Nancy Remage Evans

Context . Galactic Cepheid variable stars are fundamental calibrators of the cosmic distance scale through their period-luminosity (PL) relation. The high-precision parallaxes expected from Gaia Data Release 4 (DR4) will enable a calibration of the Milky Way PL relation with unprecedented precision. This will place stringent requirements on interstellar extinction corrections, which remain a dominant source of uncertainty. Aims . We strive to derive highly accurate extinction values for Galactic Cepheids. We present an innovative methodology relying on the analysis of neighboring field stars and apply it to a pilot sample of 16 Cepheids. Methods . We identified physically near field stars using Gaia DR3 astrometry. For these stars, we determined atmospheric parameters from medium- to high-resolution VLT/FLAMES spectroscopy. Fixing these parameters, we derived individual extinctions by fitting spectral energy distributions constructed from Gaia XP spectra and infrared photometry. We then reconstructed the three-dimensional distribution of extinction and interpolate extinction to the Cepheid position. Results . We report the extinction, A _V , total-to-selective extinction ratio, R _V , and color excess, E _{B– V} , of all 16 Cepheids. Our method achieves a mean precision of approximately 5%, improving upon typical literature values, with no significant systematic offset detected. Conclusions . We demonstrate that using field stars bypasses uncertainties associated with Cepheid variability and can provide robust extinction estimates for Galactic Cepheids.

In this paper we present a newly detected companion to the Be star, HD 52244 (B2IVnpe), using the Fine Guidance Sensors (FGS) on the Hubble Space Telescope (HST). In fall 2021, HST became momentarily unavailable to support nominal operations, and we used the operational FGS to carry out a multiplicity survey of six Be stars. We were able to resolve a companion to HD 52244, with a separation of 42.7 ± 1.1 mas (74 au) and a position angle of 144.2 ± 0.3 with a differential magnitude in the F583W filter of 1.91 ± 0.02 mag. This study presents the results of the newly detected companion of HD 52244 and lays the groundwork for future studies looking for wide or third companions to Be stars.

V659 Cen is a classical Cepheid which is part of a multiple system. Previous observations have shown that a hot companion dominates an ultraviolet spectrum and a cooler main-sequence star dominates an XMM-Newton spectrum. The Hubble Space Telescope Space Telescope Imaging Spectrograph spectra discussed here spatially resolve the components and show that the secondary in the spectroscopic binary with the Cepheid is the low-mass star, and the hottest star in the system is the outer companion. In addition a fourth star is a likely member of the system based on Gaia data. A new orbit is derived, which includes new radial velocities.

The system V350 Sgr has a classical Cepheid for the primary. Interferometry is presented for the system, and the full orbit is determined. The mass of the companion has been determined from an International Ultraviolet Explorer spectrum and comparison with the mass–temperature relation from detached eclipsing binaries. Combined with the mass of the companion (2.6 ± 0.2 M _⊙), the mass of the Cepheid is determined to be 4.7 ± 0.8 M _⊙. For systems with less complete information, Cepheid masses can be determined from a single-lined spectroscopic orbit, Gaia proper motion anomalies, and the mass of the companion from the ultraviolet. Uncertainties resulting from different approaches to mass determination are discussed and are expected to be reduced after the Gaia DR4 release. Temperatures for Morgan–Keenan standard stars from the ultraviolet are also provided.

Aims. We aim to accurately measure the dynamical mass and distance of Cepheids by combining radial velocity measurements with interferometric observations. Cepheid mass measurements are particularly necessary for solving the Cepheid mass discrepancy, while independent distance determinations provide a crucial test of the period–luminosity relation and Gaia parallaxes. Methods. We used the multi-telescope interferometric combiner, the Michigan InfraRed Combiner (MIRC) of the Center for High Angular Resolution Astronomy (CHARA) Array, to detect and measure the astrometric positions of the high-contrast companion orbiting the Galactic Cepheid SU Cygni. We also present new radial velocity measurements from ultraviolet spectra taken with the Hubble Space Telescope. The combination of interferometric astrometry with optical and ultraviolet spectroscopy provided the full orbital elements of the system, in addition to component masses and the distance to the Cepheid system. Results. We measured the mass of the Cepheid, M_A = 4.859 ± 0.058 M_⊙, and its two companions, M_Ba = 3.595 ± 0.033 M_⊙ and M_Bb = 1.546 ± 0.009 M_⊙. 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 by the tracks is higher than the measured mass of the Cepheid, which is similar to the conclusions of our previous work. We also measured the distance to the system to be 926.3 ± 5.0 pc, obtaining an unprecedented parallax precision of 6 μas (0.5%), which is the most precise and accurate distance for a Cepheid. This precision is similar to what is expected by Gaia for its last data release (DR5 in ∼2030) for single stars fainter than G = 13, but is not guaranteed for stars as bright as SU Cyg. Conclusions. We demonstrate that evolutionary models remain incapable of accurately reproducing the measured mass of Cepheids, 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 predictions from some period–luminosity relations. We find a disagreement of 0.2–0.5 mag with relations calibrated from photometry, while relations calibrated from a direct distance measurement are in better agreement.

