arXiv subjects

N. D. Richardson

Publications and source records attributed to N. D. Richardson.

At least 37 records · Page 2Linked to original sources

arXiv2016

VLTI-AMBER velocity-resolved aperture-synthesis imaging of Eta Carinae with a spectral resolution of 12000. Studies of the primary star wind and innermost wind-wind collision

Context. The mass loss from massive stars is not understood well. Eta Car is a unique object for studying the massive stellar wind during the LBV phase. It is also an eccentric binary with a period of 5.54 yr. The nature of both stars is uncertain, although we know from X-ray studies that there is a wind-wind collision whose properties change with orbital phase. Methods. Observations of Eta Car were carried out with the ESO VLTI and the AMBER instrument between approximately five and seven months before the August 2014 periastron passage. Velocity-resolved aperture-synthesis images were reconstructed from the spectrally dispersed interferograms. Interferometric studies can provide information on the binary orbit, the primary wind, and the wind collision. Results. We present velocity-resolved aperture-synthesis images reconstructed in more than 100 different spectral channels distributed across the Br Gamma 2.166 micrometer emission line. The intensity distribution of the images strongly depends on wavelength. At wavelengths corresponding to radial velocities of approximately -140 to -376 km/s measured relative to line center, the intensity distribution has a fan-shaped structure. At the velocity of -277 km/s, the position angle of the symmetry axis of the fan is ~ 126 degree. The fan-shaped structure extends approximately 8.0 mas (~ 18.8 au) to the southeast and 5.8 mas (~ 13.6 au) to the northwest, measured along the symmetry axis at the 16% intensity contour. The shape of the intensity distributions suggests that the obtained images are the first direct images of the innermost wind-wind collision zone. Therefore, the observations provide velocity-dependent image structures that can be used to test three-dimensional hydrodynamical, radiative transfer models of the massive interacting winds of Eta Car.

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arXiv2016

Dead or Alive? Long-term evolution of SN 2015bh (SNhunt275)

Supernova (SN) 2015bh (or SNhunt275) was discovered in NGC 2770 on 2015 February with an absolute magnitude of Mr ~ -13.4 mag, and was initially classified as a SN impostor. Here we present the photometric and spectroscopic evolution of SN 2015bh from discovery to late phases (~ 1 yr after). In addition, we inspect archival images of the host galaxy up to ~ 21 yr before discovery, finding a burst ~ 1 yr before discovery, and further signatures of stellar instability until late 2014. Later on, the luminosity of the transient slowly increases, and a broad light curve peak is reached after about three months. We propose that the transient discovered in early 2015 could be a core-collapse SN explosion. The pre-SN luminosity variability history, the long-lasting rise and faintness first light curve peak suggests that the progenitor was a very massive, unstable and blue star, which exploded as a faint SN because of severe fallback of material. Later on, the object experiences a sudden brightening of 3 mag, which results from the interaction of the SN ejecta with circumstellar material formed through repeated past mass-loss events. Spectroscopic signatures of interaction are however visible at all epochs. A similar chain of events was previously proposed for the similar interacting SN 2009ip.

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arXiv2016

An extensive spectroscopic time-series of three Wolf-Rayet stars. I. The lifetime of large-scale structures in the wind of WR 134

During the summer of 2013, a 4-month spectroscopic campaign took place to observe the variabilities in three Wolf-Rayet stars. The spectroscopic data have been analyzed for WR 134 (WN6b), to better understand its behaviour and long-term periodicity, which we interpret as arising from corotating interaction regions (CIRs) in the wind. By analyzing the variability of the He II $λ$5411 emission line, the previously identified period was refined to P = 2.255 $\pm$ 0.008 (s.d.) days. The coherency time of the variability, which we associate with the lifetime of the CIRs in the wind, was deduced to be 40 $\pm$ 6 days, or $\sim$ 18 cycles, by cross-correlating the variability patterns as a function of time. When comparing the phased observational grayscale difference images with theoretical grayscales previously calculated from models including CIRs in an optically thin stellar wind, we find that two CIRs were likely present. A separation in longitude of $Δϕ\simeq$ 90$^{\circ}$ was determined between the two CIRs and we suggest that the different maximum velocities that they reach indicate that they emerge from different latitudes. We have also been able to detect observational signatures of the CIRs in other spectral lines (C IV $λλ$5802,5812 and He I $λ$5876). Furthermore, a DAC was found to be present simultaneously with the CIR signatures detected in the He I $λ$5876 emission line which is consistent with the proposed geometry of the large-scale structures in the wind. Small-scale structures also show a presence in the wind, simultaneously with the larger scale structures, showing that they do in fact co-exist.

