arXiv subjects

Daniel V. Cotton

Publications and source records attributed to Daniel V. Cotton.

At least 19 recordsLinked to original sources

arXiv2026

Variable Star Polarimetry with PICSARR-2

We describe the upgraded Polarimeter using Imaging CMOS Sensor and Rotating Retarder 2 (PICSARR-2), describe its applications, and characterize its performance for stellar polarimetry on a 36-inch and 14-inch telescope. On the larger telescope in the SDSS $g^\prime$, $r^\prime$ and $i^\prime$ filters a precision of $σ_p=$ 5.7 ppm on bright stars is recorded using a fast modulation rate corresponding to frame exposures of 12 ms; accounting for the internal errors in the individual observations gives a limiting precision of $e_p=$ 1.3 ppm. Longer frame rates are required for stars with $m > 5$, but the recorded errors only underperform a photon shot noise derived extrapolation for $m > 8$, when even longer frame exposures are required. Stars as faint as $m = 11$ were observed. The position angle precision is measured as 0.0845 degrees, and there is very good agreement between observations made by both the PICSARR-2 and HIPPI-2 polarimeters. On the smaller telescope the instrument's performance approaches similar levels, and there is good cross-platform stability in the observation of standard stars. PICSARR-2 is therefore an excellent instrument to explore stellar variability due to a range of phenomena; examples from our ongoing pulsating star campaign and other variable star programs are presented.

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arXiv2026

Size, shape, density, and atmospheric limit of (50000) Quaoar revealed from 14 years of stellar occultation

We present results from 28 stellar occultations by the large Trans-Neptunian Object (50000) Quaoar registered between 2018 and 2025. By performing a joint analysis of this occultation data-set, along with other 9 published events, we were able to fit an oblate ellipsoid shape, with equatorial semi-axes, a and b of 566.1+2.5-2.2 km, and a polar semi-axis, c, of 511.2+3.6-3.7 km. It provides an equivalent volumetric diameter of 1094.4 +/- 4.6 km and polar oblateness of 0.097 +/- 0.011. Considering an absolute magnitude of H = 2.79 +/- 0.35, we derive a geometric albedo of pV = 0.125 +/- 0.038. We have derived new upper limits to the surface pressure of a CH4 atmosphere of 0.15 nbar (1-sigma) and 0.65 nbar (3-sigma). We also provide a table with the 36 new astrometric positions for Quaoar. Using the new system mass derived from Weywot's orbit around Quaoar, we calculated a density of 1.760 +/- 0.109 g/cm3. Moreover, from the derived size and rotation period (8.8394 +/- 0.0002 hours (Ortiz et al. 2003)), we calculate that, if Quaoar is in Maclaurin hydrostatic equilibrium state, it would have a density of 1.859 +/- 0.200 g/cm3. This result, within the error bars, is compatible with the value we found. Therefore, this work shows that Quaoar can be a Maclaurin object, being eligible as a dwarf planet.

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arXiv2026

One Hundred Years of Venus Polarimetry: PICSARR Observations of the Phase Curves

We report new high-precision observations of the polarization of light scattered from the atmosphere of Venus, made 100 years after the pioneering studies by Bernard Lyot. The new observations include disk-integrated observations in a range of filters as well as imaging polarimetry. We compare the new results with past observations and models. We have reproduced the 1974 modelling of the Venus polarization by Hansen and Hovenier using modern radiative transfer codes. We show that the new models are in good agreement with the originals, and enable us to calculate the polarization for wavelengths not covered by the original study and to model the polarization distribution across the disk. The new observations are in good agreement with past determinations of the size distribution of the predominant particle mode. They agree with past studies in showing variability of the phase curve between synodic cycles and also polarization variability on short timescales, particularly at higher phase angles (crescent phases). Imaging polarimetry observations show good agreement with models for the redder wavelengths. However, observations in the ultraviolet show very different polarization behavior in the polar regions (within about 30 degrees of the north and south poles). The simplest explanation of this result is that there is a larger Rayleigh scattering component in the polar regions than in the equatorial and mid-latitudes and this could be explained by a lower cloud-top height in agreement with previous spacecraft observations. These ultraviolet polarization observations are inconsistent with horizontally homogeneous atmospheric models.

