ESO: Pulsating Star Mystery Solved

[bystander]

Pulsating Star Mystery Solved
European Southern Observatory | 24 Nov 2010

This wide field view of part of the Large Magellanic Cloud was created from photographs taken in red and blue light as part of the Digitized Sky Survey 2. The remarkable double star OGLE-LMC-CEP0227 lies at the center of the picture, just one of huge numbers of faint stars. Studies of this very rare system have allowed astronomers to measure mass of a Cepheid variable star with unprecedented accuracy. [i](Credit: ESO/DSS2. Acknowledgement: Davide de Martin)[/i]

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By discovering the first double star where a pulsating Cepheid variable and another star pass in front of one another, an international team of astronomers has solved a decades-old mystery. The rare alignment of the orbits of the two stars in the double star system has allowed a measurement of the Cepheid mass with unprecedented accuracy. Up to now astronomers had two incompatible theoretical predictions of Cepheid masses. The new result shows that the prediction from stellar pulsation theory is spot on, while the prediction from stellar evolution theory is at odds with the new observations.

The new results, from a team led by Grzegorz Pietrzyński (Universidad de Concepción, Chile, Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Poland), appear in the 25 November 2010 edition of the journal Nature.
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Classical Cepheid Variables, usually called just Cepheids, are unstable stars that are larger and much brighter than the Sun. They expand and contract in a regular way, taking anything from a few days to months to complete the cycle. The time taken to brighten and grow fainter again is longer for stars that are more luminous and shorter for the dimmer ones. This remarkably precise relationship makes the study of Cepheids one of the most effective ways to measure the distances to nearby galaxies and from there to map out the scale of the whole Universe.

Unfortunately, despite their importance, Cepheids are not fully understood. Predictions of their masses derived from the theory of pulsating stars are 20–30% less than predictions from the theory of the evolution of stars. This embarrassing discrepancy has been known since the 1960s.

To resolve this mystery, astronomers needed to find a double star containing a Cepheid where the orbit happened to be seen edge-on from Earth. In these cases, known as eclipsing binaries, the brightness of the two stars dims as one component passes in front of the other, and again when it passes behind the other star. In such pairs astronomers can determine the masses of the stars to high accuracy. Unfortunately neither Cepheids nor eclipsing binaries are common, so the chance of finding such an unusual pair seemed very low. None are known in the Milky Way.
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The observers carefully measured the brightness variations of this rare object, known as OGLE-LMC-CEP0227, as the two stars orbited and passed in front of one another. They also used HARPS and other spectrographs to measure the motions of the stars towards and away from the Earth — both the orbital motion of both stars and the in-and-out motion of the surface of the Cepheid as it swelled and contracted.

This very complete and detailed data allowed the observers to determine the orbital motion, sizes and masses of the two stars with very high accuracy — far surpassing what had been done before for a Cepheid. The mass of the Cepheid is now known to about 1% and agrees exactly with predictions from the theory of stellar pulsation. However, the larger mass predicted by stellar evolution theory was shown to be significantly in error.

The much-improved mass estimate is only one outcome of this work, and the team hopes to find other examples of these remarkably useful pairs of stars to exploit the method further. They also believe that from such binary systems they will eventually be able to pin down the distance to the Large Magellanic Cloud to 1%, which would mean an extremely important improvement of the cosmic distance scale.

The dynamical mass of a classical Cepheid variable star in an eclipsing binary system G Pietrzyński et al

• Nature 468(7323) 542 (25 Nov 2010) DOI: 10.1038/nature09598

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Cepheid varying in the [b][url=http://antwrp.gsfc.nasa.gov/apod/ap960110.html]Spiral Galaxy M100[/url][/b]

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<<Classical Cepheids are population I variable stars that exhibit pulsation periods on the order of a few days to months, are 4–20 times more massive than the Sun, and up to 100,000 times more luminous. The more massive Cepheids are more luminous and have more extended envelopes. Because these envelopes are more extended and the density in their envelopes is lower, their variability period, which is proportional to the inverse square root of the density in the layer, is longer.

Cepheids are supergiants of spectral class F6 – K2 and their radii change by (~25% for the longer-period l Car) millions of kilometers during a pulsation cycle. There exists a well-defined relationship between a Cepheid variable's luminosity and pulsation period, securing Cepheids as viable standard candles for establishing the Galactic and Extragalactic Distance Scales. Over 700 classical Cepheids are known in the Milky Way Galaxy, and several thousand extragalactic Cepheids have been discovered. The Hubble Space Telescope has identified classical Cepheids in NGC 4603, which is 100 million light years distant. Some fairly bright Cepheids whose brightnesses vary enough to easily discern with the naked eye include Zeta Geminorum and Beta Doradus. The best-known star which is a Cepheid (and also the closest Cepheid to us) is the "North star", Polaris. It has a period of 3.9696 days and varies 0.05 magnitudes in brightness. The star exhibits many peculiarities.

