arXiv subjects

N. R. Evans

Publications and source records attributed to N. R. Evans.

At least 19 recordsLinked to original sources

arXiv2026

High-contrast imaging of Galactic Cepheids with VLT/SPHERE

Cepheids are key distance indicators and benchmarks for stellar evolution, yet most of them are members of binary or multiple systems. While spectroscopic surveys and Gaia proper-motion anomalies reveal a high binary fraction, the population of resolved companions remains poorly characterised. We aim to search for and characterise visual companions to bright Galactic Cepheids using high-contrast imaging and to derive quantitative limits on undetected companions to constrain the architecture of Cepheid multiple systems. We observed 47 Cepheids with SPHERE using the ZIMPOL instrument in classical imaging mode and the V, R, and I filters. The data were obtained in pupil-stabilised mode and analysed using PCA-based imaging technique. For detected companions, we injected negative fake companions in a Monte Carlo approach to measure the relative astrometry. For non-detections, synthetic companions were injected to compute 5sigma contrast curves as a function of separation. We detected companions with a signal-to-noise ratio of > 5 for 8 Cepheids, corresponding to about 17% of the sample. Our SPHERE imaging confirms previously known visual companions with improved astrometry and reveals new wide components for AP Pup, T Vel, and TX Del) at projected separations of 0.16-0.9". For the remaining Cepheids, we derived typical maximum contrasts of 10, 11, and 12mag at 0.25", 0.5", and > 1, respectively. For a sub-set of targets, these limits ruled out main sequence companions more massive than late-K dwarfs beyond 0.5". Our SPHERE survey provides the first homogeneous set of high-contrast optical constraints on wide companions of Galactic Cepheids. The low detection rate of visual companions compared to the high overall binary fraction implies that most companions inferred from radial velocities and Gaia astrometry are either closer than 20mas or significantly fainter than the limits reached here.

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arXiv2024

Multiplicity of Galactic Cepheids from long-baseline interferometry V. High-accuracy orbital parallax and mass of SU Cygni

Cepheid masses are particularly necessary to help solving the mass discrepancy, while independent distance determinations provide crucial test of the period-luminosity relation and Gaia parallaxes. We used CHARA/MIRC to measure the astrometric positions of the high-contrast companion orbiting the Cepheid SU Cygni. We also present new radial velocity measurements from the HST. The combination of interferometric astrometry with optical and ultraviolet spectroscopy provides the full orbital elements of the system, in addition to component masses and the distance to the Cepheid system. We measured the mass of the Cepheid, $M_A = 4.859\pm0.058M_\odot$, and its two companions, $M_{Ba} = 3.595 \pm 0.033 M_\odot$ and $M_{Bb} = 1.546 \pm 0.009 M_\odot$. This is the most accurate existing measurement of the mass of a Galactic Cepheid (1.2%). Comparing with stellar evolution models, we show that the mass predicted is higher than the measured mass of the Cepheid, similar to conclusions of our previous work. We also measured the distance to the system to be $926.3 \pm 5.0$pc, i.e. an unprecedented parallax precision of $6μ$as (0.5%), being the most precise and accurate distance for a Cepheid. Such precision is similar to what is expected by Gaia for the last data release (DR5 in $\sim$ 2030) for single stars fainter than G = 13, but is not guaranteed for stars as bright as SU Cyg. We demonstrated that evolutionary models remain inadequate in accurately reproducing the measured mass, often predicting higher masses for the expected metallicity, even when factors such as rotation or convective core overshooting are taken into account. Our precise distance measurement allowed us to compare prediction period-luminosity relations. We found a disagreement of 0.2-0.5 mag with relations calibrated from photometry, while relations calibrated from direct distance measurement are in better agreement.