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arXiv2024

Polarization position angle standard stars: a reassessment of $θ$ and its variability for seventeen stars based on a decade of observations

Observations of polarization position angle ($θ$) standards made from 2014 to 2023 with the High Precision Polarimetric Instrument (HIPPI) and other HIPPI-class polarimeters in both hemispheres are used to investigate their variability. Multi-band data were first used to thoroughly recalibrate the instrument performance by bench-marking against carefully selected literature data. A novel Co-ordinate Difference Matrix (CDM) approach - which combines pairs of points - was then used to amalgamate monochromatic ($g^\prime$ band) observations from many observing runs and re-determine $θ$ for 17 standard stars. The CDM algorithm was then integrated into a fitting routine and used to establish the impact of stellar variability on the measured position angle scatter. The approach yields variability detections for stars on long time scales that appear stable over short runs. The best position angle standards are $\ell$ Car, $o$ Sco, HD 154445, HD 161056 and $ι^1$ Sco which are stable to $\leq$ 0.123$^\circ$. Position angle variability of 0.27-0.82$^\circ$, significant at the 3-$σ$ level, is found for 5 standards, including the Luminous Blue Variable HD 160529 and all but one of the other B/A-type supergiants (HD 80558, HD 111613, HD 183143 and 55 Cyg), most of which also appear likely to be variable in polarization magnitude ($p$) - there is no preferred orientation for the polarization in these objects, which are all classified as $α$ Cygni variables. Despite this we make six key recommendations for observers - relating to data acquisition, processing and reporting - that will allow them to use these standards to achieve $<$ 0.1$^\circ$ precision in the telescope position angle with similar instrumentation, and allow data sets to be combined more accurately.

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arXiv2024

Epsilon Sagittarii: An Extreme Rapid Rotator with a Decretion Disk

We report high-precision multi-wavelength linear-polarization observations of the bright B9 (or A0) star $ε$ Sagittarii. The polarization shows the distinctive wavelength dependence expected for a rapidly rotating star. Analysis of the polarization data reveals an angular rotation rate $ω$ (= $Ω/Ω_{crit})$ of 0.995 or greater, the highest yet measured for a star in our galaxy. An additional wavelength-independent polarization component is attributed to electron scattering in a low-density edge-on gas disk that also produces the narrow absorption components seen in the spectrum. Several properties of the star (polarization due to a disk, occasional weak H$α$ emission, and multiple periodicities seen in space photometry) resemble those of Be stars, but the level of activity in all cases is much lower than that of typical Be stars. The stellar properties are inconsistent with single rotating-star evolutionary tracks, indicating that it is most likely a product of binary interaction. The star is an excellent candidate for observation by interferometry, optical spectropolarimetry to detect the Öhman effect, and UV polarimetry; any of which would allow its extreme rotation to be tested and its stellar properties to be refined.

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arXiv2024

Deneb is a Large Amplitude Polarimetric Variable

We write to report the discovery that Deneb is a large amplitude polarization variable. Over a ~400 d time span from August 2022 Deneb's polarization was typically around 3900 parts-per-million (ppm) in the SDSS g'-band. Yet, it varied by several hundred ppm in an irregular way on a timescale of weeks. The largest polarization change, amounting to 2500 ppm, occurred shortly after the last pulsation ``resumption'' event identified by Abt et al. (2023) in TESS photometry. The relationship between the observed polarization -- particularly corresponding to the resumption event -- and its brightness and H-alpha spectra suggests a mechanism involving density changes in its wind and/or extended atmosphere. Smaller effects due to pulsations are not ruled out and further study is recommended.

