arXiv subjects

Richard I. Anderson

Publications and source records attributed to Richard I. Anderson.

At least 73 records · Page 4Linked to original sources

arXiv2020

Tomography of cool giant and supergiant star atmospheres III. Validation of the method on VLTI/AMBER observations of the Mira star S Ori

The knowledge of the structure and dynamics of AGB-star atmospheres is crucial to better understand the mass loss. The tomographic method, that relies on the design of spectral masks containing lines forming in given ranges of optical depths in the stellar atmosphere, is an ideal technique for this purpose. It is applied to high-resolution spectro-interferometric VLTI/AMBER observations of the Mira-type AGB star S Ori. First, the interferometric visibilities are extracted at wavelengths contributing to the tomographic masks and fitted to those computed from a uniform disk model. This allows the measurement of the geometrical extent of the atmospheric layer probed by the corresponding mask. Then, we compare the observed atmospheric extension with those measured from available 1D pulsation CODEX models and 3D radiative-hydrodynamics CO5BOLD simulations. We found that while the average optical depths probed by the tomographic masks in S Ori decrease (with $<\log τ_0> = -0.45$, $-1.45$, and $-2.45$ from the innermost to the central and outermost layers), the angular diameters of these layers increase, from 10.59 $\pm$ 0.09 mas through 11.84 $\pm$ 0.17 mas, up to 14.08 $\pm$ 0.15 mas. A similar behavior is observed when the tomographic method is applied to 1D and 3D dynamical models. Thus, this study derives, for the first time, a quantitative relation between optical- and geometrical-depth scales when applied to the Mira star S Ori, or to 1D and 3D dynamical models. In the context of Mira-type stars, the knowledge of the link between the optical and geometrical depths opens the way to derive the shock-wave propagation velocity, which can not be directly observed in these stars.

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arXiv2020

GaiaNIR: Combining optical and Near-Infra-Red (NIR) capabilities with Time-Delay-Integration (TDI) sensors for a future Gaia-like mission

ESA recently called for new "Science Ideas" to be investigated in terms of feasibility and technological developments -- for technologies not yet sufficiently mature. These ideas may in the future become candidates for M or L class missions within the ESA Science Program. With the launch of Gaia in December 2013, Europe entered a new era of space astrometry following in the footsteps of the very successful Hipparcos mission from the early 1990s. Gaia is the successor to Hipparcos, both of which operated in optical wavelengths, and Gaia is two orders of magnitude more accurate in the five astrometric parameters and is surveying four orders of magnitude more stars in a vast volume of the Milky Way. The combination of the Hipparcos/Tycho-2 catalogues with the first early Gaia data release will give improved proper motions over a long ~25 year baseline. The final Gaia solution will also establish a new optical reference frame by means of quasars, by linking the optical counterparts of radio (VLBI) sources defining the orientation of the reference frame, and by using the zero proper motion of quasars to determine a non-rotating frame. A weakness of Gaia is that it only operates at optical wavelengths. However, much of the Galactic centre and the spiral arm regions, important for certain studies, are obscured by interstellar extinction and this makes it difficult for Gaia to deeply probe. Traditionally, this problem is overcome by switching to the infra-red but this was not possible with Gaia's CCDs. Additionally, to scan the entire sky and make global absolute parallax measurements the spacecraft must have a constant rotation and this requires that the CCDs operate in TDI mode, increasing their complexity.

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arXiv2020

Towards precise and accurate Cepheid chemical abundances for 1$\%$ $\mbox{H}_0$ measurement: temperature determination

One of the outstanding problems in modern physics is the origin is of accelerated expansion of the universe. High-precision determinations of the Hubble parameter $\mbox{H}_0$ at different redshifts provide direct insight into the Universe expansion: equation of state of the Dark Energy, geometry and curvature of space, sum of neutrino masses and isotropy of the Universe. We investigate the effect of chemical composition on the classical distance ladder based on Cepheids stars and Supernovae type Ia. Cepheids belonging to Magellanic Clouds represent a natural anchor for the ladder, due to their proximity and the very well known geometric distance. Accordingly, the LACES collaboration collected the largest spectroscopic sample of MCs Cepheids, containing more than 300 stars and more than 1200 medium-high resolution spectra. Abundance measurements, performed by equivalent width and curve of growth analysis, have been tested on a wide grid of synthetic spectra to quantify the systematic arising from this procedure. We focus on the errors arising from temperature measurements, using the Line Depth Ratio method. We find that in order to not bias the final results many aspects (e.g. degeneracy of the atmospheric parameters, instrumental set up etc.) should be explicitly taken into account. In particular, there is a not negligible metallicity dependence.