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arXiv2023

First Detection of Radio Emission Associated with a Classical Cepheid

We report the detection of 15 GHz radio continuum emission associated with the classical Cepheid variable star delta Cephei based on observations with the Karl G. Jansky Very Large Array. Our results constitute the first probable detection of radio continuum emission from a classical Cepheid. We observed the star at pulsation phase phi~0.43 (corresponding to the phase of maximum radius and minimum temperature) during three pulsation cycles in late 2018 and detected statistically significant emission (>5 sigma) during one of the three epochs. The observed radio emission appears to be variable at a >~10% level on timescales of days to weeks. We also present an upper limit on the 10 GHz flux density at pulsation phase phi=0.31 from an observation in 2014. We discuss possible mechanisms that may produce the observed 15 GHz emission, but cannot make a conclusive identification from the present data. The emission does not appear to be consistent with originating from a close-in, late-type dwarf companion, although this scenario cannot yet be strictly excluded. Previous X-ray observations have shown that delta Cephei undergoes periodic increases in X-ray flux during pulsation phase phi~0.43. The lack of radio detection in two out of three observing epochs at phi~0.43 suggests that either the radio emission is not linked with a particular pulsation phase, or else that the strength of the generated radio emission in each pulsation cycle is variable.

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arXiv2019

Multiplicity of Galactic Cepheids and RR Lyrae stars from Gaia DR2 -- II. Resolved common proper motion pairs

Context. The multiplicity of classical Cepheids (CCs) and RR Lyrae stars (RRLs) is still imperfectly known, particularly for RRLs. Aims. In order to complement the close-in short orbital period systems presented in Paper I, our aim is to detect the wide, spatially resolved companions of the targets of our reference samples of Galactic CCs and RRLs. Methods. Angularly resolved common proper motion pairs were detected using a simple progressive selection algorithm to separate the most probable candidate companions from the unrelated field stars. Results. We found 27 resolved, high probability gravitationally bound systems with CCs out of 456 examined stars, and one unbound star embedded in the circumstellar dusty nebula of the long-period Cepheid RS Pup. We found seven spatially resolved, probably bound systems with RRL primaries out of 789 investigated stars, and 22 additional candidate pairs. We report in particular new companions of three bright RRLs: OV And (companion of F4V spectral type), RR Leo (M0V), and SS Oct (K2V). In addition, we discovered resolved companions of 14 stars that were likely misclassified as RRLs. Conclusions. The detection of resolved non-variable companions around CCs and RRLs facilitates the validation of their Gaia DR2 parallaxes. The possibility to conduct a detailed analysis of the resolved coeval companions of CCs and old population RRLs will also be valuable to progress on our understanding of their evolutionary path.

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arXiv2019

Multiplicity of Galactic Cepheids and RR Lyrae stars from Gaia DR2 - I. Binarity from proper motion anomaly

Classical Cepheids (CCs) and RR Lyrae stars (RRLs) are important classes of variable stars used as standard candles to estimate galactic and extragalactic distances. Their multiplicity is imperfectly known, particularly for RRLs. Astoundingly, to date only one RRL has convincingly been demonstrated to be a binary, TU UMa, out of tens of thousands of known RRLs. Our aim is to detect the binary and multiple stars present in a sample of Milky Way CCs and RRLs. In the present article, we combine the Hipparcos and Gaia DR2 positions to determine the mean proper motion of the targets, and we search for proper motion anomalies (PMa) caused by close-in orbiting companions. We identify 57 CC binaries from PMa out of 254 tested stars and 75 additional candidates, confirming the high binary fraction of these massive stars. For 28 binary CCs, we determine the companion mass by combining their spectroscopic orbital parameters and astrometric PMa. We detect 13 RRLs showing a significant PMa out of 198 tested stars, and 61 additional candidates. We determine that the binary fraction of CCs is likely above 80%, while that of RRLs is at least 7%. The newly detected systems will be useful to improve our understanding of their evolutionary states. The discovery of a significant number of RRLs in binary systems also resolves the long-standing mystery of their extremely low apparent binary fraction.

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arXiv2018

Multiplicity of Galactic Cepheids from long-baseline interferometry. IV. New detected companions from MIRC and PIONIER observations