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arXiv2024

Rapid polarization variations in the O4 supergiant $ζ$ Puppis

We present time-series linear-polarization observations of the bright O4 supergiant $ζ$ Puppis. The star is found to show polarization variation on timescales of around an hour and longer. Many of the observations were obtained contemporaneously with Transiting Exoplanet Survey Satellite (TESS) photometry. We find that the polarization varies on similar timescales to those seen in the TESS light-curve. The previously reported 1.78-day photometric periodicity is seen in both the TESS and polarization data. The amplitude ratio of photometry to polarization is ~9 for the periodic component and the polarization variation is oriented along position angle ~70 deg-160 deg. Higher-frequency stochastic variability is also seen in both datasets with an amplitude ratio of ~19 and no preferred direction. We model the polarization expected for a rotating star with bright photospheric spots and find that models that fit the photometric variation produce too little polarization variation to explain the observations. We suggest that the variable polarization is more likely the result of scattering from the wind, with corotating interaction regions producing the periodic variation and a clumpy outflow producing the stochastic component. The H$α$ emission line strength was seen to increase by 10% in 2021 with subsequent observations showing a return to the pre-2018 level.

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arXiv2023

Near-UV and optical spectroscopic investigation of late-type stars from MIRA/Oliver Observing Station

Late-type stars are the most abundant in the galactic stellar population. These stars, with a similar internal structure to the Sun, are expected to have solar-like atmospheres. Investigating the stellar parameters and chemical abundances on late-type stars is essential to provide valuable constraints about stellar age, chemical evolution, and atmosphere of exoplanets. In this work, we present the study of the Near-UV and optical spectroscopic observation of three late-type stars: HR 8038, AC Her, and HD 76446, as obtained from the 36-inch MIRA/Oliver Observing Station. We derived surface temperature, gravity, metallicity, and the chemical abundances of light element Carbon in the stellar atmosphere. The elemental abundance of the Carbon for HR 8038, AC Her, and HD 76446 are derived to be 95%, 97%, and 108%, respectively, of the solar value.

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arXiv2023

Multi-wavelength aperture polarimetry of debris disc host stars

Debris discs around main sequence stars have been extensively characterised from infrared to millimetre wavelengths through imaging, spectroscopic, and total intensity (scattered light and/or thermal emission) measurements. Polarimetric observations have only been used sparingly to interpret the composition, structure, and size of dust grains in these discs. Here we present new multi-wavelength aperture polarisation observations with parts-per-million sensitivity of a sample of twelve bright debris discs, spanning a broad range of host star spectral types, and disc properties. These measurements were mostly taken with the HIgh Precision Polarimetric Instrument on the Anglo-Australian Telescope. We combine these polarisation observations with the known disc architectures and geometries of the discs to interpret the measurements. We detect significant polarisation attributable to circumstellar dust from HD 377 and HD 39060, and find tentative evidence for HD 188228 and HD 202628.

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arXiv2023

A study of the rapid rotator $ζ$ Aql: differential surface rotation?

We report new, extremely precise, photopolarimetry of the rapidly-rotating A0 main-sequence star $ζ$ Aql, covering the wavelength range $\sim$400--900nm, which reveals a rotationally-induced signal. We model the polarimetry, together with the flux distribution and line profiles, in the framework of Roche geometry with $ω$-model gravity darkening, to establish the stellar parameters. An additional constraint is provided by TESS photometry, which shows variability with a period, $P_{\rm phot}$, of 11.1 hr. Modelling based on solid-body surface rotation gives rotation periods, $P_{\rm rot}$, that are in only marginal agreement with this value. We compute new ESTER stellar-structure models to predict horizontal surface velocity fields, which depart from solid-body rotation at only the $\sim$2% level (consistent with a reasonably strong empirical upper limit on differential rotation derived from the line-profile analysis). These models bring the equatorial rotation period, $P_{\rm rot,e}$, into agreement with $P_{\rm phot}$, without requiring any 'fine tuning' (for the Gaia parallax). We confirm that surface abundances are significantly subsolar ($\mbox{[M/H]} \simeq -0.5$). The star's basic parameters are established with reasonably good precision: ${M = 2.53\pm0.16\,\mbox{M}_\odot}$, $\log{L/\mbox{L}_\odot} = 1.72\pm0.02$, $R_{\rm p} = 2.21\pm 0.02\,\mbox{R}_\odot$, $T_{\rm eff} = 9693 \pm 50~\mbox{K}$, $i = 85{^{+5}_{-7}}^\circ$, and $ω/ω_{\rm c} = 0.95\pm0.02$. Comparison with single-star, solar-abundance stellar-evolution models incorporating rotational effects shows excellent agreement (but somewhat poorer agreement for models at $\mbox{[M/H]} \simeq -0.4$).

