arXiv subjects

M. A. Hendry

Publications and source records attributed to M. A. Hendry.

At least 19 recordsLinked to original sources

arXiv2020

Constraining extra dimensions on cosmological scales with LISA future gravitational wave siren data

We investigate the idea that current cosmic acceleration could be the consequence of gravitational leakage into extra dimensions on cosmological scales rather than the result of a non-zero cosmological constant, and consider the ability of future gravitational-wave siren observations to probe this phenomenon and constrain the parameters of phenomenological models of this gravitational leakage. In theories that include additional non-compact spacetime dimensions, the gravitational leakage intro extra dimensions leads to a reduction in the amplitude of observed gravitational waves and thereby a systematic discrepancy between the distance inferred to such sources from GW and EM observations. We investigate the capability of a gravitational space interferometer such as LISA to probe this modified gravity on large scales. We find that the extent to which LISA will be able to place limits on the number of spacetime dimensions and other cosmological parameters characterising modified gravity will strongly depend on the actual number and redshift distribution of sources, together with the uncertainty on the GW measurements. A relatively small number of sources ($\sim 1$) and high measurement uncertainties would strongly restrict the ability of LISA to place meaningful constraints on the parameters in cosmological scenarios where gravity is only five-dimensional and modified at scales larger than about $\sim 4$ times the Hubble radius. Conversely, if the number of sources observed amounts to a four-year average of $\sim 27$, then in the most favourable cosmological scenarios LISA has the potential to place meaningful constraints on the cosmological parameters with a precision of $\sim 1\%$ on the number of dimensions and $\sim 7.5\%$ on the scale beyond which gravity is modified, thereby probing the late expansion of the universe up to a redshift of $\sim 8$.

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arXiv2008

The blue supergiant Sher 25 and its intriguing hourglass nebula

The blue supergiant Sher 25 is surrounded by an asymmetric, hourglass-shaped circumstellar nebula. Its structure and dynamics have been studied previously through high-resolution imaging and spectroscopy, and it appears dynamically similar to the ring structure around SN 1987A. Here we present long-slit spectroscopy of the circumstellar nebula around Sher 25, and of the background nebula of the host cluster NGC 3603. We perform a detailed nebular abundance analysis to measure the gas-phase abundances of oxygen, nitrogen, sulphur, neon and argon. The oxygen abundance in the circumstellar nebula (12 + log[O/H] = 8.61 +/- 0.13 dex) is similar to that in the background nebula (8.56 +/- 0.07), suggesting the composition of the host cluster is around solar. However, we confirm that the circumstellar nebula is very rich in nitrogen, with an abundance of 8.91 +/- 0.15, compared to the background value of 7.47 +/- 0.18. A new analysis of the stellar spectrum with the FASTWIND model atmosphere code suggests that the photospheric nitrogen and oxygen abundances in Sher 25 are consistent with the nebular results. While the nitrogen abundances are high, when compared to stellar evolutionary models they do not unambiguously confirm that the star has undergone convective dredge-up during a previous red supergiant phase. We suggest that the more likely scenario is that the nebula was ejected from the star while it was in the blue supergiant phase. The star's initial mass was around 50 M_sun, which is rather too high for it to have had a convective envelope stage as a red supergiant. Rotating stellar models that lead to mixing of core-processed material to the stellar surface during core H-burning can quantitatively match the stellar results with the nebula abundances.

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arXiv2006

SN 2004A: Another Type II-P Supernova with a Red Supergiant Progenitor

We present a monitoring study of SN 2004A and probable discovery of a progenitor star in pre-explosion HST images. The photometric and spectroscopic monitoring of SN 2004A show that it was a normal Type II-P which was discovered in NGC 6207 about two weeks after explosion. We compare SN 2004A to the similar Type II-P SN 1999em and estimate an explosion epoch of 2004 January 6. We also calculate three new distances to NGC 6207 of 21.0 +/-4.3, 21.4 +/-3.5 and 25.1 +/-1.7Mpc. The former was calculated using the Standard Candle Method (SCM) for SNe II-P, and the latter two from the Brightest Supergiants Method (BSM). We combine these three distances with existing kinematic distances, to derive a mean value of 20.3 +/-3.4Mpc. Using this distance we estimate that the ejected nickel mass in the explosion is 0.046(+0.031,-0.017) Msolar. The progenitor of SN 2004A is identified in pre-explosion WFPC2 F814W images with a magnitude of mF814W = 24.3 +/-0.3, but is below the detection limit of the F606W images. We show that this was likely a red supergiant (RSG) with a mass of 9(+3,-2) Msolar. The object is detected at 4.7 sigma above the background noise. Even if this detection is spurious, the 5 sigma upper limit would give a robust upper mass limit of 12 Msolar for a RSG progenitor. These initial masses are very similar to those of two previously identified RSG progenitors of the Type II-P SNe 2004gd 8(+4,-2) Msolar and 2005cs 9(+3,-2) Msolar).

