3,216 research outputs found
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 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 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 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 sectorâs 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
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