Computer Science – Sound
Scientific paper
Oct 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004eso..pres...25.&link_type=abstract
ESO Press Release, 10/2004
Computer Science
Sound
Scientific paper
VLTI Watches the Changing Size of Bright Southern Cepheids
Summary
Taking advantage of the very high spatial resolution provided by the Very Large Telescope Interferometer, a team of French and Swiss astronomers [1] has measured directly the change in angular diameter of four southern Cepheid variable stars over their pulsation cycle.
When combined with spectroscopic radial velocity measurements, this allowed the astronomers to measure very accurately the distances of these stars in a quasi-geometrical way, and to calibrate the zero-point of the Cepheid Period-Luminosity empirical law.
These observations constitute a fundamental step towards an independent verification of the extragalactic distance scale by interferometry.
PR Photo 30a/04: Observation Techniques of the Baade-Wesselink Method. PR Photo 30b/04: Paranal Platform and VLTI Baselines Used. PR Photo 30c/04: Pulsation of the Cepheid Variable L Car. (VINCI/VLTI) PR Photo 30d/04: Period-Luminosity relation for Cepheids. (VINCI/VLTI)
Cepheids and the cosmic distance ladder
It is very difficult to measure the distance to an astronomical object. In fact, this is one of the greatest challenges facing astronomers. There is indeed no accurate, direct way to determine the distance to galaxies beyond the Milky Way: astronomers first determine the distance to nearby stars in our galaxy as accurately as possible and then use a series of other techniques that reach progressively further into space to estimate distances to more distant systems. This process is often referred as the "cosmic distance ladder".
Over the years, a number of different distance estimators have been found. One of these is a particular class of stars known as Cepheid variables. They are used as one of the first "steps" on this cosmic distance ladder.
Cepheids are rare and very luminous stars whose luminosity varies in a very regular way. They are named after the star Delta Cephei in the constellation of Cepheus, the first known variable star of this particular type and bright enough to be easily seen with the unaided eye.
In 1912, American astronomer Henrietta Leavitt observed 20 variable stars of the Cepheid-type in the Small Magellanic Cloud (SMC), one of the closest galaxies to the Milky Way. For all purposes, these stars are all at the same distance (the size of the SMC is negligible compared to its much larger distance from us). Apparently brighter stars in this group are thus also intrinsically brighter (more luminous). Henrietta Leavitt discovered a basic relation between the intrinsic brightness and the pulsation period of Cepheid variable stars in the SMC and showed that intrinsically brighter Cepheids have longer periods.
This relation is now known as the "Period-Luminosity relation" and is an important way to derive the distance to stars of this type. By measuring the period of a Cepheid star, its intrinsic brightness can be deduced and from the observed apparent brightness, the distance may then be calculated. In this way, Cepheid stars are used by astronomers as one of the "standard candles" in the Universe. They act either as distance indicators themselves or are used to calibrate other distance indicators.
The Cepheid stars have taken on an even more important role since the Hubble Space Telescope Key Project on the extragalactic distance scale relies completely on them for the calibration of distance indicators to reach cosmologically large distances. In other words, if the calibration of the Cepheid Period-Luminosity relation were wrong, the entire extragalactic distance scale and with it, the rate of cosmic expansion and the related acceleration, as well as the estimated age of the Universe, would also be off.
A main problem is thus to calibrate as accurately as possible the Period-Luminosity relation for nearby Cepheids. This requires measuring their distances with the utmost precision, a truly daunting task. And this is where interferometry now enters the picture.
The Baade-Wesselink method
ESO PR Photo 30a/04
ESO PR Photo 30a/04
Title
[Preview - JPEG: 400 x 345 pix - 104k] [Normal - JPEG: 800 x 690 pix - 229k]
Caption of ESO PR Photo 30a/04: The two observation techniques used for the interferometric version of the Baade-Wesselink method are high-resolution spectroscopy (left) and interferometry (right). The former provides the radial velocity curve over the pulsation cycle of the star. When integrated, this in turn provides the linear radius variation of the star (in metres). The interferometric observations document variation of the star's angular radius. The ratio of these two quantities gives the distance of the Cepheid.
Independent determinations of the distance of variable stars make use of the so-called Baade-Wesselink method, named after astronomers Walter Baade (1893 - 1960) and Adriaan Wesselink (1909 - 1995). With this classical method, the variation of the angular diameter of a Cepheid variable star is inferred from the measured changes in brightness (by means of model atmosphere calculations) as it pulsates. Spectroscopy is then used to measure the corresponding radial velocity variations, hence providing the linear distance over which the star's outer layers have moved. By dividing the angular and linear measures, the distance to the star is obtained.
This sounds straightforward. However, it would obviously be much better to measure the variation of the radius directly and not to rely on model atmosphere calculations. But here the main problem is that, despite their apparent brightness, all Cepheids are situated at large distances. Indeed, the closest Cepheid star (excluding the peculiar star Polaris), Delta Cephei, is more than 800 light-years away. Even the largest Cepheids in the sky subtend an angle of only 0.003 arcsec. To observe this is similar to view a two-storey house on the Moon. And what astronomers want to do is to measure the change of the stars' sizes, amounting to only a fraction of this!
Such an observing feat is only possible with long-baseline interferometry. Also on this front, the VLT Interferometer is now opening a new field of observational astrophysics.
Three VLTI baselines
ESO PR Photo 30b/04
ESO PR Photo 30b/04
Title
[Preview - JPEG: 400 x 345 pix - 112k] [Normal - JPEG: 800 x 690 pix - 276k]
Caption: ESO PR Photo 30b/04 is a view of the Paranal platform with the three baselines used for the VLTI observations of Cepheids (in red).
ESO PR Photo 30c/04
ESO PR Photo 30c/04
Title
[Preview - JPEG: 400 x 367 pix - 88k] [Normal - JPEG: 800 x 734 pix - 180k]
Caption: ESO PR Photo 30c/04 shows the VINCI observations of the pulsation of the Cepheid variable L Car (P = 35.5 days, red dots) and the adjusted radius curve (green line), as deduced from the integration of the radial velocity measured on this star over its pulsation period.
Some time ago, an undaunted team of French and Swiss astronomers [1] started a major research programme aimed at measuring the distance to several Cepheids by means of the above outlined Baade-Wesselink interferometric method. For these observations they combined sets of two beams - one set from the two VLTI Test Siderostats with 0.35m aperture and the other set from two Unit Telescopes (Antu and Melipal; 8.2m mirrors) - with the VINCI (VLT Interferometer Commissioning Instrument) facility. Three VLTI baselines were used for this programme with, respectively, 66, 140 and 102.5m ground length. ESO PR Photo 30b/04 shows the respective positions on the VLTI platform. The observations were made in the near-infrared K-band.
A total of 69 individual angular diameter measurements were obtained with the VLTI, over more than 100 hours of total telescope time, distributed over 68 nights; the largest angular diameter measured was 0.0032 arcsec (L Car at maximum).
Seven Cepheids observable from Paranal Observatory were selected for this programme: X and W Sagittarii, Eta Aquilae, Beta Doradus, Zeta Gemini, Y Ophiocus and L Carinae. Their periods range from 7 to 35.5 days, a fairly wide interval and an important advantage to properly calibrate the Period-Luminosity r
No affiliations
No associations
LandOfFree
Measuring Cosmic Distances with Stellar Heart Beats does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Measuring Cosmic Distances with Stellar Heart Beats, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Measuring Cosmic Distances with Stellar Heart Beats will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-843114