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arXiv2023

PICSARR: high-precision polarimetry using CMOS image sensors

We have built and tested a compact, low-cost, but very-high-performance astronomical polarimeter based on a continuously rotating half-wave plate and a high-speed imaging detector. The polarimeter is suitable for small telescopes up to ~1 m in aperture. The optical system provides very high transmission over a wide wavelength range from the atmospheric UV cutoff to ~1000 nm. The high-quantum-efficiency, low-noise and high-speed of the detectors enable bright stars to be observed with high-precision as well as polarization imaging of extended sources. We have measured the performance of the instrument on 20 cm and 60 cm aperture telescopes. We show some examples of the type of science possible with this instrument. The polarimeter is particularly suited to studies of the wavelength dependence and time variability of the polarization of stars and planets.

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arXiv2022

Polarimetric detection of nonradial oscillation modes in the beta Cephei star beta Crucis

Here we report the detection of polarization variations due to nonradial modes in the beta Cephei star beta Crucis. In so doing we confirm 40-year-old predictions of pulsation-induced polarization variability and its utility in asteroseismology for mode identification. In an approach suited to other beta Cep stars, we combine polarimetry with space-based photometry and archival spectroscopy to identify the dominant nonradial mode in polarimetry, f2, as l = 3, m = -3 (in the m-convention of Dziembowski) and determine the stellar axis position angle as 25 (or 205) +/- 8 deg. The rotation axis inclination to the line of sight was derived as approx. 46 deg. from combined polarimetry and spectroscopy, facilitating identification of additional modes and allowing for asteroseismic modelling. This reveals a star of 14.5 +/- 0.5 Solar masses and a convective core containing approx. 28% of its mass -- making beta Crucis the most massive star with an asteroseismic age.

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arXiv2022

A study of the F-giant star $θ$ Scorpii A: a post-merger rapid rotator?

We report high-precision observations of the linear polarization of the F1$\,$III star $θ$ Scorpii. The polarization has a wavelength dependence of the form expected for a rapid rotator, but with an amplitude several times larger than seen in otherwise similar main-sequence stars. This confirms the expectation that lower-gravity stars should have stronger rotational-polarization signatures as a consequence of the density dependence of the ratio of scattering to absorption opacities. By modelling the polarization, together with additional observational constraints (incorporating a revised analysis of Hipparcos astrometry, which clarifies the system's binary status), we determine a set of precise stellar parameters, including a rotation rate $ω\, (= Ω/Ω_{\rm c})\ge 0.94$, polar gravity $\log{g_p} = 2.091 ^{+0.042}_{-0.039}$ (dex cgs), mass $3.10 ^{+0.37}_{-0.32}$ solar masses, and luminosity $\log(L/Lsun) =3.149^{+0.041}_{-0.028}$. These values are incompatible with evolutionary models of single rotating stars, with the star rotating too rapidly for its evolutionary stage, and being undermassive for its luminosity. We conclude that $θ$ Sco A is most probably the product of a binary merger.

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arXiv2021

Near-Infrared atmospheric modelling of Jupiter's South Equatorial Belt (SEB) observed with AAT/IRIS2

Near-Infrared spectra of Jupiter's South Equatorial Belt (SEB) with AAT/IRIS2 in H and K bands at a resolving power of R~2400 have been obtained. By creating line-by-line radiative transfer models with the latest improved spectral line data for ammonia and methane (HITRAN2016), we derive best models of cloud/haze parameters in Jupiter's South Equatorial Belt. The modelled spectra fit the observations well except for small, isolated discrepancies in the trough region of H2-H2 collision-induced-absorption around 2.08 μm and the methane absorption level between 2.16 and 2.19 μm in K band and at the high pressure methane window between 1.596 to 1.618 μm in H band.

