Measuring Cosmic Distances with Stellar Heart Beats (Forwarded)

ESO Education and Public Relations Dept.

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Contacts:

Pierre Kervella
Observatoire de Paris-Meudon
France
Phone: +33 1 45 07 79 66
Email:

Denis Mourard
Observatoire de la Côte d'Azur
France
Phone: +33 4 93 40 54 92
Email:

For immediate release: 29 October 2004

ESO Press Release 25/04

Measuring Cosmic Distances with Stellar Heart Beats

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

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 30c/04

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 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 relation.

The distances to four of the stars (Eta Aql, W Sgr, Beta Dor
and L Car) were derived using the interferometric Baade-
Wesselink method, as their pulsation is detected by the VLTI.
ESO PR Photo 30c/04 shows the angular diameter measurements
and the fitted radius curve of L Car (P = 35.5 days); this
measures its distance with a relative precision better than
5%.

For the remaining three objects of the sample (X Sgr,
Zeta Gem and Y Oph), a hybrid method was applied to derive
their distances, based on their average angular diameter
and pre-existing estimations of their linear diameters.

The new calibration

ESO PR Photo 30d/04

Caption: ESO PR Photo 30d/04 represents the Period-
Luminosity relation in the V band, as deduced from the
interferometric observations of Cepheids and the HST
parallax measurement of Delta Cep. The green line is
the fitted P-L relation, assuming the slope from previous
authors (Gieren et al.; 1998, ApJ, 496, 17). The agreement
between the model and the measurements is excellent, in
particular for the high-precision measurements of
Delta Cep and L Car.

Combining the distances measured by this programme with the
apparent magnitudes of the stars, the astronomers determined
the absolute magnitude (intrinsic brightness) of these stars
and arrived at a very precise calibration of the zero-point
of the Period-Luminosity relation (assuming the slope from
previous work).

It turned out that this new and independently derived value
of the zero-point is exactly the same as the one obtained
during previous work based on a large number of relatively
low-precision Cepheid distance measurements by the ESA
Hipparcos astrometric satellite. The agreement between these
two independent, geometrical calibrations is remarkable and
greatly increases the confidence in the cosmic distance
scale now in use.

Prospects with AMBER

With 1.8m Auxiliary Telescopes soon to be ready on the VLTI
platform, the astronomers will be able to observe many more
Cepheids with a precision at least as good as the present
high-precision VINCI observations of L Car. In addition,
the future AMBER instrument will extend the VLTI capabilities
toward shorter wavelengths (J and H bands), providing even
higher spatial resolution than what is now possible with
VINCI (K band).

The combined effect of these two improvements will be to
extend significantly the accessible sample of Cepheids. It
is expected that the distances to more than 30 Cepheids will
then be measurable with a precision better than 5%. This
will provide a high precision calibration of both the
reference point (down to +/- 0.01 mag) and the slope of
the Galactic Cepheid Period-Luminosity.

More information

The information contained in this press release is based on
a series of three research articles which are being published
by the European research journal "Astronomy & Astrophysics"
by P. Kervella and collaborators (Paper I : 2004, A&A, 416,
941, Paper II : 2004, A&A, 423, 327 and Paper III : in press).
The present press release is published exactly three years
after the first observations with two 8.2-m VLT Unit
Telescopes and the VLTI with VINCI were achieved, cf. ESO
PR 23/01.

Note

[1]: The team consists of Pierre Kervella and Vincent Coudé
du Foresto at the Paris Observatory in France, David Bersier
of the Space Telescope Science Institute (USA), Nicolas
Nardetto and Denis Mourard (Observatoire de la Côte d'Azur,
France), and Pascal Fouqué (Observatoire Midi-Pyré né es,
France).

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