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arXiv2016

Wolf-Rayet stars in the Small Magellanic Cloud: II. Analysis of the binaries

Massive WR stars are evolved massive stars characterized by strong mass-loss. Hypothetically, they can form either as single stars or as mass donors in close binaries. About 40% of the known WR stars are confirmed binaries, raising the question as to the impact of binarity on the WR population. By performing a spectral analysis of all multiple WR systems in the SMC, we obtain the full set of stellar parameters for each individual component. Mass-luminosity relations are tested, and the importance of the binary evolution channel is assessed. The spectral analysis is performed with the PoWR model atmosphere code by superimposing model spectra that correspond to each component. Evolutionary channels are constrained using the BPASS evolution tool. Significant Hydrogen mass fractions (0.1 - 0.4) are detected in all WN components. A comparison with mass-luminosity relations and evolutionary tracks implies that the majority of the WR stars in our sample are not chemically homogeneous. The WR component in the binary AB 6 is found to be very luminous (Log L ~ 6.3 [Lsun]) given its orbital mass (~10 Msun), presumably because of observational contamination by a third component. Evolutionary paths derived for our objects suggest that Roche lobe overflow had occurred in most systems, affecting their evolution. However, the implied initial masses are large enough for the primaries to have entered the WR phase, regardless of binary interaction. Together with the results for the putatively single SMC WR stars, our study suggests that the binary evolution channel does not dominate the formation of WR stars at SMC metallicity.

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arXiv2016

He II $λ$4686 emission from the massive binary system in $η$ Car: constraints to the orbital elements and the nature of the periodic minima

η Carinae is an extremely massive binary system in which rapid spectrum variations occur near periastron. Most notably, near periastron the He II $λ4686$ line increases rapidly in strength, drops to a minimum value, then increases briefly before fading away. To understand this behavior, we conducted an intense spectroscopic monitoring of the He II $λ4686$ emission line across the 2014.6 periastron passage using ground- and space-based telescopes. Comparison with previous data confirmed the overall repeatability of EW(He II $λ4686$), the line radial velocities, and the timing of the minimum, though the strongest peak was systematically larger in 2014 than in 2009 by 26%. The EW(He II $λ4686$) variations, combined with other measurements, yield an orbital period $2022.7\pm0.3$ d. The observed variability of the EW(He II $λ4686$) was reproduced by a model in which the line flux primarily arises at the apex of the wind-wind collision and scales inversely with the square of the stellar separation, if we account for the excess emission as the companion star plunges into the hot inner layers of the primary's atmosphere, and including absorption from the disturbed primary wind between the source and the observer. This model constrains the orbital inclination to $135^\circ$-$153^\circ$, and the longitude of periastron to $234^\circ$-$252^\circ$. It also suggests that periastron passage occurred on $T_0 = 2456874.4\pm1.3$ d. Our model also reproduced EW(He II $λ4686$) variations from a polar view of the primary star as determined from the observed He II $λ4686$ emission scattered off the Homunculus nebula.

