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Topic Name: Taking Advantage of Light Echoes A Team of Astronomers Measure the Distance of a Cepheid using ESO Telescope

Category: STAR (Space, Telecommunications & Radioscience)

Research persons: Pierre Kervella

Location: European Organisation for Astronomical Research, Switzerland

Details

Taking advantage of the presence of light echoes, a team of astronomers have used an European Organisation for Astronomical Research telescope to measure, at the 1% precision level, the distance of a Cepheid - a class of variable stars that constitutes one of the first steps in the cosmic distance ladder.

"Our measurements with ESO's New Technology Telescope at La Silla allow us to obtain the most accurate distance to a Cepheid," says Pierre Kervella, lead-author of the paper reporting the result.

Cepheids are pulsating stars that have been used as distance indicators since almost a hundred years. The new accurate measurement is important as, contrary to many others, it is purely geometrical and does not rely on hypotheses about the physics at play in the stars themselves.

The team of astronomers studied RS Pup, a bright Cepheid star located towards the constellation of Puppis ('the Stern') and easily visible with binoculars. RS Pup varies in brightness by almost a factor of five every 41.4 days. It is 10 times more massive than the Sun, 200 times larger, and on average 15 000 times more luminous.

RS Pup is the only Cepheid to be embedded in a large nebula, which is made of very fine dust that reflects some of the light emitted by the star.

Because the luminosity of the star changes in a very distinctive pattern, the presence of the nebula allows the astronomers to see light echoes and use them to measure the distance of the star.

"The light that travelled from the star to a dust grain and then to the telescope arrives a bit later than the light that comes directly from the star to the telescope," explains Kervella. "As a consequence, if we measure the brightness of a particular, isolated dust blob in the nebula, we will obtain a brightness curve that has the same shape as the variation of the Cepheid, but shifted in time."

This delay is called a 'light echo', by analogy with the more traditional echo, the reflection of sound by, for example, the bottom of a well.

By monitoring the evolution of the brightness of the blobs in the nebula, the astronomers can derive their distance from the star: it is simply the measured delay in time, multiplied by the velocity of light (300 000 km/s). Knowing this distance and the apparent separation on the sky between the star and the blob, one can compute the distance of RS Pup.

From the observations of the echoes on several nebular features, the distance of RS Pup was found to be 6500 light years, plus or minus 90 light years.

"Knowing the distance to a Cepheid star with such an accuracy proves crucial to the calibration of the period-luminosity relation of this class of stars," says Kervella. "This relation is indeed at the basis of the distance determination of galaxies using Cepheids."

RS Pup is thus distant by about a quarter of the distance between the Sun and the Centre of the Milky Way. RS Pup is located within the Galactic plane, in a very populated region of our Galaxy.

Note for Cepheid
A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity. The namesake and prototype of these variables is the star Delta Cephei, discovered to be variable by John Goodricke in 1784.
Because of this correlation (discovered and stated by Henrietta Swan Leavitt in 1908 and given precise mathematical form by her in 1912), a Cepheid variable can be used as a standard candle to determine the distance to its host cluster or galaxy. Since the period-luminosity relation can be calibrated with great precision using the nearest Cepheid stars, the distances found with this method are among the most accurate available.
A Cepheid is usually a population I giant yellow star, pulsing regularly by expanding and contracting, resulting in a regular oscillation of its luminosity. The luminosity of cepheid stars range from 103 to 104 times that of the Sun. Because Cepheids are from population I, they are sometimes called Type I Cepheids, while the similar (but belonging to population II) W Virginis variables are known as Type II Cepheids.
The exact mass of Cepheids with given brightness or oscillations is not known to any great precision, but astronomers hope to gather data for this from the newly-discovered third star of the Polaris system.
The variation in luminosity is caused by a cycle of ionization of helium in the star's atmosphere, followed by expansion and deionization. While ionized, the atmosphere is more opaque to light. This cycle has a period equal to the star's dynamical time scale, therefore giving information on the mean density of the body as well as its luminosity.

Note for Nebula
A nebula is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of a star's cycle. Originally nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way. Nebulae often form star-forming regions, such as in the Eagle Nebula. This nebula is depicted in one of NASA's most famous images, the "Pillars of Creation". In these regions the formations of gas, dust and other materials 'clump' together to form larger masses, which attract further matter, and eventually will become big enough to form stars. The remaining materials are then believed to form planets, and other solar system objects.
Many nebulae form from the gravitational collapse of diffuse gas in the interstellar medium or ISM. As the material collapses under its own weight, massive stars may form in the centre, and their ultraviolet radiation ionises the surrounding gas, making it visible at optical wavelengths. An example of this type of nebula is the Rosette Nebula or the Pelican Nebula. The size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas, and the number of stars formed can vary too. As the sites of star formation, the formed stars are sometimes known as a young, loose cluster.
Some nebulae are formed as the result of supernova explosions, the death throes of massive, short-lived stars. The material thrown off from the supernova explosion is ionised by the supernova remnant. One of the best examples of this is the Crab Nebula, in Taurus. It is the result of a recorded supernova in the year 1054 and at the centre of the nebula is a neutron star, created during the explosion.
Other nebulae may form as planetary nebulae. This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8-10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost a sufficient amount of material, its temperature increases and the ultraviolet radiation it emits is capable of ionizing the surrounding nebula that it has thrown off.

