arXiv subjects

Lynn D. Matthews

Publications and source records attributed to Lynn D. Matthews.

At least 37 records · Page 2Linked to original sources

arXiv2020

X-ray Observations of the Peculiar Cepheid V473 Lyr Identify A Low-Mass Companion

V473 Lyr is a classical Cepheid which is unique in having substantial amplitude variations with a period of approximately 3.3 years, thought to be similar to the Blazhko variations in RR Lyrae stars. We obtained an {\it XMM-Newton} observation of this star to followup a previous detection in X-rays. Rather than the X-ray burst and rapid decline near maximum radius seen in $δ$ Cephei itself, the X-ray flux in V473 Lyr remained constant for a third of the pulsation cycle covered by the observation. Thus the X-rays are most probably not produced by the changes around the pulsation cycle. The X-ray spectrum is soft (kT = 0.6 keV), with X-ray properties which are consistent with a young low mass companion. Previously there was no evidence of a companion in radial velocities or in {\it Gaia} and {\it Hipparcos} proper motions. While this rules out companions which are very close or very distant, a binary companion at a separation between 30 and 300 AU is possible. This is an example of an X-ray observation revealing evidence of a low mass companion, which is important in completing the mass ratio statistics of binary Cepheids. Furthermore, the detection of a young X-ray bright companion is a further indication that the Cepheid (primary) is a Population I star, even though its pulsation behavior differs from other classical Cepheids.

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arXiv2019

Studying Black Holes on Horizon Scales with VLBI Ground Arrays

High-resolution imaging of supermassive black holes is now possible, with new applications to testing general relativity and horizon-scale accretion and relativistic jet formation processes. Over the coming decade, the EHT will propose to add new strategically placed VLBI elements operating at 1.3mm and 0.87mm wavelength. In parallel, development of next-generation backend instrumentation, coupled with high throughput correlation architectures, will boost sensitivity, allowing the new stations to be of modest collecting area while still improving imaging fidelity and angular resolution. The goal of these efforts is to move from imaging static horizon scale structure to dynamic reconstructions that capture the processes of accretion and jet launching in near real time.

astro-ph.IM↗

arXiv2019

First M87 Event Horizon Telescope Results and the Role of ALMA

In April 2019, the Event Horizon Telescope (EHT) collaboration revealed the first image of the candidate super-massive black hole (SMBH) at the centre of the giant elliptical galaxy Messier 87 (M87). This event-horizon-scale image shows a ring of glowing plasma with a dark patch at the centre, which is interpreted as the shadow of the black hole. This breakthrough result, which represents a powerful confirmation of Einstein's theory of gravity, or general relativity, was made possible by assembling a global network of radio telescopes operating at millimetre wavelengths that for the first time included the Atacama Large Millimeter/ submillimeter Array (ALMA). The addition of ALMA as an anchor station has enabled a giant leap forward by increasing the sensitivity limits of the EHT by an order of magnitude, effectively turning it into an imaging array. The published image demonstrates that it is now possible to directly study the event horizon shadows of SMBHs via electromagnetic radiation, thereby transforming this elusive frontier from a mathematical concept into an astrophysical reality. The expansion of the array over the next few years will include new stations on different continents - and eventually satellites in space. This will provide progressively sharper and higher-fidelity images of SMBH candidates, and potentially even movies of the hot plasma orbiting around SMBHs. These improvements will shed light on the processes of black hole accretion and jet formation on event-horizon scales, thereby enabling more precise tests of general relativity in the truly strong field regime.

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arXiv2019

Exploring Regularized Maximum Likelihood Reconstruction for Stellar Imaging with the ngVLA

The proposed next-generation Very Large Array (ngVLA) will enable the imaging of astronomical sources in unprecedented detail by providing an order of magnitude improvement in sensitivity and angular resolution compared with radio interferometers currently operating at 1.2--116~GHz. However, the current ngVLA array design results in a highly non-Gaussian dirty beam that may make it difficult to achieve high-fidelity images with both maximum sensitivity and maximum angular resolution using traditional CLEAN deconvolution methods. This challenge may be overcome with regularized maximum-likelihood (RML) methods, a new class of imaging techniques developed for the Event Horizon Telescope. RML methods take a forward-modeling approach, directly solving for the images without using either the dirty beam or the dirty map. Consequently, this method has the potential to improve the fidelity and effective angular resolution of images produced by the ngVLA. As an illustrative case, we present ngVLA imaging simulations of stellar radio photospheres performed with both multi-scale (MS-) CLEAN and RML methods implemented in the CASA and SMILI packages, respectively. We find that both MS-CLEAN and RML methods can provide high-fidelity images recovering most of the representative structures for different types of stellar photosphere models. However, RML methods show better performance than MS-CLEAN for various stellar models in terms of goodness-of-fit to the data, residual errors of the images, and in recovering representative features in the ground truth images. Our simulations support the feasibility of transformative stellar imaging science with the ngVLA, and simultaneously demonstrate that RML methods are an attractive choice for ngVLA imaging.