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arXiv2015

Characterizing the Rigidly Rotating Magnetosphere Stars HD 345439 and HD 23478

The SDSS III APOGEE survey recently identified two new $σ$ Ori E type candidates, HD 345439 and HD 23478, which are a rare subset of rapidly rotating massive stars whose large (kGauss) magnetic fields confine circumstellar material around these systems. Our analysis of multi-epoch photometric observations of HD 345439 from the KELT, SuperWASP, and ASAS surveys reveals the presence of a $\sim$0.7701 day period in each dataset, suggesting the system is amongst the faster known $σ$ Ori E analogs. We also see clear evidence that the strength of H-alpha, H I Brackett series lines, and He I lines also vary on a $\sim$0.7701 day period from our analysis of multi-epoch, multi-wavelength spectroscopic monitoring of the system from the APO 3.5m telescope. We trace the evolution of select emission line profiles in the system, and observe coherent line profile variability in both optical and infrared H I lines, as expected for rigidly rotating magnetosphere stars. We also analyze the evolution of the H I Br-11 line strength and line profile in multi-epoch observations of HD 23478 from the SDSS-III APOGEE instrument. The observed periodic behavior is consistent with that recently reported by Sikora and collaborators in optical spectra.

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arXiv2015

The X-ray Lightcurve of the Supermassive star eta Carinae, 1996--2014

Eta Carinae is the nearest example of a supermassive, superluminous, unstable star. Mass loss from the system is critical in shaping its circumstellar medium and in determining its ultimate fate. Eta Car currently loses mass via a dense, slow stellar wind and possesses one of the largest mass loss rates known. It is prone to episodes of extreme mass ejection via eruptions from some as-yet unspecified cause; the best examples of this are the large-scale eruptions which occurred in 19th century. Eta Car is a colliding wind binary in which strong variations in X-ray emission and in other wavebands are driven by the violent collision of the wind of eta Car-A and the fast, less dense wind of an otherwise hidden companion star. X-ray variations are the simplest diagnostic we have to study the wind-wind collision and allow us to measure the state of the stellar mass loss from both stars. We present the X-ray lightcurve over the last 20 years from ROSAT observations and monitoring with the Rossi X-ray Timing Explorer and the X-ray Telescope on the Swift satellite. We compare and contrast the behavior of the X-ray emission from the system over that timespan, including surprising variations during the 2014 X-ray minimum.

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arXiv2015

A Coordinated X-ray and Optical Campaign on the Nearest Massive Eclipsing Binary, Delta Ori Aa: I. Overview of the X-ray Spectrum

We present an overview of four phase-constrained Chandra HETGS X-ray observations of Delta Ori A. Delta Ori A is actually a triple system which includes the nearest massive eclipsing spectroscopic binary, Delta Ori Aa, the only such object which can be observed with little phase-smearing with the Chandra gratings. Since the fainter star, Delta Ori Aa2, has a much lower X-ray luminosity than the brighter primary, Delta Ori A provides a unique system with which to test the spatial distribution of the X-ray emitting gas around Delta Ori Aa1 via occultation by the photosphere of and wind cavity around the X-ray dark secondary. Here we discuss the X-ray spectrum and X-ray line profiles for the combined observation, having an exposure time of nearly 500 ksec and covering nearly the entire binary orbit. Companion papers discuss the X-ray variability seen in the Chandra spectra, present new space-based photometry and ground-based radial velocities simultaneous with the X-ray data to better constrain the system parameters, and model the effects of X-rays on the optical and UV spectrum. We find that the X-ray emission is dominated by embedded wind shock emission from star Aa1, with little contribution from the tertiary star Ab or the shocked gas produced by the collision of the wind of Aa1 against the surface of Aa2. We find a similar temperature distribution to previous X-ray spectrum analyses. We also show that the line half-widths are about $0.3-0.5\times$ the terminal velocity of the wind of star Aa1. We find a strong anti-correlation between line widths and the line excitation energy, which suggests that longer-wavelength, lower-temperature lines form farther out in the wind. Our analysis also indicates that the ratio of the intensities of the strong and weak lines of \ion{Fe}{17} and \ion{Ne}{10} are inconsistent with model predictions, which may be an effect of resonance scattering

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arXiv2015

A Coordinated X-ray and Optical Campaign of the Nearby Massive Binary $δ$ Orionis Aa: II. X-ray Variability