We aim at detecting and characterizing the main-sequence companions of a sample of known and suspected Galactic binary Cepheids. We used the multi-telescope interferometric combiners MIRC and PIONIER to detect and measure the astrometric positions of the high-contrast companions orbiting 16 bright Galactic Cepheids. We made use of the CANDID algorithm to search for the companions and set detection limits. We also present new high-precision RVs which were used to fit radial pulsation and orbital velocities. We report the detection of the companions orbiting the Cepheids U Aql, BP Cir, and S Mus, possible detections for FF Aql, Y Car, BG Cru, X Sgr, V350 Sgr, and V636 Sco, while no component is detected around U Car, YZ Car, T Mon, R Mus, S Nor, W Sgr and AH Vel. For U Aql and S Mus, we performed a preliminary orbital fit combining astrometry with new high-precision RVs, providing the full set of orbital and pulsation parameters. Assuming the distance, we estimated preliminary masses of M(U Aql) = 4.97+/-0.62Msun and M(S Mus) = 4.63+/-0.99Msun. For YZ Car, W Sgr, V350 Sgr, and V636 Sco, we revised the spectroscopic orbits using new high-precision RVs, while we updated the pulsation parameters for BP Cir, BG Cru, S Nor and AH Vel. Our interferometric observations also provide measurements of the angular diameters, that can be used in a Baade-Wesselink type analysis. We have now several astrometric detections of Cepheid companions. When RVs of the companions will be available, such systems will provide accurate and independent masses and distances. Orbital parallaxes with an accuracy <5% will be particularly useful for a better calibration of the P-L relation. The final Gaia parallaxes will be also particularly helpful for single-line spectroscopic systems, where mass and distance are degenerate. Mass measurements are necessary for a better understanding of the age and evolution of Cepheids

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arXiv2018

A geometrical 1% distance to the short-period binary Cepheid V1334 Cygni

Cepheid stars play a considerable role as extragalactic distances indicators, thanks to the simple empirical relation between their pulsation period and their luminosity. They overlap with that of secondary distance indicators, such as Type Ia supernovae, whose distance scale is tied to Cepheid luminosities. However, the Period-Luminosity (P-L) relation still lacks a calibration to better than 5%. Using an original combination of interferometric astrometry with optical and ultraviolet spectroscopy, we measured the geometrical distance d = 720.35+/-7.84 pc of the 3.33 d period Cepheid V1334 Cyg with an unprecedented accuracy of +/-1 %, providing the most accurate distance for a Cepheid. Placing this star in the P-L diagram provides an independent test of existing period-luminosity relations. We show that the secondary star has a significant impact on the integrated magnitude, particularly at visible wavelengths. Binarity in future high precision calibrations of the P-L relations is not negligible, at least in the short-period regime. Subtracting the companion flux leaves V1334 Cyg in marginal agreement with existing photometric-based P-L relations, indicating either an overall calibration bias or a significant intrinsic dispersion at a few percent level. Our work also enabled us to determine the dynamical masses of both components, M1 = 4.288 +/- 0.133 Msun (Cepheid) and M2 = 4.040 +/- 0.048 Msun (companion), providing the most accurate masses for a Galactic binary Cepheid system.

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arXiv2017

Dynamical masses of Cepheids from the GAIA parallaxes

The mass of a Cepheid is a fundamental parameter for studying the pulsation and evolution of intermediate-mass stars. But determining this variable has been a long-standing problem for decades. Detecting the companions (by spectroscopy or imaging) is a difficult task because of the brightness of the Cepheids and the close orbit of the components. So most of the Cepheid masses are derived using stellar evolution or pulsation modeling, but they differ by 10-20%. Measurements of dynamical masses offer the unique opportunity to make progress in resolving this mass discrepancy. The first problem in studying binary Cepheids is the high contrast between the components for wavelengths longer than 0.5 μm, which make them single-line spectroscopic binaries. In addition, the close orbit of the companions (< 40 mas) prevents us from spatially resolving the systems with a single-dish 8m-class telescope. A technique able to reach high spatial resolution and high- dynamic range is long-baseline interferometry. We have started a long-term program that aims at detecting, monitoring and characterizing physical parameters of the Cepheid companions. The GAIA parallaxes will enable us to combine interferometry with single-line velocities to provide unique dynamical mass measurements of Cepheids.

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arXiv2017

Four years' interferometric observations of Galactic binary Cepheids

We give an update on our long-term program of Galactic Cepheids started in 2012, whose goal is to measure the visual orbits of Cepheid companions. Using the VLTI/PIONIER and CHARA/MIRC instruments, we have now detected several com- panions, and we already have a good orbital coverage for several of them. By combining interferometry and radial velocities, we can now derive all the orbital elements of the systems, and we will be soon able to estimate the Cepheid masses.

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arXiv2017

The MOST view of Cepheids

The MOST space telescope observed four Cepheid variables so far, all of different subtypes. Here we summarize the results obtained and the possible ways to continue to study these stars.