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arXiv2019

Calibration of ALMA as a phased array: ALMA observations during the 2017 VLBI campaign

We present a detailed description of the special procedures for calibration and quality assurance of Atacama Large Millimeter/submillimeter Array (ALMA) observations in Very Long Baseline Interferometry (VLBI) mode. These procedures are required to turn the phased ALMA array into a fully calibrated VLBI station. As an illustration of these methodologies, we present full-polarization observations carried out with ALMA as a phased array at 3mm (Band 3) and 1.3mm (Band 6) as part of Cycle-4. These are the first VLBI science observations conducted with ALMA and were obtained during a 2017 VLBI campaign in concert with other telescopes worldwide as part of the Global mm-VLBI Array (GMVA, April 1-3) and the Event Horizon Telescope (EHT, April 5-11) in ALMA Bands 3 and 6, respectively.

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arXiv2019

Astro2020: Unlocking the Secrets of Late-Stage Stellar Evolution and Mass Loss through Radio Wavelength Imaging

During the late phases of evolution, low-to-intermediate mass stars like our Sun undergo periods of extensive mass loss, returning up to 80% of their initial mass to the interstellar medium. This mass loss profoundly affects the stellar evolutionary history, and the resulting circumstellar ejecta are a primary source of dust and heavy element enrichment in the Galaxy. However, many details concerning the physics of late-stage stellar mass loss remain poorly understood, including the wind launching mechanism(s), the mass loss geometry and timescales, and the mass loss histories of stars of various initial masses. These uncertainties have implications not only for stellar astrophysics, but for fields ranging from star formation to extragalactic astronomy and cosmology. Observations at centimeter, millimeter, and submillimeter wavelengths that resolve the radio surfaces and extended atmospheres of evolved stars in space, time, and frequency are poised to provide groundbreaking new insights into these questions in the coming decade.

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arXiv2019

Astro2020: Molecular Masers as Probes of the Dynamic Atmospheres of Dying Stars

More than half of the dust and heavy element enrichment in galaxies originates from the winds and outflows of evolved, low-to-intermediate mass stars on the asymptotic giant branch (AGB). However, numerous details of the physics of late-stage stellar mass loss remain poorly understood, ranging from the wind launching mechanism(s) to the geometry and timescales of the mass loss. One of the major challenges to understanding AGB winds is that the AGB evolutionary phase is characterized by the interplay between highly complex and dynamic processes, including radial pulsations, shocks, magnetic fields, opacity changes due to dust and molecule formation, and large-scale convective flows. Collectively, these phenomena lead to changes in the observed stellar properties on timescales of days to years. Probing the complex atmospheric physics of AGB stars therefore demands exquisite spatial resolution, coupled with temporal monitoring over both short and long timescales. Observations of the molecular maser lines that arise in the winds and outflows of AGB stars using very long baseline interferometry (VLBI) offer one of the most powerful tools available to measure the atmospheric dynamics, physical conditions, and magnetic fields with ultra-high spatial resolution (i.e., up to tens of microarcseconds, corresponding to ~0.002R* at d~150pc), coupled with the ability to track features and phenomena on timescales of days to years. Observational advances in the coming decade will enable contemporaneous observations of an unprecedented number of maser transitions spanning centimeter to submillimeter wavelengths. In evolved stars, observations of masers within the winds and outflows are poised to provide groundbreaking new insights into the atmospheric physics and mass-loss process.

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arXiv2018

Science with an ngVLA: Evolved Stars

This chapter reviews some of the expected contributions of the ngVLA to the understanding of the late evolutionary stages of low-to-intermediate mass stars, including asymptotic giant branch (AGB) stars, post-AGB stars, and pre-planetary nebulae. Such objects represent the ultimate fate of stars like the Sun, and the stellar matter they lose to their immediate vicinity contributes significantly to the chemical enrichment of galaxies. Topics addressed in this chapter include continuum imaging of radio photospheres, studies of circumstellar envelopes in both thermal and nonthermal lines, and the investigation of the transition stages from the AGB to planetary nebulae using radio wavelength diagnostics. The authors gratefully acknowledge contributions to the content of this chapter from members of the evolved star community.

