LMC6041IM/NOPB 其他数据使用手册 - TI（德州仪器）

arXiv:1508.02725v1 [astro-ph.SR] 11 Aug 2015

Astronomy & Astrophysics

manuscript no. 1812

c

 ESO 2015

August 13, 2015

Letter to the Editor

The strange evolution of the Large Magellanic Cloud Cepheid

OGLE-LMC-CEP1812

Hilding R. Neilson

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Argelander-Institut f¨ur Astronomie, Universit¨at Bonn, Auf dem H¨ugel 71, D-53121 Bonn, Germany

4

Department of Physics & Astronomy, East Tennessee State University, Box 70652, Johnson City, TN 37614 USA

Classical Cepheids are key probes of both stellar astrophysics and cosmology as standard candles and pulsating variable stars. It is

important to understand Cepheids in unprecedented detail in preparation for upcoming GAIA, JWST and extremely-large telescope

observations. Cepheid eclipsing binary stars are ideal tools for achieving this goal, however there are currently only three known

systems. One of those systems, OGLE-LMC-CEP1812, raises new questions about the evolution of classical Cepheids because of

an apparent age discrepancy between the Cepheid and its red giant companion. We show that the Cepheid component is actually the

product of a stellar merger of two main sequence stars that has since evolved across the Hertzsprung gap of the HR diagram. This

Cepheid Leavitt law more than a century ago (Leavitt 1908).

They have been used to measure the Hubble constant (Hubble

1929; Riess et al. 2011; Freedman et al. 2012) to a precision of

2% as well as to constrain the detailed physics of stellar struc-

ture and evolution (e.g., Bono et al. 2000), yet there still exist a

number of mysteries regarding these stars.

One such challenge is the detailed calibration of the Cepheid

Leavittlaw, i.e., the period-luminosity(PL) relation. In the forth-

coming era of the James Webb Space Telescope, it is expected

that we will be able to measure the Hubble constant to less than

Cepheids for which distances are known.

One enduring uncertainty is the decades-old ‘Cepheid mass

discrepancy’ (Cox 1980), where Cepheid masses measured us-

ing stellar evolutionand pulsation calculations differ by about 10

– 20% (Keller 2008). Bono et al. (2006) suggested four possible

resolutions to this discrepancy: changes in the radiative opac-

ities, rotation, convective core overshooting in main sequence

progenitors and stellar mass loss. Neilson et al. (2011) showed

that pulsation-driven mass loss can partly explain the mass dis-

crepancy, while Anderson et al. (2014) proposed rotation as an

discrepancy is to independently measure Cepheid masses.

Cepheid distances and masses can be measured in eclipsing

binary systems. Evans et al. (2005) suggested that about 35% of

all Galactic Cepheids are in spectroscopic binary systems, but

none are known to be in eclipsing binary systems. Evans et al.

(2013) presented a catalogue of binary companions detected

using ultraviolet spectral observations, where the companions

are all about 2 M

, inferring a larger binary fraction of 60%.

Evans et al. (2015) report radial velocity measurements which

LMC (Soszynski et al. 2008). These eclipsing binary systems

provide unique mass and distance estimates that can be com-

pared to estimates using Cepheid evolution and pulsation mod-

els. Thesehelp to resolvethe Cepheid mass discrepancyandcon-

strain the PL relation in greater detail.

Pietrzy´nski et al. (2010) presented detailed observations of

one such eclipsing binary, OGLE-LMC-CEP0227 (CEP0227),

with a 310 day orbit and a mass ratio between the components

of q = 1.00± 0.01. One binary component is a classical Cepheid

and the companion a red giant star. The authors measured a