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arXiv2020

Cepheids under the magnifying glass -- not so simple, after all!

Classical Cepheids are blue loop stars that have famously been dubbed "magnifying glasses of stellar evolution" and have been studied for a long time. As more and more precise observations of Cepheids are secured over ever-increasing temporal baselines, our ignorance of the physics governing these crucial stars is becoming increasingly clear. Thus, it is time to turn up the magnification and investigate the limitations of our understanding of classical pulsators. Of course, classical Cepheids are also standard candles thanks to the Leavitt law that allows to measure distances in the nearby Universe. Nowadays, Cepheids serve as the backbone of a precisely calibrated distance ladder that allows to measure Hubble's constant (H0) to better than 2%, thus providing crucial constraints for precision cosmology. The recently established discord among H0 values measured in the late-time Universe and inferred from observations of the early Universe requires utmost diligence in estimating systematic uncertainties in order to strengthen the significance of the results. In this presentation I focus on two main aspects of recent Cepheid-related research. First, I present the Geneva Cepheid Radial Velocity survey (GE-CeRVS) and an update on the modulated spectroscopic variability exhibited by the 4 d Cepheid QZ Normae based on 8 years of monitoring. Then, I discuss some efforts directed towards a 1% H0 measurement needed for understanding the cosmological implications of discordant H0 values. Finally, I argue that now is a particularly opportune time to leverage the synergies between stellar physics and observational cosmology.

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arXiv2019

Towards a 1% Measurement of the Hubble Constant: Accounting for Time Dilation in Variable Star Light Curves

Assessing the significance and implications of the recently established Hubble tension requires the comprehensive identification, quantification, and mitigation of uncertainties and/or biases affecting $H_0$ measurements. Here, we investigate the previously overlooked distance scale bias resulting from the interplay between redshift and Leavitt laws in an expanding Universe: Redshift-Leavitt bias (RLB). Redshift dilates oscillation periods of pulsating stars residing in supernova-host galaxies relative to periods of identical stars residing in nearby (anchor) galaxies. Multiplying dilated $\log{P}$ with Leavitt Law slopes leads to underestimated absolute magnitudes, overestimated distance moduli, and a systematic error on $H_0$. Emulating the SH0ES distance ladder, we estimate an associated $H_0$ bias of $(0.27 \pm 0.01)$% and obtain a corrected $H_0 = (73.70 \pm 1.40) \rm{km s^{-1} Mpc^{-1}}$. RLB becomes increasingly relevant as distance ladder calibrations pursue greater numbers of ever more distant galaxies hosting both Cepheids (or Miras) and type-Ia supernovae. The measured periods of oscillating stars can readily be corrected for heliocentric redshift (e.g. of their host galaxies) in order to ensure $H_0$ measurements free of RLB.

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arXiv2019

A stellar relic filament in the Orion star forming region

We report the discovery of the oldest stellar substructure in the Orion star forming region (OSFR), the Orion relic filament. The relic filament is physically associated with the OSFR as demonstrated by Gaia DR2 photometry and astrometry, as well as targeted radial velocity follow-up observations of a bright sub-sample of proper-motion selected candidate members. Gaia DR2 parallaxes place the Orion relic filament in the more distant part of the OSFR, approx. 430pc from the Sun. Given its age, velocity dispersion, spatial extent, and shape, it is not possible for the Orion relic filament to have formed as a single stellar cluster, even taking into account residual gas expulsion. The relic filament is also too young to be a tidal stream, since Galactic tides act on much longer time scales of order 100 Myr. It therefore appears likely that the structure formed from a molecular cloud filament similar to Orion A in the OSFR and retained its morphology despite decoupling from its natal gas. Hence, the Orion relic filament bears witness to the short-lived evolutionary phase between gas removal and dispersion due to shears and tides, and provides crucial new insights into how stars are formed in molecular clouds.