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arXiv2018

Radio Stars: from kHz to THz

Advances in technology and instrumentation have now opened up virtually the entire radio spectrum to the study of stars. An international workshop "Radio Stars: from kHz to THz" was held at the Massachusetts Institute of Technology Haystack Observatory on 2017 November 1-3 to enable the discussion of progress in solar and stellar astrophysics enabled by radio wavelength observations. Topics covered included the Sun as a radio star, radio emission from hot and cool stars (from the pre- to post-main-sequence), ultracool dwarfs, stellar activity, stellar winds and mass loss, planetary nebulae, cataclysmic variables, classical novae, and the role of radio stars in understanding the Milky Way. This article summarizes meeting highlights along with some contextual background information.

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arXiv2015

Modeling the HI 21-cm line profile from circumstellar shells around red giants

We present HI line profiles for various models of circumstellar shells around red giants. In the calculations we take into account the effect of the background at 21 cm, and show that in some circumstances it may have an important effect on the shape and intensity of the observed line profiles. We show that self-absorption should also be considered depending on the mass loss rate and the temperature reached by circumstellar gas. HI emission from circumstellar shells has been mostly reported from stars with mass loss rates around 10$^{-7}$ solar masses per year. We discuss the possible reasons for the non detection of many sources with larger mass loss rates that are hallmarks of the end of the AGB phase. Although radiative transfer effects may weaken the line emission, they cannot alone account for this effect. Therefore, it seems likely that molecular hydrogen, rather than atomic hydrogen, dominates the composition of matter expelled by stars at the end of their evolution on the Asymptotic Giant Branch. However sensitive HI observations can still yield important information on the kinematics and physical properties of the circumstellar material at large distances from central stars with heavy mass loss, despite the low abundance of atomic hydrogen.

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arXiv2013

Radio Stars and Their Lives in the Galaxy

This paper summarizes the three-day international workshop "Radio Stars and Their Lives in the Galaxy", held at the Massachusetts Institute of Technology Haystack Observatory on 2012 October 3-5. The workshop was organized to provide a forum for the presentation and discussion of advances in stellar and solar astrophysics recently (or soon to be) enabled by the latest generation of state-of-the-art observational facilities operating from meter to submillimeter wavelengths. The meeting brought together both observers and theorists to discuss how radio wavelength observations are providing new and unique insights into the workings of stars and their role in the Galactic ecosystem. Topics covered included radio emission from hot and cool stars (from the pre- to post-main-sequence), the Sun as a radio star, circumstellar chemistry, planetary nebulae, white dwarf binaries and novae, supernova progenitors, and radio stars as probes of the Galaxy.

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arXiv2013

Science with the Murchison Widefield Array

Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the Southern Hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21 cm emission from the epoch of reionisation in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

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arXiv2011

The Stability of Low Surface Brightness Disks Based on Multi-Wavelength Modeling

To investigate the structure and composition of the dusty interstellar medium (ISM) of low surface brightness (LSB) disk galaxies, we have used multiwavelength photometry to construct spectral energy distributions for three low-mass, edge-on LSB galaxies. We use Monte Carlo radiation transfer codes that include the effects of transiently heated small grains and polycyclic aromatic hydrocarbon molecules to model and interpret the data. We find that unlike the high surface brightness galaxies previously modeled, the dust disks appear to have scale heights equal to or exceeding their stellar scale heights. This result supports the findings of previous studies that low mass disk galaxies have dust scale heights comparable to their stellar scale heights and suggests that the cold ISM of low mass, LSB disk galaxies may be stable against fragmentation and gravitational collapse. This may help to explain the lack of observed dust lanes in edge-on LSB galaxies and their low current star formation rates.

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arXiv2011

First Spectroscopic Imaging Observations of the Sun at Low Radio Frequencies with the Murchison Widefield Array Prototype

We present the first spectroscopic images of solar radio transients from the prototype for the Murchison Widefield Array (MWA), observed on 2010 March 27. Our observations span the instantaneous frequency band 170.9-201.6 MHz. Though our observing period is characterized as a period of `low' to `medium' activity, one broadband emission feature and numerous short-lived, narrowband, non-thermal emission features are evident. Our data represent a significant advance in low radio frequency solar imaging, enabling us to follow the spatial, spectral, and temporal evolution of events simultaneously and in unprecedented detail. The rich variety of features seen here reaffirms the coronal diagnostic capability of low radio frequency emission and provides an early glimpse of the nature of radio observations that will become available as the next generation of low frequency radio interferometers come on-line over the next few years.