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arXiv2016

V473 Lyr, a modulated, period-doubled Cepheid, and U TrA, a double-mode Cepheid observed by MOST

Space-based photometric measurements first revealed low-amplitude irregularities in the pulsations of Cepheid stars, but their origins and how commonly they occur remain uncertain. To investigate this phenomenon, we present MOST space telescope photometry of two Cepheids. V473 Lyrae is a second-overtone, strongly modulated Cepheid, while U Trianguli Australis is a Cepheid pulsating simultaneously in the fundamental mode and first overtone. The nearly continuous, high-precision photometry reveals alternations in the amplitudes of cycles in V473 Lyr, the first case of period doubling detected in a classical Cepheid. In U TrA, we tentatively identify one peak as the $f_X$ or 0.61-type mode often seen in conjunction with the first radial overtone in Cepheids, but given the short length of the data, we cannot rule out that it is a combination peak instead. Ground-based photometry and spectroscopy were obtained to follow two modulation cycles in V473 Lyr and to better specify its physical parameters. The simultaneous data yield the phase lag parameter (the phase difference between maxima in luminosity and radial velocity) of a second-overtone Cepheid for the first time. We find no evidence for a period change in U TrA or an energy exchange between the fundamental mode and the first overtone during the last 50 years, contrary to earlier indications. Period doubling in V473 Lyr provides a strong argument that mode interactions do occur in some Cepheids and we may hypothesise that it could be behind the amplitude modulation, as recently proposed for Blazhko RR Lyrae stars.

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arXiv2016

BRITE observations of classical Cepheids

We briefly summarize the BRITE observations of classical Cepheids. Possible detection of modulation in a fundamental mode Cepheid, T Vul, and of additional non-radial modes in first overtone Cepheids, DT Cyg and V1334 Cyg, are reported.

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arXiv2016

A Search for Mass Loss on the Cepheid Instability Strip using HI 21-cm Line Observations

We present the results of a search for HI 21-cm line emission from the circumstellar environments of four Galactic Cepheids (RS Pup, X Cyg, $ζ$ Gem, and T Mon) based on observations with the Karl G. Jansky Very Large Array. The observations were aimed at detecting gas associated with previous or ongoing mass loss. Near the long-period Cepheid T Mon, we report the detection of a partial shell-like structure whose properties appear consistent with originating from an earlier epoch of Cepheid mass loss. At the distance of T Mon, the nebula would have a mass (HI+He) of $\sim0.5M_{\odot}$, or $\sim$6\% of the stellar mass. Assuming that one-third of the nebular mass comprises swept-up interstellar gas, we estimate an implied mass-loss rate of ${\dot M}\sim (0.6-2)\times10^{-5} M_{\odot}$ yr$^{-1}$. No clear signatures of circumstellar emission were found toward $ζ$ Gem, RS Pup, or X Cyg, although in each case, line-of-sight confusion compromised portions of the spectral band. For the undetected stars, we derive model-dependent $3σ$ upper limits on the mass-loss rates, averaged over their lifetimes on the instability strip, of $<(0.3-6)\times10^{-6} M_{\odot}$ yr$^{-1}$ and estimate the total amount of mass lost to be less than a few per cent of the stellar mass.

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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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arXiv2014

Observations of Cepheids with the MOST satellite: Contrast between Pulsation Modes

The quantity and quality of satellite photometric data strings is revealing details in Cepheid variation at very low levels. Specifically, we observed a Cepheid pulsating in the fundamental mode and one pulsating in the first overtone with the Canadian MOST satellite. The 3.7-d period fundamental mode pulsator (RT Aur) has a light curve that repeats precisely, and can be modeled by a Fourier series very accurately. The overtone pulsator (SZ Tau, 3.1 d period) on the other hand shows light curve variation from cycle to cycle which we characterize by the variations in the Fourier parameters. We present arguments that we are seeing instability in the pulsation cycle of the overtone pulsator, and that this is also a characteristic of the O-C curves of overtone pulsators. On the other hand, deviations from cycle to cycle as a function of pulsation phase follow a similar pattern in both stars, increasing after minimum radius. In summary, pulsation in the overtone pulsator is less stable than that of the fundamental mode pulsator at both long and short timescales.