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arXiv2005

Microlensing of Circumstellar Envelopes. III. Line profiles from stellar winds in homologous expansion

This paper examines line profile evolution due to the linear expansion of circumstellar material obsverved during a microlensing event. This work extends our previous papers on emission line profile evolution from radial and azimuthal flow during point mass lens events and fold caustic crossings. Both ''flavours'' of microlensing were shown to provide effective diagnostics of bulk motion in circumstellar envelopes. In this work a different genre of flow is studied, namely linear homologous expansion, for both point mass lenses and fold caustic crossings. Linear expansion is of particular relevance to the effects of microlensing on supernovae at cosmological distances. We derive line profiles and equivalent widths for the illustrative cases of pure resonance and pure recombination lines, modelled under the Sobolev approximation. The efficacy of microlensing as a diagnostic probe of the stellar environs is demonstrated and discussed.

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arXiv2005

Determination of Cepheid parameters by light-curve template-fitting

We describe techniques to characterise the light-curves of regular variable stars by applying principal component analysis (PCA) to a training set of high quality data, and to fit the resulting light-curve templates to sparse and noisy photometry to obtain parameters such as periods, mean magnitudes etc. The PCA approach allows us to efficiently represent the multi-band light-curve shapes of each variable, and hence quantitatively describe the average behaviour of the sample as a smoothly varying function of period, and also the range of variation around this average. In this paper we focus particularly on the utility of such methods for analysing HST Cepheid photometry, and present simulations which illustrate the advantages of our PCA template-fitting approach. These are: accurate parameter determination, including light-curve shape information; simultaneous fitting to multiple passbands; quantitative error analysis; objective rejection of variables with non Cepheid-like light-curves or those with potential period aliases. We also use PCA to confirm that Cepheid light-curve shapes are systematically different (at the same period) between the Milky Way (MW) and the Large and Small Magellanic Clouds (LMC, SMC), and consider whether light-curve shape might therefore be used to estimate the mean metallicities of Cepheid samples, thus allowing metallicity corrections to be applied to derived distance estimates.

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arXiv2005

The VLT-FLAMES Survey of Massive Stars: Observations in the Galactic Clusters NGC 3293, NGC 4755 and NGC 6611

We introduce a new survey of massive stars in the Galaxy and the Magellanic Clouds using the Fibre Large Array Multi-Element Spectrograph (FLAMES) instrument at the Very Large Telescope (VLT). Here we present observations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph (R ~ 25,000), in fields centered on the open clusters NGC 3293, NGC 4755 and NGC 6611. These data are supplemented by a further 50 targets observed with the Fibre-Fed Extended Range Optical Spectrograph (FEROS, R = 48,000). Following a description of our scientific motivations and target selection criteria, the data reduction methods are described; of critical importance the FLAMES reduction pipeline is found to yield spectra that are in excellent agreement with less automated methods. Spectral classifications and radial velocity measurements are presented for each star, with particular attention paid to morphological peculiarities and evidence of binarity. These observations represent a significant increase in the known spectral content of NGC 3293 and NGC 4755, and will serve as standards against which our subsequent FLAMES observations in the Magellanic Clouds will be compared.

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arXiv2005

A Study of the Type II-P Supernova 2003gd in M74

We present photometric and spectroscopic data of the type II-P supernova 2003gd, which was discovered in M74 close to the end of its plateau phase. SN 2003gd is the first type II supernova to have a directly confirmed red supergiant progenitor. We compare SN 2003gd with SN 1999em, a similar type II-P supernova, and estimate an explosion date of 18th March 2003. We determine a reddening towards the supernova of E(B-V) = 0.14+/-0.06, using three different methods. We also calculate three new distances to M74 of 9.6+/-2.8 Mpc, 7.7+/-1.7 Mpc and 9.6+/-2.2 Mpc. The former was estimated using the Standardised Candle Method (SCM), for type II supernovae, and the latter two using the Brightest Supergiants Method (BSM). When combined with existing kinematic and BSM distance estimates, we derive a mean value of 9.3+/-1.8 Mpc. SN 2003gd was found to have a lower tail luminosity compared to other ``normal'' type II-P SNe bringing into question the nature of this supernova. We present a discussion concluding that this is a ``normal'' type II-P supernova which is consistent with the observed progenitor mass of 8(+4/-2) Mo.