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arXiv2019

Consistent radial velocities of classical Cepheids from the cross-correlation technique

Accurate radial velocities ($v_{\rm rad}$) of Cepheids are mandatory within the context of distance measurements via the Baade-Wesselink technique. The most common $v_{\rm rad}$ derivation method consists in cross-correlating the observed spectrum with a binary template and measuring a velocity on the resulting profile. Yet for Cepheids, the spectral lines selected within the template as well as the way of fitting the cross-correlation function (CCF) have a significant impact on the measured $v_{\rm rad}$. We detail the steps to compute consistent Cepheid CCFs and $v_{\rm rad}$, and we characterise the impact of Cepheid spectral properties and $v_{\rm rad}$ computation method on the resulting line profiles. We collected more than 3900 high-resolution spectra from seven different spectrographs of 64 classical Cepheids. These spectra were standardised through a single process on pre-defined wavelength ranges. We built six correlation templates selecting un-blended lines of different depths from a synthetic Cepheid spectrum, on three different wavelength ranges from 390 to 800 nm. Each spectrum was cross-correlated with these templates to build the corresponding CCFs. We derived a set of line profile observables as well as three different $v_{\rm rad}$ measurements from each CCF. This study confirms that both the template wavelength range, its mean line depth and width, and the $v_{\rm rad}$ computation method significantly impact the $v_{\rm rad}$. Deriving more robust Cepheid $v_{\rm rad}$ time series require to minimise the asymmetry of the line profile and its impact on the $v_{\rm rad}$. Centroid $v_{\rm rad}$, that exhibit slightly smaller amplitudes but significantly smaller scatter than Gaussian or biGaussian $v_{\rm rad}$, should thus be favoured. Stronger lines are also less asymmetric and lead to more robust $v_{\rm rad}$ than weaker lines.

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arXiv2019

Probing Polaris' Puzzling Radial Velocity Signals - Pulsational (In-)Stability, Orbital Motion, and Bisector Variations

We investigate temporally changing variability amplitudes and the multi-periodicity of the type-I Cepheid Polaris using 161 high-precision radial velocity (RV) and bisector inverse span (BIS) measurements based on optical spectra recorded using Hermes at the 1.2 m Flemish Mercator telescope on La Palma, Canary Islands, Spain. Using an empirical template fitting method, we show that Polaris' RV amplitude has been stable to within $\sim 30 \rm{m s}^{-1}$ between September 2011 and November 2018. We apply the template fitting method to publicly accessible, homogeneous RV data sets from the literature and provide an updated solution of Polaris' eccentric 29.3 yr orbit. While the inferred pulsation-induced RV amplitudes differ among individual data sets, we find no evidence for time-variable RV amplitudes in any of the separately considered, homogeneous data sets. Additionally, we find that increasing photometric amplitudes determined using SMEI photometry are likely spurious detections due to as yet ill-understood systematic effects of instrumental origin. Given this confusing situation, further analysis of high-quality homogeneous data sets with well-understood systematics is required to confidently establish whether Polaris' variability amplitude is subject to change over time. We confirm periodic bisector variability periods of 3.97 d and 40.22 d using Hermes BIS measurements and identify a third signal at a period of 60.17 d. Although the 60.17 d signal dominates the BIS periodogram, we caution that this signal may not be independent of the 40.22 d signal. Finally, we show that the 40.22 d signal cannot be explained by stellar rotation. Further long-term, high-quality spectroscopic monitoring is required to unravel the complete set of Polaris' periodic signals, which has the potential to provide unprecedented insights into the evolution of Cepheid variables.

