Tuning in on Cepheids: Radial velocity amplitude modulations. A source of systematic uncertainty for Baade-Wesselink distances

[Abridged] I report the discovery of modulations in radial velocity (RV) curves of four Galactic classical Cepheids and investigate their impact as a systematic uncertainty for Baade-Wesselink distances. Highly precise Doppler measurements were obtained using the Coralie high-resolution spectrograph since 2011. Particular care was taken to sample all phase points in order to very accurately trace the RV curve during multiple epochs and to search for differences in linear radius variations derived from observations obtained at different epochs. Different timescales are sampled, ranging from cycle-to-cycle to months and years. The unprecedented combination of excellent phase coverage obtained during multiple epochs and high precision enabled the discovery of significant modulation in the RV curves of the short-period s-Cepheids QZ Normae and V335 Puppis, as well as the long-period fundamental mode Cepheids l Carinae and RS Puppis. The modulations manifest as shape and amplitude variations that vary smoothly on timescales of years for short-period Cepheids and from one pulsation cycle to the next in the long-period Cepheids. The order of magnitude of the effect ranges from several hundred m/s to a few km/s. The resulting difference among linear radius variations derived using data from different epochs can lead to systematic errors of up to 15% for Baade-Wesselink-type distances, if the employed angular and linear radius variations are not determined contemporaneously. The different natures of the Cepheids exhibiting modulation in their RV curves suggests that this phenomenon is common. The observational baseline is not yet sufficient to conclude whether these modulations are periodic. To ensure the accuracy of Baade-Wesselink distances, angular and linear radius variations should always be determined contemporaneously.Comment: 7 pages, 5 figures, 1 table. Accepted for publication in A&A letter

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.Comment: 6 pages, 4 figures, 2 tables. Accepted for publication in A&A letter

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.Comment: 18 pages, 13 figures, accepted for publication in A&A. Version 2 with minor cosmetic edit

Portfolio Implications of Apartment Investing

In this article, we examine the portfolio implications of apartment investing. In particular, we explore the sector’s relative stability, liquidity, and current market outlook. In general, we find support for many of the advantages attributed to apartments relative to other property types. The apartment sector has historically offered high risk-adjusted returns and a relatively low correlation with other property sectors. These features, combined with the attractive demographics and stable space market fundamentals, suggest that the current environment should be favorable for apartment investing. However, the popularity of the sector, aggressive rent growth assumptions, and potential limitations on future immigration provide sources of performance risk.

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.Comment: 5 pages, 4 figures, proceedings of the 22nd Los Alamos Stellar Pulsation Conference "Wide-field variability surveys: a 21st-century perspective" held in San Pedro de Atacama, Chile, Nov. 28 - Dec. 2, 201

Amplitude Modulation of Cepheid Radial Velocity Curves as a Systematic Source of Uncertainty for Baade-Wesselink Distances

I report on the recent discovery of modulation in the radial velocity curves in four classical Cepheids. This discovery may enable significant improvements in the accuracy of Baade-Wesselink distances by revealing a not previously considered systematic source of uncertaint