Cepheid masses continue to be important tests of evolutionary tracks for intermediate-mass stars as well as important predictors of their future fate. For systems where the secondary is a B star, Hubble Space Telescope ultraviolet spectra have been obtained. From these spectra a temperature can be derived, and from this a mass of the companion M ₂. Once Gaia DR4 is available, proper motions can be used to determine the inclination of the orbit. Combining mass of the companion, M ₂, the mass function from the ground-based orbit of the Cepheid and the inclination produces the mass of the Cepheid, M ₁. The Cepheid system FN Vel is used here to demonstrate this approach and what limits can be put on the Cepheid mass for inclination between 50° and 130°.

Classical Cepheids provide valuable insights into the evolution of stellar multiplicity among intermediate-mass stars. Here, we present a systematic investigation of single-lined spectroscopic binaries (SB1s) based on high-precision velocities measured by the VELOcities of CEpheids (VELOCE) project. We detected 76 (29%) SB1 systems among the 258 Milky Way Cepheids in the first VELOCE data release, 32 (43%) of which were not previously known to be SB1 systems. We determined 30 precise and three tentative orbital solutions, 18 (53%) of which are reported for the first time. This large set of Cepheid orbits provides a detailed view of the eccentricity e and orbital period P_orb distribution among evolved intermediate-mass stars, ranging from e ∈ [0.0, 0.8] and P_orb ∈ [240, 9000] d. The orbital motion on timescales exceeding the 11 yr VELOCE baseline was investigated using a template-fitting technique applied to literature data. Particularly interesting objects include (a) R Cru, the Cepheid with the shortest orbital period in the Milky Way (∼238 d); (b) ASAS J103158−5814.7, a short-period overtone Cepheid exhibiting time-dependent pulsation amplitudes as well as orbital motion; and (c) 17 triple systems with outer visual companions, among other interesting objects. Most VELOCE Cepheids (21/23) that exhibit evidence of a companion based on a Gaia proper motion anomaly are also spectroscopic binaries, whereas the remaining do not exhibit significant (> 3σ) orbital radial velocity variations. Gaia quality flags, notably the renormalized unit weight error (RUWE), do not allow Cepheid binaries to be identified reliably although statistically the average RUWE of SB1 Cepheids is slightly higher than that of non-SB1 Cepheids. A comparison with Gaia photometric amplitudes in G-, Bp, and Rp also does not allow one to identify spectroscopic binaries among the full VELOCE sample, indicating that the photometric amplitudes in this wavelength range are not sufficiently informative of companion stars.

The Cepheid AW Per is a component in a multiple system with a long-period orbit. The radial velocities of Griffin cover the 38 yr orbit well. An extensive program of interferometry with the Center for High Angular Resolution Astronomy array is reported here, from which the long-period orbit is determined. In addition, a Hubble Space Telescope high-resolution spectrum in the ultraviolet demonstrates that the companion is itself a binary with nearly equal-mass components. These data combined with a distance from Gaia provide a mass of the Cepheid (primary) of M ₁ = 6.79 ± 0.85 M _⊙. The combined mass of the secondary is M _S = 8.79 ± 0.50 M _⊙. The accuracy of the mass will be improved after the fourth Gaia data release, expected in approximately two years.

The 30 yr orbit of the Cepheid Polaris has been followed with observations by the Center for High Angular Resolution Astronomy (CHARA) Array from 2016 through 2021. An additional measurement has been made with speckle interferometry at the Apache Point Observatory. Detection of the companion is complicated by its comparative faintness—an extreme flux ratio. Angular diameter measurements appear to show some variation with pulsation phase. Astrometric positions of the companion were measured with a custom grid-based model-fitting procedure and confirmed with the CANDID software. These positions were combined with the extensive radial velocities (RVs) discussed by Torres to fit an orbit. Because of the imbalance of the sizes of the astrometry and RV data sets, several methods of weighting are discussed. The resulting mass of the Cepheid is 5.13 ± 0.28 M _⊙. Because of the comparatively large eccentricity of the orbit (0.63), the mass derived is sensitive to the value found for the eccentricity. The mass combined with the distance shows that the Cepheid is more luminous than predicted for this mass from evolutionary tracks. The identification of surface spots is discussed. This would give credence to the identification of a radial velocity variation with a period of approximately 120 days as a rotation period. Polaris has some unusual properties (rapid period change, a phase jump, variable amplitude, and unusual polarization). However, a pulsation scenario involving pulsation mode, orbital periastron passage, and low pulsation amplitude can explain these characteristics within the framework of pulsation seen in Cepheids.