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arXiv2004

Detection of a red supergiant progenitor star of a type II-plateau supernova

We present the discovery of a red supergiant star that exploded as supernova 2003gd in the nearby spiral galaxy M74. The Hubble Space Telescope (HST) and the Gemini Telescope imaged this galaxy 6 to 9 months before the supernova explosion and subsequent HST images confirm the positional coincidence of the supernova with a single,resolved star which is an 8 +4/-2 solar mass red supergiant. This confirms both stellar evolution models and supernova theories which predict that type II-Plateau supernovae have cool red supergiants as their immediate progenitor stars.

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arXiv2003

The extra-galactic Cepheid distance scale from LMC and Galactic period-luminosity relations

In this paper, we recalibrate the Cepheid distance to some nearby galaxies observed by the HST Key Project (KP) and the Sandage-Tammann-Saha (STS) group. We use much of the KP methodology in our analysis but apply new techniques, based on Fourier methods to estimate the mean magnitudes to published Cepheid data. We also apply different calibrating PL relations to estimate Cepheid distances, and investigate the sensitivity of the distance moduli to the adopted calibrating PL relation. We re-determine the OGLE LMC PL relations using a more conservative approach and also study the effect of using Galactic PL relations on the distance scale. For the KP galaxies, we find good agreement with an average discrepancy of -0.002 and 0.075mag. when using the LMC and Galaxy, respectively, as a calibrating PL relation. However, the Cepheid distance to STS galaxies are shorter by -0.178mag. and -0.108mag. on average again when using the LMC and Galaxy PL relations, respectively. We also calculate the distance to these galaxies using PL relations at maximum light and find very good agreement with mean light PL distances. However, after correcting for metallicity effects, the difference between the distance moduli obtained using the two sets of calibrating PL relations becomes negligible. This suggests that Cepheids in the LMC and Galaxy do follow different PL relations, at least at the median periods of the target galaxies.

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arXiv2002

The microlensing signatures of starspots

Point lens microlensing events with impact parameter close to the source stellar radius allow the observer to study the surface brightness profile of the lensed source. We have examined the effect of photospheric star spots on multicolour microlensing lightcurves and investigated the detectability of such spots in different wavebands as a function of spot temperature, position, radius and lens trajectories. We include the effect of limb darkening and spot projection as a function of position on the stellar disc. In particular we apply the updated, state-of-the-art NEXTGEN stellar atmosphere models of Hauschildt et al which predict very strong limb darkening and which are likely to be applicable to the source stars considered here. Our results indicate that star spots generally give a clear signature for transit events. Moreover, this signature is strongly suppressed by limb darkening for spots close to the limb, although the spots may still be clearly detected for favourable lens trajectories. It is also clear that intensive temporal sampling throughout the duration of the transit is necessary in order for such events to be effective as a tool for imaging stellar photospheres. Nonetheless, with sufficiently well sampled light curves of good photometric precision, microlensing can indeed place useful constraints on the presence or otherwise of photospheric starspots.

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arXiv2002

Gravitational microlensing as a test of stellar model atmospheres

We present calculations illustrating the potential of gravitational microlensing to discriminate between classical models of stellar surface brightness profiles and the recently computed ``Next Generation'' models of Hauschildt et al. These spherically-symmetric models include a much improved treatment of molecular lines in the outer atmospheres of cool giants -- stars which are very typical sources in Galactic bulge microlensing events. We show that the microlensing signatures of intensively monitored point and fold caustic crossing events are readily able to distinguish between NextGen and the classical models, provided a photometric accuracy of 0.01 magnitudes is reached. This accuracy is now routinely achieved by alert networks, and hence current observations can discriminate between such model atmospheres, providing a unique insight on stellar photospheres.

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arXiv2001

On the use of Principal Component Analysis in analysing Cepheid light curves

We show how Principal Component Analysis can be used to analyse the structure of Cepheid light curves. This method is more efficient than Fourier analysis at bringing out changes in light curve shape as a function of period. Using this technique, we study the shape of fundamental and first overtone mode Cepheid light curves in the Galaxy, LMC and SMC over a wide period range. For fundamentals, we find evidence for structural changes at $\log P \approx 1.55, 2.1$. It is suggested that the feature at $\log P \approx 2.1$ is associated with a resonance in the Cepheid normal mode spectrum. For overtones, we recover the Z shape in the $R_{21}$ period plane and reproduce the metallicity dependence of this Z shape.