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arXiv2018

Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant

We present HST photometry of a selected sample of 50 long-period, low-extinction Milky Way Cepheids measured on the same WFC3 F555W, F814W, and F160W-band photometric system as extragalactic Cepheids in SN Ia hosts. These bright Cepheids were observed with the WFC3 spatial scanning mode in the optical and near-infrared to mitigate saturation and reduce pixel-to-pixel calibration errors to reach a mean photometric error of 5 millimags per observation. We use the new Gaia DR2 parallaxes and HST photometry to simultaneously constrain the cosmic distance scale and to measure the DR2 parallax zeropoint offset appropriate for Cepheids. We find a value for the zeropoint offset of -46 +/- 13 muas or +/- 6 muas for a fixed distance scale, higher than found from quasars, as expected, for these brighter and redder sources. The precision of the distance scale from DR2 has been reduced by a factor of 2.5 due to the need to independently determine the parallax offset. The best fit distance scale is 1.006 +/- 0.033, relative to the scale from Riess et al 2016 with H0=73.24 km/s/Mpc used to predict the parallaxes photometrically, and is inconsistent with the scale needed to match the Planck 2016 CMB data combined with LCDM at the 2.9 sigma confidence level (99.6%). At 96.5% confidence we find that the formal DR2 errors may be underestimated as indicated. We identify additional error associated with the use of augmented Cepheid samples utilizing ground-based photometry and discuss their likely origins. Including the DR2 parallaxes with all prior distance ladder data raises the current tension between the late and early Universe route to the Hubble constant to 3.8 sigma (99.99 %). With the final expected precision from Gaia, the sample of 50 Cepheids with HST photometry will limit to 0.5% the contribution of the first rung of the distance ladder to the uncertainty in the Hubble constant.

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arXiv2018

On Cepheid Distance Scale Bias due to Stellar Companions and Cluster Populations

State-of-the art photometric measurements of extragalactic Cepheids account for the mean additional light due to chance superposition of Cepheids on crowded backgrounds through the use of artificial star measurements. However, light from stars physically associated with Cepheids may bias relative distance measurements if the changing spatial resolution along the distance ladder significantly alters the amount of associated blending. We have identified two regimes where this phenomenon may occur: Cepheids in wide binaries and open clusters. We estimate stellar association bias using the photometric passbands and reddening-free Wesenheit magnitudes used to set up the distance scale. For wide binaries, we rely on Geneva stellar evolution models in conjunction with detailed statistics on intermediate-mass binary stars. For the impact of cluster stars, we have compiled information on the clustered Cepheid fraction and measured the typical cluster contribution in M31 via deep HST imaging provided by the PHAT project. We find that the dominant effect on the distance scale comes from Cepheids in clusters, despite cluster Cepheids being a relatively rare phenomenon. Wide binaries have a negligible effect of 0.004% on $H_0$ for long-period Cepheids observed in the near-infrared or when considering Wesenheit magnitudes. We estimate that blending due to cluster populations has previously resulted in a 0.23% overestimate of $H_0$. Correcting for this bias, we obtain $H_0 = 73.07 \pm 1.76 km s^{-1} Mpc^{-1}$, which remains in $3.3σ$ tension with the Planck value. We conclude that stellar association bias does not constitute a limit for measuring $H_0$ with an accuracy of 1%.

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arXiv2018

A Near-Infrared Period-Luminosity Relation for Miras in NGC 4258, an Anchor for a New Distance Ladder

We present year-long, near-infrared Hubble Space Telescope WFC3 observations of Mira variables in the water megamaser host galaxy NGC 4258. Miras are AGB variables that can be divided into oxygen- (O-) and carbon- (C-) rich subclasses. Oxygen-rich Miras follow a tight (scatter $\sim 0.14$ mag) Period-Luminosity Relation (PLR) in the near-infrared and can be used to measure extragalactic distances. The water megamaser in NGC 4258 gives a geometric distance to the galaxy accurate to 2.6% that can serve to calibrate the Mira PLR. We develop criteria for detecting and classifying O-rich Miras with optical and NIR data as well as NIR data alone. In total, we discover 438 Mira candidates that we classify with high confidence as O-rich. Our most stringent criteria produce a sample of 139 Mira candidates that we use to measure a PLR. We use the OGLE-III sample of O-rich Miras in the LMC to obtain a relative distance modulus, $μ_{4258} - μ_{LMC} = 10.95 \pm 0.01 $ (statistical) $\pm 0.06 $ (systematic) mag in good agreement with the relative distance determined using Cepheids. These results demonstrate the feasibility of discovering and characterizing Miras using the near-infrared with the Hubble Space Telescope and the upcoming James Webb Space Telescope and using them to measure extragalactic distances and determine the Hubble constant.

