galois groups by fully exploring the Schubert problems on $\text{Gr}(4,9)$, the smallest Grassmannian for which they are not currently known. We also discover sets of Schubert conditions for any sufficiently large Grassmannian that imply the Galois group of a Schubert problem is much smaller than the full symmetric group.

math.AG↗

arXiv2017

Magnetic Proximity Effect in Pt/CoFe2O4 Bilayers

We observe the magnetic proximity effect (MPE) in Pt/CoFe2O4 bilayers grown by molecular beam epitaxy. This is revealed through angle-dependent magnetoresistance measurements at 5 K, which isolate the contributions of induced ferromagnetism (i.e. anisotropic magnetoresistance) and spin Hall effect (i.e. spin Hall magnetoresistance) in the Pt layer. The observation of induced ferromagnetism in Pt via AMR is further supported by density functional theory calculations and various control measurements including insertion of a Cu spacer layer to suppress the induced ferromagnetism. In addition, anomalous Hall effect measurements show an out-of-plane magnetic hysteresis loop of the induced ferromagnetic phase with larger coercivity and larger remanence than the bulk CoFe2O4. By demonstrating MPE in Pt/CoFe2O4, these results establish the spinel ferrite family as a promising material for MPE and spin manipulation via proximity exchange fields.

cond-mat.mtrl-sci↗

arXiv2017

SPIRITS: Uncovering Unusual Infrared Transients With Spitzer

We present an ongoing, systematic search for extragalactic infrared transients, dubbed SPIRITS --- SPitzer InfraRed Intensive Transients Survey. In the first year, using Spitzer/IRAC, we searched 190 nearby galaxies with cadence baselines of one month and six months. We discovered over 1958 variables and 43 transients. Here, we describe the survey design and highlight 14 unusual infrared transients with no optical counterparts to deep limits, which we refer to as SPRITEs (eSPecially Red Intermediate Luminosity Transient Events). SPRITEs are in the infrared luminosity gap between novae and supernovae, with [4.5] absolute magnitudes between -11 and -14 (Vega-mag) and [3.6]-[4.5] colors between 0.3 mag and 1.6 mag. The photometric evolution of SPRITEs is diverse, ranging from < 0.1 mag/yr to > 7 mag/yr. SPRITEs occur in star-forming galaxies. We present an in-depth study of one of them, SPIRITS 14ajc in Messier 83, which shows shock-excited molecular hydrogen emission. This shock may have been triggered by the dynamic decay of a non-hierarchical system of massive stars that led to either the formation of a binary or a proto-stellar merger.

astro-ph.HE↗

arXiv2016

Strongly maximal intersection-complete neural codes on grids are convex

The brain encodes spacial structure through a combinatorial code of neural activity. Experiments suggest such codes correspond to convex areas of the subject's environment. We present an intrinsic condition that implies a neural code may correspond to a convex space and give a bound on the minimal dimension underlying such a realization.

math.CO↗

arXiv2016

SPIRITS 15c and SPIRITS 14buu: Two Obscured Supernovae in the Nearby Star-Forming Galaxy IC 2163

SPIRITS---SPitzer InfraRed Intensive Transients Survey---is an ongoing survey of nearby galaxies searching for infrared (IR) transients with Spitzer/IRAC. We present the discovery and follow-up observations of one of our most luminous ($M_{[4.5]} = -17.1\pm0.4$ mag, Vega) and red ($[3.6] - [4.5] = 3.0 \pm 0.2$ mag) transients, SPIRITS 15c. The transient was detected in a dusty spiral arm of IC 2163 ($D\approx35.5$ Mpc). Pre-discovery ground-based imaging revealed an associated, shorter-duration transient in the optical and near-IR (NIR). NIR spectroscopy showed a broad ($\approx 8400$ km s$^{-1}$), double-peaked emission line of He I at $1.083 μ$m, indicating an explosive origin. The NIR spectrum of SPIRITS 15c is similar to that of the Type IIb SN 2011dh at a phase of $\approx 200$ days. Assuming $A_V = 2.2$ mag of extinction in SPIRITS 15c provides a good match between their optical light curves. The IR light curves and the extreme $[3.6]-[4.5]$ color cannot be explained using only a standard extinction law. Another luminous ($M_{4.5} = -16.1\pm0.4$ mag) event, SPIRITS 14buu, was serendipitously discovered in the same galaxy. The source displays an optical plateau lasting $\gtrsim 80$ days, and we suggest a scenario similar to the low-luminosity Type IIP SN 2005cs obscured by $A_V \approx 1.5$ mag. Other classes of IR-luminous transients can likely be ruled out in both cases. If both events are indeed SNe, this may suggest $\gtrsim 18\%$ of nearby core-collapse SNe are missed by currently operating optical surveys.

astro-ph.HE↗

arXiv2016

Third Parameter Classification of Transients and Novae Ejecta as Ballistically Ejected Globules