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arXiv2000

Analytic Inversion of Emission Lines of Arbitrary Optical Depth for the Structure of Supernova Ejecta

We derive a method for inverting emission line profiles formed in supernova ejecta. The derivation assumes spherical symmetry and homologous expansion (i.e., $v(r) \propto r$), is analytic, and even takes account of occultation by a pseudo-photosphere. Previous inversion methods have been developed which are restricted to optically thin lines, but the particular case of homologous expansion permits an analytic result for lines of {\it arbitrary} optical depth. In fact, we show that the quantity that is generically retrieved is the run of line intensity $I_λ$ with radius in the ejecta. This result is quite general, and so could be applied to resonance lines, recombination lines, etc. As a specific example, we show how to derive the run of (Sobolev) optical depth $τ_λ$ with radius in the case of a pure resonance scattering emission line.

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arXiv2000

The radio luminosity of persistent X-ray binaries

We summarise all the reported detections of, and upper limits to, the radio emission from persistent (i.e. non-transient) X-ray binaries. A striking result is a common mean observed radio luminosity from the black hole candidates (BHCs) in the Low/Hard X-ray state and the neutron-star Z sources on the horizontal X-ray branch. This implies a common mean intrinsic radio luminosity to within a factor of twenty five (or less, if there is significant Doppler boosting of the radio emission). Unless coincidental, these results imply a physical mechanism for jet formation which requires neither a black hole event horizon or a neutron star surface. As a whole the populations of Atoll and X-ray pulsar systems are less luminous by factors of >5 and >10 at radio wavelengths than the BHCs and Z sources (while some Atoll sources have been detected, no high-field X-ray pulsar has ever been reliably detected as a radio source). We suggest that all of the persistent BHCs and the Z sources generate, at least sporadically, an outflow with physical dimensions > 1e12cm, i.e. significantly larger than the binary separations of most of the systems. We compare the physical conditions of accretion in each of the types of persistent X-ray binary and conclude that a relatively low (<1e10 G) magnetic field associated with the accreting object, and a high (>0.1 Eddington) accretion rate and/or dramatic physical change in the accretion flow, are required for formation of a radio-emitting outflow or jet.

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arXiv1999

A robust method for measuring the Hubble parameter

We obtain a robust, non-parametric, estimate of the Hubble constant from galaxy linear diameters calibrated using HST Cepheid distances. Our method is independent of the parametric form of the diameter function and the spatial distribution of galaxies and is insensitive to Malmquist bias. We include information on the galaxy rotation velocities; unlike Tully-Fisher, however, we retain a fully non-parametric treatment. We find $H_0=66\pm6$ km/s/Mpc, somewhat larger than previous results using galaxy diameters.

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arXiv1998

The Polarisation Signatures of Microlensing

It has already been shown that microlensing can give rise to a non-zero variable polarisation signal. Here we use realistic simulations to demonstrate the additional information that can be gained from polarimetric observations of lensing events.

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arXiv1998

Microlensing of Extended Stellar Sources

We investigate the feasibility of reconstructing the radial intensity profile of extended stellar sources by inverting their microlensed light curves. Using a simple, linear, limb darkening law as an illustration, we show that the intensity profile can be accurately determined, at least over the outer part of the stellar disc, with realistic light curve sampling and photometric errors. The principal requirement is that the impact parameter of the lens be less than or equal to the stellar radius. Thus, the analysis of microlensing events provides a powerful method for testing stellar atmosphere models.

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arXiv1998

Microlensing of Circumstellar Envelopes I. Simplified considerations for diagnosing radial and azimuthal flow

This paper presents first results on the line profile shapes from a circumstellar envelope in bulk motion as modified by a microlensing event. Only geometrically and optically thin spherical shells in uniform expansion or rotation are considered here so as to emphasise the information content available in the profile shapes. In particular it is demonstrated that for the case of expansion, the line emission can increase by significant factors and the time variation of the profile shape is symmetric about line centre. For uniform rotation the line emission also increases significantly, but the time evolution of the profile shape is distinctly asymmetric. Thus, microlensing is seen to yield information about the velocity field in the extended envelope. We elaborate on (a) the observational advantages of tailoring microlensing programs toward detecting extended circumstellar envelopes, (b) the use of multiline observations to infer other properties of the envelopes, such as the ionization stratification, (c) the use of the continuum excess emission at infrared wavelengths as a means of probing the envelope structure, and (d) the use of polarisation for constraining the properties of ``clumpy'' winds.

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