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arXiv2014

Searching for visual companions of close Cepheids. VLT/NACO lucky imaging of Y~Oph, FF~Aql, X~Sgr, W~Sgr and $η$~Aql

Aims: High-resolution imaging in several photometric bands can provide color and astrometric information of the wide-orbit component of Cepheid stars. Such measurements are needed to understand the age and evolution of pulsating stars. In addition, binary Cepheids have the potential to provide direct and model-independent distances and masses. Methods: We used the NAOS-CONICA adaptive optics instrument (NACO) in the near-infrared to perform a deep search for wide components around the classical Cepheids, Y~Oph, FF~Aql, X~Sgr, W~Sgr, and $η$~Aql, within a field of view (FoV) of $1.7"\times 1.7"$ ($3.4"\times 3.4"$ for $η$~Aql). Results: We were able to reach contrast $ΔH = 5$-8\,mag and $ΔK_\mathrm{s} = 4$-7\,mag in the radius range $r > 0.2"$, which enabled us to constrain the presence of wide companions. For Y~Oph, FF~Aql, X~Sgr, W~Sgr, and $η$~Aql at $r > 0.2"$, we ruled out the presence of companions with a spectral type that is earlier than a B7V, A9V, A9V, A1V, and G5V star, respectively. For $0.1"< r < 0.2"$, no companions earlier than O9V, B3V, B4V, B2V, and B2V star, respectively, are detected. A component is detected close to $η$~Aql at projected separation $ρ= 654.7 \pm 0.9$\,mas and a position angle $PA = 92.8 \pm 0.1^\circ$. We estimated its dereddened apparent magnitude to be $m_H^0 = 9.34 \pm 0.04$ and derived a spectral type that ranges between an F1V and F6V star. Additional photometric and astrometric measurements are necessary to better constrain this star and check its physical association to the $η$~Aql system.

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arXiv2014

Subtle flickering in Cepheids

Fundamental mode classical Cepheids have light curves which repeat accurately enough that we can watch them evolve (change period). The new level of accuracy and quantity of data with the {\it Kepler} and {\it MOST} satellites probes this further. An intriguing result was found in the long time-series of Kepler data for V1154 Cyg the one classical Cepheid (fundamental mode, P = 4.9$^d$) in the field, which has short term changes in period ($\simeq$20 minutes), correlated for $\simeq$10 cycles (period jitter). To follow this up, we obtained a month long series of observations of the fundamental mode Cepheid RT Aur and the first overtone pulsator SZ Tau. RT Aur shows the traditional strict repetition of the light curve, with the Fourier amplitude ratio $R_1/R_2$ remaining nearly constant. The light curve of SZ Tau, on the other hand, fluctuates in amplitude ratio at the level of approximately $50\%$. Furthermore prewhitening the RT Aur data with 10 frequencies reduces the Fourier spectrum to noise. For SZ Tau, considerable power is left after this prewhitening in a complicated variety of frequencies.

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arXiv2013

The Kepler-SEP Mission: Harvesting the South Ecliptic Pole large-amplitude variables with Kepler

As a response to the white paper call, we propose to turn Kepler to the South Ecliptic Pole (SEP) and observe thousands of large amplitude variables for years with high cadence in the frame of the Kepler-SEP Mission. The degraded pointing stability will still allow observing these stars with reasonable (probably better than mmag) accuracy. Long-term continuous monitoring already proved to be extremely helpful to investigate several areas of stellar astrophysics. Space-based missions opened a new window to the dynamics of pulsation in several class of pulsating variable stars and facilitated detailed studies of eclipsing binaries. The main aim of this mission is to better understand the fascinating dynamics behind various stellar pulsational phenomena (resonances, mode coupling, chaos, mode selection) and interior physics (turbulent convection, opacities). This will also improve the applicability of these astrophysical tools for distance measurements, population and stellar evolution studies. We investigated the pragmatic details of such a mission and found a number of advantages: minimal reprogramming of the flight software, a favorable field of view, access to both galactic and LMC objects. However, the main advantage of the SEP field comes from the large sample of well classified targets, mainly through OGLE. Synergies and significant overlap (spatial, temporal and in brightness) with both ground- (OGLE, LSST) and space-based missions (GAIA, TESS) will greatly enhance the scientific value of the Kepler-SEP mission. GAIA will allow full characterization of the distance indicators. TESS will continuously monitor this field for at least one year, and together with the proposed mission provide long time series that cannot be obtained by other means. If Kepler-SEP program is successful, there is a possibility to place one of the so-called LSST "deep-drilling" fields in this region.

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