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arXiv2010

HI and CO in the circumstellar environment of the S-type star RS Cnc

This paper presents interferometric and single-dish CO observations along with HI data obtained for the oxygen-rich semi-regular variable RS Cnc, in order to probe its circumstellar environment at different scales. With the Plateau de Bure Interferometer and the IRAM 30-m telescope, we detect both the CO(1-0) and the CO(2-1) rotational lines from RS Cnc. The line profiles are composite, with two components of half-width ~2 km/s and ~8 km/s respectively. Whereas the narrow velocity component seems to originate from an equatorial disk in the central part of the CO envelope, the broad component reveals a bipolar structure, with a north-south velocity gradient. In addition, we obtain new HI data on the source and around it in a field of almost 1 square degree. The HI line is centered at v_LSR = 7 km/s in agreement with CO observations. A new reduction process reveals a complex extended structure in the northwest direction, of estimated size ~18 arcmin, with a PA (~310 degrees) opposite to the direction of the stellar proper motion (~140 degrees). We derive an HI mass of ~3 10^-2 M_sun for this structure. Based on a non-spherical simulation, we find that this structure is consistent with arising from the interaction of the star undergoing mass loss at an average rate of ~10^-7 M_sun.yr^-1 over ~2-3 10^5 years with the interstellar medium. This work illustrates the powerful complementarity of CO and HI observations with regard to a more complete description of circumstellar environments around AGB stars.

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arXiv2009

Dark matter dominance at all radii in the superthin galaxy UGC 7321

We model the shape and density profile of the dark matter halo of the low surface brightness, superthin galaxy UGC 7321, using the observed rotation curve and the HI scale height data as simultaneous constraints. We treat the galaxy as a gravitationally coupled system of stars and gas, responding to the gravitational potential of the dark matter halo. An isothermal halo of spherical shape with a core density in the range of 0.039 - 0.057 M_sun/pc^3 and a core radius between 2.5 - 2.9 kpc, gives the best fit to the observations for a range of realistic gas parameters assumed. We find that the best-fit core radius is only slightly higher than the stellar disc scale length (2.1 kpc), unlike the case of the high surface brightness galaxies where the halo core radius is typically 3-4 times the disc scale length of the stars. Thus our model shows that the dark matter halo dominates the dynamics of the low surface brightness, superthin galaxy UGC 7321 at all radii, including the inner parts of the galaxy.

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arXiv2007

HI Imaging Observations of Superthin Galaxies. II. IC2233 and the Blue Compact Dwarf NGC2537

We have used the VLA to image the HI 21-cm line emission in the edge-on Sd galaxy IC2233 and the blue compact dwarf NGC2537. We also present new optical B,R, and H alpha imaging of IC2233 obtained with the WIYN telescope. Despite evidence of localized massive star formation, IC2233 has a low surface brightness disk with a low global star formation rate (~0.05 M_sun/yr), and no significant 21-cm radio continuum emission. The HI and ionized gas disks of IC2233 are clumpy and vertically distended, with scale heights comparable to the young stars. Both the stellar and HI disks of IC2233 appear flared, and we also find a vertically extended, rotationally anomalous HI component extending to z~2.4 kpc. The HI disk exhibits a mild lopsidedness as well as a global corrugation pattern with a period of ~7 kpc and an amplitude of ~150 pc. To our knowledge, this is the first time corrugations of the gas disk have been reported in an external galaxy; these undulations may be linked to bending instabilities or to underlying spiral structure and suggest that the disk is largely self-gravitating. Lying at a projected distance of 16.7' from IC2233, NGC2537 has an HI disk with a bright, tilted inner ring and a flocculent, dynamically cold outer region that extends to ~3.5D_25. Although NGC2537 is rotationally-dominated, it shows significant turbulence near its center. The inner rotation curve rises steeply, implying a strong central mass concentration. Our data indicate that IC2233 and NGC2537 do not constitute a bound pair and most likely lie at different distances. We also find no compelling evidence of a recent minor merger in either galaxy, suggesting that both are examples of small disk galaxies evolving in relative isolation. (Abridged)

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arXiv2007

Photometry of Polar-Ring Galaxies

We have obtained photometry in B and R for seven confirmed or probable polar-ring galaxies from the Polar-Ring Catalog of Whitmore et al. (1990). The rings show a range of colors from B-R \approx 0.6 to B-R \approx 1.7. The bluest rings have bright HII regions, which are direct evidence for recent star formation. The minimum age of the reddest ring, that in PRC B-20, is somewhat uncertain because of a lack of knowledge of the internal reddening and metallicity, but appears to be at least 1.2 Gyr. As such, this ring is likely to be stable for at least several rotation periods. This ring is an excellent candidate for future studies that might better determine if it is truly old.

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