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arXiv2021

Polarization of hot Jupiter systems: a likely detection of stellar activity and a possible detection of planetary polarization

We present high-precision linear polarization observations of four bright hot Jupiter systems ($τ$ Boo, HD 179949, HD 189733 and 51 Peg) and use the data to search for polarized reflected light from the planets. The data for 51 Peg are consistent with a reflected light polarization signal at about the level expected with 2.8$σ$ significance and a false alarm probability of 1.9 per cent. More data will be needed to confirm a detection of reflected light in this system. HD 189733 shows highly variable polarization that appears to be most likely the result of magnetic activity of the host star. This masks any polarization due to reflected light, but a polarization signal at the expected level of $\sim$20 ppm cannot be ruled out. $τ$ Boo and HD 179949 show no evidence for polarization due to reflected light. The results are consistent with the idea that many hot Jupiters have low geometric albedos. Conclusive detection of polarized reflected light from hot Jupiters is likely to require further improvements in instrument sensitivity.

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arXiv2020

Polarimetric and radiative transfer modelling of HD 172555

The debris disc around HD 172555 was recently imaged in near-infrared polarised scattered light by the Very Large Telescope's Spectro-Polarimetric High-contrast Exoplanet REsearch instrument. Here we present optical aperture polarisation measurements of HD 172555 by the HIgh Precision Polarimetric Instrument (HIPPI), and its successor HIPPI-2 on the Anglo-Australian Telescope. We seek to refine constraints on the disc's constituent dust grains by combining our polarimetric measurements with available infrared and millimetre photometry to model the scattered light and continuum emission from the disc. We model the disc using the 3D radiative transfer code Hyperion, assuming the orientation and extent of the disc as obtained from the SPHERE observation. After correction for the interstellar medium contribution, our multi-wavelength HIPPI/-2 observations (both magnitude and orientation) are consistent with the recent SPHERE polarisation measurement with a fractional polarisation $p = 62.4 \pm 5.2$~ppm at 722.3 nm, and a position angle $θ= 67 \pm 3^{\circ}$. The multi-wavelength polarisation can be adequately replicated by compact, spherical dust grains (i.e. from Mie theory) that are around 1.2 $μ$m in size, assuming astronomical silicate composition, or 3.9 $μ$m assuming a composition derived from radiative transfer modelling of the disc. We were thus able to reproduce both the spatially resolved disc emission and polarisation with a single grain composition model and size distribution.

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arXiv2020

Phase-locked polarization by photospheric reflection in the semidetached eclipsing binary $μ^1$ Sco

We report the detection of phase-locked polarization in the bright ($m_V$=2.98-3.24) semidetached eclipsing binary $μ^1$ Sco (HD 151890). The phenomenon was observed in multiple photometric bands using two different HIPPI-class (HIgh Precision Polarimetric Instrument)polarimeters with telescopes ranging in size from 35-cm to 3.9-m. The peak-to-trough amplitude of the polarization is wavelength dependent and large, $\sim$700 parts-per-million in green light, and is easily seen with even the smallest telescope. We fit the polarization phase curve with a SYNSPEC/VLIDORT polarized radiative transfer model and a Wilson-Devinney geometric formalism, which we describe in detail. Light from each star reflected by the photosphere of the other, together with a much smaller contribution from tidal distortion and eclipse effects, wholly accounts for the polarization amplitude. In the past polarization in semidetached binaries has been attributed mostly to scattering from extra-stellar gas. Our new interpretation facilitates determining masses of such stars in non-eclipsing systems.

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arXiv2020

Polarization measurements of the polluted white dwarf G29-38

We have made high precision polarimetric observations of the polluted white dwarf G29-38 with the HIgh Precision Polarimetric Instrument 2. The observations were made at two different observatories -- using the 8.1-m Gemini North Telescope and the 3.9-m Anglo AustralianTelescope -- and are consistent with each other. After allowing for a small amount of interstellar polarization, the intrinsic linear polarization of the system is found to be 275.3 +/- 31.9 parts-per-million at a position angle of 90.8 +/- 3.8 degrees in the SDSS g' band. We compare the observed polarization with the predictions of circumstellar disc models. The measured polarization is small in the context of the models we develop which only allows us to place limits on disc inclination and Bond albedo for optically thin disc geometries. In this case either the inclination is near face-on or the albedo is small -- likely in the range 0.05 to 0.15 -- which is in line with other debris disc measurements. A preliminary search for the effects of G29-38's pulsations in the polarization signal produced inconsistent results. This may be caused by beating effects, indicate a clumpy dust distribution, or be a consequence of measurement systematics.

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