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arXiv2018

Homing in on Polaris: A 7 M$_\odot$ first-overtone Cepheid entering the instability strip for the first time

A recently presented HST/FGS parallax measurement of the Polaris system has been interpreted as evidence for the Cepheid Polaris Aa to be pulsating in the second overtone. An age discrepancy between components A and B has been noted and discussed in terms of a stellar merger. Here I show that the new parallax of Polaris is consistent with a simpler interpretation of Polaris as a $7 \rm{M}_\odot,$ first-overtone, classical Cepheid near the hot boundary of the first instability strip crossing. This picture is anchored to rates of period change, the period-luminosity relation, the location in color-magnitude space, the interferometrically determined radius, spectroscopic N/C and N/O enhancements, and a dynamical mass measurement. The detailed agreement between models and data corroborates the physical association between the Cepheid and its visual companion as well as the accuracy of the HST parallax. The age discrepancy between components A and B is confirmed and requires further analysis, for example to investigate the possibility of stellar mergers in an evaporating birth cluster of which the Polaris triple system would be the remaining core.

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arXiv2018

Radial Velocity Observations of Classical Pulsating Stars

I concisely review the history, applications, and recent developments pertaining to radial velocity (RV) observations of classical pulsating stars. The focus lies on type-I (classical) Cepheids, although the historical overview and most technical aspects are also relevant for RR Lyrae stars and type-II Cepheids. The presence and impact of velocity gradients and different experimental setups on measured RV variability curves are discussed in some detail. Among the recent developments, modulated spectral line variability results in modulated RV curves that represent an issue for Baade-Wesselink-type distances and the detectability of spectroscopic companions, as well as challenges for stellar models. Spectral line asymmetry (e.g. via the bisector inverse span; BIS) provides a useful tool for identifying modulated spectral variability due to perturbations of velocity gradients. The imminent increase in number of pulsating stars observed with time series RVs by Gaia and ever increasing RV zero-point stability hold great promise for high-precision velocimetry to continue to provide new insights into stellar pulsations and their interactions with atmospheres.

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arXiv2017

Gaia's Cepheids and RR Lyrae Stars and Luminosity Calibrations Based on Tycho-Gaia Astrometric Solution

Gaia Data Release 1 contains parallaxes for more than 700 Galactic Cepheids and RR Lyrae stars, computed as part of the Tycho-Gaia Astrometric Solution (TGAS). We have used TGAS parallaxes, along with literature ($V, I, J, {K_\mathrm{s}}, W_1$) photometry and spectroscopy, to calibrate the zero point of the Period-Luminosity and Period-Wesenheit relations of classical and type II Cepheids, and the near-infrared Period-Luminosity, Period-Luminosity-Metallicity and optical Luminosity-Metallicity relations of RR Lyrae stars. In this contribution we briefly summarise results obtained by fitting these basic relations adopting different techniques that operate either in parallax or distance (absolute magnitude) space.