Note for Milky Way
The Milky Way, is a barred spiral galaxy that is part of the Local Group of galaxies. Although the Milky Way is one of billions of galaxies in the observable universe, the Galaxy has special significance to humanity as it is the home galaxy of the planet Earth. The Milky Way galaxy is visible from Earth as a band of light in the night sky, and it is the appearance of this band of light which has inspired the name for our galaxy.
Some sources hold that, strictly speaking, the term Milky Way should refer exclusively to the observation of the band of light, while the full name Milky Way Galaxy, or alternatively the Galaxy should be used to describe our galaxy as an astrophysical whole. It is unclear how widespread the usage of this convention is, however, and the term "Milky Way" is routinely used in either context.
The Milky Way looks brightest in the direction of the constellation of Sagittarius, toward the galactic center. Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic relative to the galactic plane. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that our Solar System lies close to the galactic plane. The Milky Way has a relatively low surface brightness, making it difficult to see from any urban or suburban location suffering from light pollution.

About La Silla Observatory
La Silla Observatory is an astronomical observatory in Chile with eighteen telescopes. Nine of these telescopes were built by the European Southern Observatory (ESO) organisation, and several of the others are partly maintained by ESO. The observatory is one of the largest in the Southern Hemisphere.
La Silla is a 2400 m high mountain, bordering the southern extremity of the Atacama Desert in Chile. It is located about 160 km north of La Serena, 27 km south of Las Campanas Observatory, and 100 km north of Cerro Tololo Observatory.
Originally known as Cinchado, the mountain was renamed La Silla (the saddle) after its shape. It rises quite isolated and remote from any artificial light and dust sources (astronomy's worst enemies). La Silla was the first observatory in Chile used by ESO. Its history is full of optimism and disappointments, ups and downs, since its beginnings in the 1950's until the middle of the 1970's when the observatory became a reality.

The team is composed of Pierre Kervella and Guy Perrin (LESIA, Observatoire de Paris, France), Antoine Mérand (Center for High Angular Resolution Astronomy, Atlanta, Georgia, USA), László Szabados (Konkoly Observatory, Budapest, Hungary), Pascal Fouqué (Observatoire Midi-Pyrénées, Toulouse, France), David Bersier (Liverpool John Moores University, UK), and Emanuela Pompei (ESO).

In figure 1, This colour composite image is based on observations made with the 3.6-m ESO New Technology Telescope (NTT) installed at the La Silla Observatory (Chile) and equipped with the ESO Multi-Mode Instrument (EMMI). The image is based on data obtained through B, V, and R-filters. In order to avoid a heavy saturation of the detector, RS Pup was positioned in the gap between the detectors.

In figure 2, In order to better vizualise the light echo phenomenon, the astronomers computed a continuous movie of the progression of the light echoes in the nebula (using a spline interpolation of the measured epochs for each pixel). The variation of the luminosity of RS Pup is visible on these images from the changing extension of the wings of its 'Point Spread Function' (PSF). The morphological changes of the nebula with time are considerable.

In figure 3, The determination of the distance to RS Pup, following the method of the American astronomer Robert Havlen, is based on the measurement of the phase difference between the variation of the star and the variation of isolated nebular features. Because the luminosity of the star changes in a very distinctive pattern, the presence of the nebula allows the astronomers to see light echoes and use them to measure the distance of the star. The light that travelled from the star to a dust grain and then to the telescope arrives a bit later than the light that comes directly from the star to the telescope. As a consequence, if we measure the brightness of a particular, isolated dust blob in the nebula, we will obtain a brightness curve that has the same shape as the variation of the Cepheid, but shifted in time. This delay is called a 'light echo', by analogy with the more traditional echo, the reflection of sound by, for example, the bottom of a well. By monitoring the evolution of the brightness of the blobs in the nebula, the astronomers can derive their distance from the star: it is simply the measured delay in time, multiplied by the velocity of light (300 000 km/s). Knowing this distance and the apparent separation on the sky between the star and the blob, one can compute the distance of RS Pup. This artist's illustration is not to scale.

In figure 4, This artist's impression shows the location of the Cepheid star RS Pup in our Galaxy, as determined by the astronomers using data from ESO's NTT. From the observations of the echoes on several nebular features, the distance of RS Pup was found to be 6500 light years, with an uncertainty of 90 light years. RS Pup is distant by about a quarter of the distance between the Sun and the Centre of the Milky Way. RS Pup is located within the Galactic plane, in a very populated region of our Galaxy. 

In figure 5, The nebula around RS Pup seen at 6 different epochs, corresponding to different phases of its 41.4 day cycle, as indicated in the top left part of each image. By monitoring both the light variation of the star (not visible on these images as it is positioned in the gap between the detectors) and the the light echoes from several features in the nebula, it is possible to determine the distance of the star. The changes in the nebula are for example very easy to detect in the middle part, just above and right to the two close foreground stars.

In figure 6, This short animation illustrates the principle of the light echo phenomenon used to determine the distance of the Cepheid star RS Pup