The two nearby galaxies - M31 and M33, are stepping stones to most of
our current effort to understand the evolving universe at large
scales. They are essential to the calibration of the extragalactic
distance scale and for constraining population synthesis models for
early galaxy formation and evolution. There is one simple requirement
for all this - accurate distances.
Detached eclipsing binaries (DEBs) have the potential to
establish distances to M31 and M33 with an unprecedented accuracy of
better than 5% and possibly to better than 1%. These distances are now
known to no better than 10-15%, as there are discrepancies of 0.2-0.3
mag between various distance indicators. Eclipsing binaries offer a
single step distance determination to these galaxies and may therefore
provide an accurate zero point calibration - a major step towards very
accurate determination of the Hubble constant. DEBs have not
been used so far as distance indicators to M31 and M33. Only now
availability of large format CCD detectors and inexpensive CPUs makes
it possible to organize a massive search for periodic variables, which
is producing a handful of good DEB candidates. These can then be
observed with the powerful new 6.5-10 meter telescopes.
As a first step of the DIRECT project, between September 1996
and November 1999 we obtained about 200 full/partial nights on the
F. L. Whipple Observatory (FLWO) 1.2-meter telescope and the
Michigan-Dartmouth-MIT (MDM) 1.3-meter telescope to search for
detached eclipsing binaries and Cepheids in the M31 and the M33
galaxies. The second step involved obtaining precise light curves for
2 selected targets in M33 with the Kitt Peak 2.1 meter over 27 nights
in 1999 and 2001. In 2002-2004, we obtained a total of 4 nights on the
Keck II 10 meter telescope and 19 hours on the 8 meter Gemini
telescope for the spectroscopic follow-up of the brightest DEB (V=19.5
mag). With infrared photometry from Gemini we constrained the
extinction to the system and measured a 6% distance to M33: 964+/-54
kpc (24.92 +/- 0.12 mag).
The massive photometry provides us with good light curves for
DEBs and for Cepheid variables, which is essential to the
parallel project to derive direct Baade-Wesselink distances to
Cepheids in M31 and M33. With both Cepheids and eclipsing binaries the
distances to M31 and M33 will be free of any intermediate steps (hence
the DIRECT).
Invited talk
The DIRECTProject by Macri, was published in the
proceedings of the IAU Colloquium 193, "Variable stars in the Local
Group", D.W. Kurtz & K. Pollard, eds. (2004)
(also in the astro-ph
preprint server)
We can also make
available the CCD images for the fields which were processed by us,
i.e. at this point seven fields M31A,
M31B, M31C, M31D, M31F, M33A, M33B. Because of the size (>1
GBy of data per field), we do not keep them permanently on the WWW, so
please write to kstanek@cfa.harvard.edu to
arrange the transfer.
Have a look at our posters (along with the poster-making software
assemble) from the 189th AAS meeting in Toronto (January
1997)
See some lightcurves of variables we
detect in M31 and M33
DIRECT fields in 1996 and 1997; below the field
M31F in color
January 2007
Paper The First DIRECT Distance Determination to a Detached Eclipsing Binary in M33 by Bonanos, Stanek, Kudritzki, Macri et al. was published in the Astrophysical Journal (November 2006) (also in the astro-ph preprint server)
Paper The DIRECT Project: Influence of Blending on the Cepheid Distance Scale. II. Cepheids in M33 by Mochejska et al. was submitted to the Astronomical Journal (March 2001) (also in the astro-ph preprint server)
Have a look at our posters (along with the poster-making software assemble) from the 189th AAS meeting in Toronto (January 1997)
See some lightcurves of variables we detect in M31 and M33
Paper Two Confirmed Cataclysmic Variables in the Old Stellar Cluster NGC 6791 by Kaluzny et al., partly based on the data collected during the 1996 DIRECT season, appeared on Dec. 10th, 1997 in the Astrophysical Journal
- Optical Gravitational Lensing Experiment