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arXiv2017

Mass and p-factor of the type II Cepheid OGLE-LMC-T2CEP-098 in a binary system

We present the results of a study of the type II Cepheid ($P_{puls} = 4.974 d$) in the eclipsing binary system OGLE-LMC-T2CEP-098 ($P_{orb} = 397.2 d$). The Cepheid belongs to the peculiar W Vir group, for which the evolutionary status is virtually unknown. It is the first single-lined system with a pulsating component analyzed using the method developed by Pilecki et al. (2013). We show that the presence of a pulsator makes it possible to derive accurate physical parameters of the stars even if radial velocities can be measured for only one of the components. We have used four different methods to limit and estimate the physical parameters, eventually obtaining precise results by combining pulsation theory with the spectroscopic and photometric solutions. The Cepheid radius, mass and temperature are $25.3 \pm 0.2 R_\odot$, $1.51 \pm 0.09 M_\odot$ and $5300 \pm 100 K$, respectively, while its companion has similar size ($26.3 R_\odot$), but is more massive ($6.8 M_\odot$) and hotter ($9500 K$). Our best estimate for the p-factor of the Cepheid is $1.30 \pm 0.03$. The mass, position on the period-luminosity diagram, and pulsation amplitude indicate that the pulsating component is very similar to the Anomalous Cepheids, although it has a much longer period and is redder in color. The very unusual combination of the components suggest that the system has passed through a mass transfer phase in its evolution. More complicated internal structure would then explain its peculiarity.

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arXiv2017

How Rotation Affects Masses and Ages of Classical Cepheids

Classical Cepheid variable stars are both sensitive astrophysical laboratories and accurate cosmic distance tracers. We have recently investigated how the evolutionary effects of rotation impact the properties of these important stars and here provide an accessible overview of some key elements as well as two important consequences. Firstly, rotation resolves the long-standing Cepheid mass discrepancy problem. Second, rotation increases main sequence lifetimes, i.e, Cepheids are approximately twice as old as previously thought. Finally, we highlight the importance of the short-period ends of Cepheid period distributions as indicators for model adequacy.

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arXiv2017

Toward a renewed Galactic Cepheid distance scale from Gaia and optical interferometry

Through an innovative combination of multiple observing techniques and mod- eling, we are assembling a comprehensive understanding of the pulsation and close environment of Cepheids. We developed the SPIPS modeling tool that combines all observables (radial velocimetry, photometry, angular diameters from interferometry) to derive the relevant physical parameters of the star (effective temperature, infrared ex- cess, reddening,...) and the ratio of the distance and the projection factor d/p. We present the application of SPIPS to the long-period Cepheid RS Pup, for which we derive p = 1.25 +/- 0.06. The addition of this massive Cepheid consolidates the existing sample of p-factor measurements towards long-period pulsators. This allows us to conclude that p is constant or mildly variable around p = 1.29 +/- 0.04 (+/-3%) as a function of the pulsation period. The forthcoming Gaia DR2 will provide a considerable improvement in quantity and accuracy of the trigonometric parallaxes of Cepheids. From this sample, the SPIPS modeling tool will enable a robust calibration of the Cepheid distance scale.

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arXiv2017

Observational calibration of the projection factor of Cepheids - III. The long-period Galactic Cepheid RS Puppis

The projection factor (p-factor) is an essential component of the classical Baade-Wesselink (BW) technique, that is commonly used to determine the distances to pulsating stars. It is a multiplicative parameter used to convert radial velocities into pulsational velocities. As the BW distances are linearly proportional to the p-factor, its accurate calibration for Cepheids is of critical importance for the reliability of their distance scale. We focus on the observational determination of the p-factor of the long-period Cepheid RS Pup (P = 41.5 days). This star is particularly important as this is one of the brightest Cepheids in the Galaxy and an analog of the Cepheids used to determine extragalactic distances. An accurate distance of 1910 +/- 80 pc (+/- 4.2%) has recently been determined for RS Pup using the light echoes propagating in its circumstellar nebula. We combine this distance with new VLTI/PIONIER interferometric angular diameters, photometry and radial velocities to derive the p-factor of RS Pup using the code Spectro-Photo-Interferometry of Pulsating Stars (SPIPS). We obtain p = 1.250 +/- 0.064 (+/-5.1%), defined for cross-correlation radial velocities. Together with measurements from the literature, the p-factor of RS Pup confirms the good agreement of a constant p = 1.293 +/- 0.039 (+/-3.0%) model with the observations. We conclude that the p-factor of Cepheids is constant or mildly variable over a broad range of periods (3.7 to 41.5 days).

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