We present time-resolved and phase-resolved variability studies of an extensive X-ray high-resolution spectral dataset of the $δ$ Orionis Aa binary system. The four observations, obtained with Chandra ACIS HETGS, have a total exposure time of ~479 ks and provide nearly complete binary phase coverage. Variability of the total X-ray flux in the range 5-25 $Å$ is confirmed, with maximum amplitude of about +/-15% within a single ~125 ks observation. Periods of 4.76d and 2.04d are found in the total X-ray flux, as well as an apparent overall increase in flux level throughout the 9-day observational campaign. Using 40 ks contiguous spectra derived from the original observations, we investigate variability of emission line parameters and ratios. Several emission lines are shown to be variable, including S XV, Si XIII, and Ne IX. For the first time, variations of the X-ray emission line widths as a function of the binary phase are found in a binary system, with the smallest widths at phase=0.0 when the secondary $δ$ Orionis Aa2 is at inferior conjunction. Using 3D hydrodynamic modeling of the interacting winds, we relate the emission line width variability to the presence of a wind cavity created by a wind-wind collision, which is effectively void of embedded wind shocks and is carved out of the X-ray-producing primary wind, thus producing phase-locked X-ray variability.

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arXiv2015

The Expanding Fireball of Nova Delphini 2013

A classical nova occurs when material accreting onto the surface of a white dwarf in a close binary system ignites in a thermonuclear runaway. Complex structures observed in the ejecta at late stages could result from interactions with the companion during the common envelope phase. Alternatively, the explosion could be intrinsically bipolar, resulting from a localized ignition on the surface of the white dwarf or as a consequence of rotational distortion. Studying the structure of novae during the earliest phases is challenging because of the high spatial resolution needed to measure their small sizes. Here we report near-infrared interferometric measurements of the angular size of Nova Delphini 2013, starting from one day after the explosion and continuing with extensive time coverage during the first 43 days. Changes in the apparent expansion rate can be explained by an explosion model consisting of an optically thick core surrounded by a diffuse envelope. The optical depth of the ejected material changes as it expands. We detect an ellipticity in the light distribution, suggesting a prolate or bipolar structure that develops as early as the second day. Combining the angular expansion rate with radial velocity measurements, we derive a geometric distance to the nova of 4.54 +/- 0.59 kpc from the Sun.

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arXiv2015

A coordinated X-ray and Optical Campaign of the Nearest Massive Eclipsing Binary, $δ$ Orionis Aa: IV. A multiwavelength, non-LTE spectroscopic analysis

Eclipsing systems of massive stars allow one to explore the properties of their components in great detail. We perform a multi-wavelength, non-LTE analysis of the three components of the massive multiple system $δ$ Ori A, focusing on the fundamental stellar properties, stellar winds, and X-ray characteristics of the system. The primary's distance-independent parameters turn out to be characteristic for its spectral type (O9.5 II), but usage of the ${\rm \it Hipparcos}$ parallax yields surprisingly low values for the mass, radius, and luminosity. Consistent values follow only if $δ$ Ori lies at about twice the ${\rm \it Hipparcos}$ distance, in the vicinity of the $σ$-Orionis cluster. The primary and tertiary dominate the spectrum and leave the secondary only marginally detectable. We estimate the V-band magnitude difference between primary and secondary to be $ΔV \approx 2.\!\!^{\rm m}8$. The inferred parameters suggest the secondary is an early B-type dwarf ($\approx$ B1 V), while the tertiary is an early B-type subgiant ($\approx$ B0 IV). We find evidence for rapid turbulent velocities ($\sim 200$ km ${\rm s}^{-1}$) and wind inhomogeneities, partially optically thick, in the primary's wind. The bulk of the X-ray emission likely emerges from the primary's stellar wind ($\log L_{\text{X}} / L_{\text{Bol}} \approx -6.85$), initiating close to the stellar surface at $R_0 \sim 1.1\,R_*$. Accounting for clumping, the mass-loss rate of the primary is found to be $\log \dot{M} \approx -6.4\,[M_\odot\,{\rm yr}^{-1}]$, which agrees with hydrodynamic predictions, and provides a consistent picture along the X-ray, UV, optical and radio spectral domains.