A third parameter, in addition to luminosity and rate of brightness decline, derived from the spectra of transients is suggested as a means of more accurately classifying objects in outburst. Principal component analysis of the spectra of transients is suggested as the best way to determine the third parameter. A model is suggested for novae ejecta that is based on ballistic ejection of an ensemble of globules. The distribution in sizes of the clouds should determine basic characteristics of the ejecta, including the short term brightness fluctuations of novae in decline and the location of dust formation.

astro-ph.SR↗

arXiv2016

A Systematic Study of Mid-Infrared Emission from Core-Collapse Supernovae with SPIRITS

We present a systematic study of mid-infrared (mid-IR) emission from 141 nearby supernovae (SNe) observed with the InfraRed Array Camera (IRAC) on Spitzer.These SNe reside in one of the 190 galaxies within 20 Mpc drawn from the ongoing SPIRITS program. We detect 8 Type Ia SNe and 36 core-collapse SNe. All Type I SNe become undetectable within 3 years of explosion. About 22$\pm$11% of Type II SNe continue to be detected at late-times. Dust luminosity, temperature, and a lower liit on mass are obtained by fitting the SED using photometry with IRAC bands 1 and 2. The mass estimate does not distinguish between pre-existing and newly produced dust. We observe warm dust masses between $10^{-2}$ and $10^{-6}$ $\rm M_{\odot}$ and dust temperatures from 200 K to 1280 K.We present detailed case studies of two extreme Type II-P SNe: SN 2011ja and 2014bi. SN 2011ja was over-luminous ([4.5] = -15.6 mag) at 900 days post-explosion accompanied by the growing dust mass. This suggests either an episode of dust formation or an intensifying CSM interactions heating up pre-existing dust. SN 2014bi showed a factor of 10 decrease in dust mass over one month suggesting either an episode of dust destruction or a fading source of dust heating. A rebrightening of the Type Ib SN 2014C is observed and attributed to CSM interactions. This observation adds to a small number of stripped-envelope SNe that have mid-IR excess. The observations suggest that the CSM shell around SN 2014C is originated from an LBV-like eruption roughly 100 years before the explosion. We also report detections of SN1974E, 1979C, 1980K, 1986J, and 1993J detected more than 20 years post-explosion. The number of outlying SNe identified in this work demonstrates the power of late time mid-IR observations of a large sample of SNe to identify events with unusual evolution.

astro-ph.SR↗

arXiv2013

Novae Ejecta as Discrete Adiabatically Expanding Globules

Available data for novae show that the X-ray and visible spectral regions correlate with each other as they evolve. Large differences in ionization exist simultaneously in the two wavelength regimes, and a straightforward model is proposed that explains the characteristics observed in both spectral regimes. Its key features are (1) ejected blobs of very high density gas from the white dwarf that expand to create within each clump a wide range of emitting density, ionization, and velocity, and (2) a more homogeneous circumbinary envelope of gas that is produced by secondary star mass loss. The relative mass loss rates from the two stars determine whether the He/N or the Fe II visible spectrum predominates during decline, when hard X-rays are detected, and when the white dwarf can be detected as a super soft X-ray source.

astro-ph.HE↗

arXiv2012

Origin of the `He/N' and `Fe II' Spectral Classes of Novae

The spectra of postoutburst novae display either He+N or Fe II lines as the most prominent non-Balmer lines at maximum light. Spectral diagnostics indicate physical conditions for 'He/N' spectra that are consistent with their origin in the white dwarf (WD) ejecta, whereas 'Fe II' spectra point to their formation in a large circumbinary envelope of gas whose origin is the secondary star. A determining parameter for which of the two types of spectra predominates may be the binary mass ratio Msec/MWD. The increasing fraction of novae that are observed to be 'hybrid' objects, where both classes of spectra appear sequentially, is explained by changing parameters in the two emitting regions during the postoutburst decline. We argue that most novae may be hybrids that show both types of spectra during decline. The emission line intensity ratio O I λ8446/λ7773 is suggested as a good density diagnostic for the ejecta, and a finding list of emission lines identified in recent spectroscopic surveys of novae is presented as an aid to future line identification work.

astro-ph.SR↗

arXiv2012

GK Per (Nova Persei 1901): HST Imagery and Spectroscopy of the Ejecta, and First Spectrum of the Jet-Like Feature