On September 10, 1784 Edward Pigott detected the variability of Eta Aquilae, the first known representative of the class of Cepheid variables. However, the namesake for classical Cepheids is the star Delta Cephei, discovered to be variable by John Goodricke a few months later. The period-luminosity relation of Cepheids was discovered in 1908 by Henrietta Swan Leavitt in an investigation of thousands of variable stars in the Magellanic Clouds. She published it in 1912 with further evidence.

Cepheid variables are divided into at least two subclasses: classical Cepheids (also called Delta Cephei variables or Population I Cepheids) and Type II Cepheids (also called Population II Cepheids and historically termed W Virginis variables). The former are young massive population I stars, whereas the latter are older, less massive, fainter population II stars. Classical Cepheids and Type II Cepheids follow different period-luminosity relationships. The luminosity of Type II Cepheids is, on average, less than classical Cepheids by about 1.5 magnitudes (but still brighter than RR Lyrae stars). The RR Lyrae stars were recognized fairly early (by the 1930s) as being a separate class of variable, due in part to their short periods. RR Lyrae stars are associated with older globular clusters and can be found at any galactic latitude, whereas classical Cepheids are associated with the galactic plane given they are younger and more massive. Initial studies of Cepheid variable distances were complicated by the admixture of classical Cepheids, RR Lyrae variables, and W Virginis variables. In 1942 Walter Baade realized that the Cepheids in the Andromeda Galaxy were of two populations. All these stars lie within the instability strip of the Hertzsprung–Russell diagram of stellar types. The variable stars may be used as a standard candles for measuring distances within our own galaxy, but trace different structures owing to their different ages.

In 1915 Harlow Shapley used Cepheids to place initial constraints on the size and shape of the Milky Way, and of the placement of our Sun within it. In 1924 Edwin Hubble discovered Cepheid variables in the Andromeda galaxy. That settled the Island Universe debate, concerning the question of whether the Milky Way and the Universe were synonymous, or was the Milky Way merely one in a plethora of galaxies that constitutes the Universe. Combining his calculations based on Cepheids of distances of galaxies with Vesto Slipher's measurements of the speed at which the galaxies recede from us, in 1929 Hubble and Milton L. Humason formulated what is now known as Hubble's law. They discovered that the Universe is expanding (see the expansion of the Universe). HST observations of classical Cepheid variables have enabled firmer constraints on Hubble's law. Cepheids have also been used to clarify many characteristics of our galaxy, for example: the Sun's height above the galactic plane, the distance to the galactic center, and local galactic spiral structure.

The accepted explanation for the pulsation of Cepheids is called the Eddington valve, or κ-mechanism, where the Greek letter κ (kappa) denotes gas opacity. Helium is the gas thought to be most active in the process. Doubly-ionized helium (helium whose atoms are missing two electrons) is more opaque than singly-ionized helium. The more helium is heated, the more ionized it becomes. At the dimmest part of a Cepheid's cycle, the ionized gas in the outer layers of the star is opaque, and so is heated by the star's radiation, and due to the increased temperature, begins to expand. As it expands, it cools, and so becomes less ionized and therefore more transparent, allowing the radiation to escape. Then the expansion stops, and reverses due to the star's gravitational attraction. The process then repeats. The mechanics of the pulsation as a heat-engine was proposed in 1917 by Arthur Stanley Eddington (who wrote at length on the dynamics of Cepheids), but it was not until 1953 that S. A. Zhevakin identified ionized helium as a likely valve for the engine.

The period-luminosity relationship has been calibrated by many astronomers throughout the twentieth century, beginning with Hertzsprung. The distance to a Cepheid follows from the period-luminosity relation. Calibrating the period-luminosity relation has been problematic. However, a firm Galactic calibration was established by Benedict et al. 2007 using precise HST parallaxes for 10 nearby classical Cepheids. Also, in 2008, ESO astronomers estimated with a precision within 1% the distance to the Cepheid RS Puppis, using light echos from a nebula in which it is embedded. However, that latter finding has been actively debated in the literature. A calibration was published by Michael Feast and Robin Catchpole in 1997 using trigonometric parallaxes determined by the Hipparcos satellite. The relationship between a Population I Cepheid's period P, and its luminosity, measured as its mean absolute magnitude Mv was:



Chief among the uncertainties tied to the Cepheid distance scale are: the nature of the period-luminosity relation in various passbands, the impact of metallicity on both the zero-point and slope of those relations, and the effects of photometric contamination (blending) and a changing (typically unknown) extinction law on Cepheid distances. These unresolved matters have resulted in cited values for the Hubble constant ranging between 60 km/s/Mpc and 80 km/s/Mpc. Resolving this discrepancy is one of the foremost problems in astronomy since the cosmological parameters of the Universe may be constrained by supplying a precise value of the Hubble constant.>>