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arXiv2014

The Multiplicity of Massive Stars: A High Angular Resolution Survey with the HST Fine Guidance Sensor

We present the results of an all-sky survey made with the Fine Guidance Sensor on Hubble Space Telescope to search for angularly resolved binary systems among the massive stars. The sample of 224 stars is comprised mainly of Galactic O- and B-type stars and Luminous Blue Variables, plus a few luminous stars in the Large Magellanic Cloud. The FGS TRANS mode observations are sensitive to detection of companions with an angular separation between 0."01 and 1."0 and brighter than $\triangle m = 5$. The FGS observations resolved 52 binary and 6 triple star systems and detected partially resolved binaries in 7 additional targets (43 of these are new detections). These numbers yield a companion detection frequency of 29% for the FGS survey. We also gathered literature results on the numbers of close spectroscopic binaries and wider astrometric binaries among the sample, and we present estimates of the frequency of multiple systems and the companion frequency for subsets of stars residing in clusters and associations, field stars, and runaway stars. These results confirm the high multiplicity fraction, especially among massive stars in clusters and associations. We show that the period distribution is approximately flat in increments of log P. We identify a number of systems of potential interest for long term orbital determinations, and we note the importance of some of these companions for the interpretation of the radial velocities and light curves of close binaries that have third companions.

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arXiv2013

The H-band Emitting Region of the Luminous Blue Variable P Cygni: Spectrophotometry and Interferometry of the Wind

We present the first high angular resolution observations in the nearinfrared H-band (1.6 microns) of the Luminous Blue Variable star P Cygni. We obtained six-telescope interferometric observations with the CHARA Array and the MIRC beam combiner. These show that the spatial flux distribution is larger than expected for the stellar photosphere. A two component model for the star (uniform disk) plus a halo (two-dimensional Gaussian) yields an excellent fit of the observations, and we suggest that the halo corresponds to flux emitted from the base of the stellar wind. This wind component contributes about 45% of the H-band flux and has an angular FWHM = 0.96 mas, compared to the predicted stellar diameter of 0.41 mas. We show several images reconstructed from the interferometric visibilities and closure phases, and they indicate a generally spherical geometry for the wind. We also obtained near-infrared spectrophotometry of P Cygni from which we derive the flux excess compared to a purely photospheric spectral energy distribution. The H-band flux excess matches that from the wind flux fraction derived from the two component fits to the interferometry. We find evidence of significant near-infrared flux variability over the period from 2006 to 2010 that appears similar to the variations in the H-alpha emission flux from the wind. Future interferometric observations may be capable of recording the spatial variations associated with temporal changes in the wind structure.

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arXiv2013

A CHARA Array Survey of Circumstellar Disks around Nearby Be-type Stars

We report on a high angular resolution survey of circumstellar disks around 24 northern sky Be stars. The K-band continuum survey was made using the CHARA Array long baseline interferometer (baselines of 30 to 331 m). The interferometric visibilities were corrected for the flux contribution of stellar companions in those cases where the Be star is a member of a known binary or multiple system. For those targets with good uv coverage, we used a four parameter Gaussian elliptical disk model to fit the visibilities and to determine the axial ratio, position angle, K-band photospheric flux contribution, and angular diameter of the disk major axis. For the other targets with relatively limited uv coverage, we constrained the axial ratio, inclination angle, and or disk position angle where necessary in order to resolve the degeneracy between possible model solutions. We also made fits of the ultraviolet and infrared spectral energy distributions to estimate the stellar angular diameter and infrared flux excess of each target. The mean ratio of the disk diameter (measured in K-band emission) to stellar diameter (from SED modeling) is 4.4 among the 14 cases where we reliably resolved the disk emission, a value which is generally lower than the disk size ratio measured in the higher opacity Halpha emission line. We estimated the equatorial rotational velocity from the projected rotational velocity and disk inclination for 12 stars, and most of these stars rotate close to or at the critical rotational velocity.