We have imaged the ejecta of GK Persei (Nova Persei 1901 A.D.) with the Hubble Space Telescope (HST), revealing hundreds of cometary-like structures. One or both ends of the structures often show a brightness enhancement relative to the structures' middle sections, but there is no simple regularity to their morphologies (in contrast with the Helix nebula). Some of the structures' morphologies suggest the presence of slow-moving or stationary material with which the ejecta is colliding, while others suggest shaping from a wind emanating from GK Per itself. A detailed expansion map of the nova's ejecta was created by comparing HST images taken in successive years. WFPC2 narrowband images and STIS spectra demonstrate that the physical conditions in the ejecta vary strongly on spatial scales much smaller than those of the ejecta. Directly measuring accurate densities and compositions, and hence masses of this and other nova shells, will demand data at least as resolved spatially as those presented here. The filling factor the ejecta is < 1%, and the nova ejecta mass must be less than $10^{-4} \Msun$. A few of the nebulosities vary in brightness by up to a factor of two on timescales of one year. Finally, we present the deepest images yet obtained of a jet-like feature outside the main body of GK Per nebulosity, and the first spectrum of that feature. Dominated by strong, narrow emission lines of [NII], [OII], [OIII], and [SII], this feature is probably a shock due to ejected material running into stationary ISM, slowly moving ejecta from a previous nova episode, or circum-binary matter present before 1901. An upper limit to the mass of the jet is of order a few times $10^{-6} \Msun$. The jet might be an important, or even dominant mass sink from the binary system. The jet's faintness suggests that similar features could easily have been missed in other cataclysmic binaries.

astro-ph.SR↗

arXiv2011

Episodic mass transfer: A trigger for nova outbursts?

High resolution spectra of postoutburst novae show multiple components of ejected gas that are kinematically distinct. We interpret the observations in terms of episodes of enhanced mass transfer originating from the secondary star that result in the formation of discrete components of circumbinary gas and accretion onto the WD that trigger nova outbursts. In this picture the concordance between absorption line velocities and emission line widths in most novae occurs as a result of the collision of the expanding nova ejecta with a larger mass of surrounding circumbinary gas. One implication of this model is that much of the accreted gas remains on the WD, leading to a secular increase in WD mass over each outburst event. Alternative scenarios to explain nova spectral evolution are possible that do not invoke circumbinary gas and a possible test of different models is proposed.

astro-ph.SR↗

arXiv2010

The 2010 early outburst spectrum of the recurrent nova U Scorpii

Aims. We present optical spectra of the fast recurrent nova U Sco during its recent outburst, obtained within 24 hr of maximum light. Methods. We use medium resolution (R~4000) spectra taken with the with the MagE spectrograph on the Magellan (Clay) 6.5m telescope of the Las Campanas Observatories. Results. The spectrum is notable for its lack of a low ionization transient heavy element absorption system that is visible in the large majority of novae near maximum light. We suggest that this may be due to the dominance of inner Lagrangian L1 mass transfer and the absence of a circumbinary gas reservoir in this object

astro-ph.SR↗

arXiv2010

Novae Ejecta as Colliding Shells

Following on our initial absorption-line analysis of fifteen novae spectra we present additional evidence for the existence of two distinct components of novae ejecta having different origins. As argued in Paper I one component is the rapidly expanding gas ejected from the outer layers of the white dwarf by the outburst. The second component is pre-existing outer, more slowly expanding circumbinary gas that represents ejecta from the secondary star or accretion disk. We present measurements of the emission-line widths that show them to be significantly narrower than the broad P Cygni profiles that immediately precede them. The emission profiles of novae in the nebular phase are distinctly rectangular, i.e., strongly suggestive of emission from a relatively thin, roughly spherical shell. We thus interpret novae spectral evolution in terms of the collision between the two components of ejecta, which converts the early absorption spectrum to an emission-line spectrum within weeks of the outburst. The narrow emission widths require the outer circumbinary gas to be much more massive than the white dwarf ejecta, thereby slowing the latter's expansion upon collision. The presence of a large reservoir of circumbinary gas at the time of outburst is suggestive that novae outbursts may sometime be triggered by collapse of gas onto the white dwarf, as occurs for dwarf novae, rather than steady mass transfer through the inner Lagrangian point.

astro-ph.HE↗

arXiv2008

Transient Heavy Element Absorption Systems in Novae: Episodic Mass Ejection from the Secondary Star

A high-resolution spectroscopic survey of postoutburst novae reveals short-lived heavy element absorption systems in a majority of novae near maximum light, having expansion velocities of 400-1000 km/s and velocity dispersions between 35-350 km/s. A majority of systems are accelerated outwardly, and they all progressively weaken and disappear over timescales of weeks. A few of the systems having narrow, deeper absorption reveal a rich spectrum of singly ionized Sc, Ti, V, Cr, Fe, Sr, Y, Zr, and Ba lines. Analysis of the richest such system, in Nova LMC 2005, shows the excitation temperature to be 104 K and elements lighter than Fe to have abundance enhancements over solar values by up to an order of magnitude. The gas causing the absorption systems must be circumbinary and its origin is most likely mass ejection from the secondary star. The absorbing gas pre-exists the outburst and may represent episodic mass transfer events from the secondary star that initiate the nova outburst(s). If SNe Ia originate in single degenerate binaries, such absorption systems could be detectable before maximum light

astro-ph↗