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arXiv2012

The relationship between gamma Cassiopeiae's X-ray emission and its circumstellar environment

γCas is the prototypical classical Be star and is best known for its variable hard X-ray emission. To elucidate the reasons for this emission, we mounted a multiwavelength campaign in 2010 centered around 4 XMM observations. The observational techniques included long baseline optical interferometry (LBOI), monitoring by an Automated Photometric Telescope and Halpha observations. Because gamma Cas is also known to be in a binary, we measured Halpha radial velocities and redetermined its period as 203.55+/-0.2 days and an eccentricity near zero. The LBOI observations suggest that the star's decretion disk was axisymmetric in 2010, has an inclination angle near 45^o, and a larger radius than previously reported. The Be star began an "outburst" at the beginning of our campaign, made visible by a disk brightening and reddening during our campaign. Our analyses of the new high resolution spectra disclosed many attributes found from spectra obtained in 2001 (Chandra) and 2004 (XMM). As well as a dominant hot 14 keV thermal component, these familiar ones included: (i) a fluorescent feature of Fe K stronger than observed at previous times, (ii) strong lines of N VII and Ne XI lines indicative of overabundances, and (iii) a subsolar Fe abundance from K-shell lines but a solar abundance from L-shell ions. We also found that 2 absorption columns are required to fit the continuum. While the first one maintained its historical average of 1X10^21 cm^-2, the second was very large and doubled to 7.4X10^23 cm^-2 during our X-ray observations. Although we found no clear relation between this column density and orbital phase, it correlates well with the disk brightening and reddening both in the 2010 and earlier observations. Thus, the inference from this study is that much (perhaps all?) of the X-ray emission from this source originates behind matter ejected by gamma Cas into our line of sight.

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arXiv2011

Spectroscopy and Interferometry of Luminous Blue Variables

We report on the preliminary results of a three-year monitoring campaign of Galactic and Magellanic LBVs with both moderate and high resolution spectroscopy. We have collected more than 500 moderate-resolution spectra of 20 stars in the red portion of the optical spectrum, which includes the wind-sensitive transitions of H$α$ and He I 5876 and 6678 Angstroms. We summarize our detailed study of 15 years of high resolution H$α$ spectroscopy of the prototypical luminous blue variable, P Cygni. We report on the discovery of discrete absorption components in P Cygni's H$α$ profile, and we discuss their relationship to structure in the wind. These results are compared to our recent high resolution interferometric imaging of the $H$-band emitting region surrounding the star. We discuss recent observations of $η$ Car, highlighting its unusual recovery from the previous periastron passage. Our results on HDE 326823 indicate that the star has a short period binary orbit, with Roche Lobe overflow onto an unseen massive companion.

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arXiv2011

Recent Results form the CHARA Array

The CHARA Array is a six 1-m telescope optical and near infrared interferometer located at the Mount Wilson Observatory in southern California and operated by the Center for High Angular Resolution Astronomy of Georgia State University. The CHARA Array has been in regular scientific operation since 2005 and now has over 55 publications in the refereed literature, including two in Science and one in Nature. The Array now supports seven beam combiners ranging from 0.5 microns up to 2.3 microns and combing from 2 to 4 beams at a time. An upgrade to a full 6 beam combiner is now underway and fringes with all six telescopes were achieved soon after the meeting.We present some of the more recent results from the CHARA-Array.

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arXiv2010

A Five-year Spectroscopic and Photometric Campaign on the Prototypical alpha Cygni Variable and A-type Supergiant Star Deneb

Deneb is often considered the prototypical A-type supergiant, and is one of the visually most luminous stars in the Galaxy. A-type supergiants are potential extragalactic distance indicators, but the variability of these stars needs to be better characterized before this technique can be considered reliable. We analyzed 339 high resolution echelle spectra of Deneb obtained over the five-year span of 1997 through 2001 as well as 370 Stromgren photometric measurements obtained during the same time frame. Our spectroscopic analysis included dynamical spectra of the H-alpha profile, H-alpha equivalent widths, and radial velocities measured from Si II 6347, 6371. Time-series analysis reveals no obvious cyclic behavior that proceeds through multiple observing seasons, although we found a suspected 40 day period in two, non-consecutive observing seasons. Some correlations are found between photometric and radial velocity data sets, and suggest radial pulsations at two epochs. No correlation is found between the variability of the H-alpha profiles and that of the radial velocities or the photometry. Lucy (1976) found evidence that Deneb was a long period single-lined spectroscopic binary star, but our data set shows no evidence for radial velocity variations caused by a binary companion.

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