http://www.ast.cam.ac.uk/ioa/wikis/gsaw ... /Main_Page
First, some word about GAIA. As most of you know, HYPPARCOS was a satellite which provide a huge catalog of astrometric position of stars. It turned out that the basic photometry of HYPPARCOS did bring a lot of useful information to the scientifical community.
Based on this success, GAIA is a new satellite which will scan the sky for 5 years. A difference with ground based survey is that GAIA is in space and will do a 360° survey, everywhere! It will also go very deep and it will provide very accurate astrometrical position of about a billion stars (1% of our galaxy). Mag 15 stars position will be accurate to 9-26µas (micro arc seconds!) depending on spectal type. Mag 20 stars to 100-330µas.
But GAIA will also measure photometry for stars (and moving non extended objects such as asteroids or compact comets) with a fairly good accuracy. Again, this is a 360° survey of our all surrounding sky while ground based survey are often limited to some regions (OGLE: southern hemisphere) or Catalina Sky Survey avoiding I believe the galatic plane. COROT and KELPER satellites focused on small sky region. So this will be a unique catalog of photometry (G-band) on all those objects!
And because GAIA is continuous spining around, objects will be measured several times. Minimum 40 times, some up to 200 times, most stars will be measured 70 yimes over the 5y period. So it will detect changes in magnitude... I'll come back to that as this is the goal of the GAIA Science Alert group.
GAIA also includes additional instruments. The design is that any object is scanned as the satellite spins (like drift-scan with our CCDs). First CCDs columns will detect the stars (contrary to HYPPARCOS which was based on a preset catalog of position, GAIA will collect anything that comes in front of his instrument!). Well, there is some limitations there. Basically, the system will detect an object because flux will rise. If it doesn't fall down quick enough, it is not a star (extended object) or a bright (Mag <6) star which saturates. Those will not be measured so GAIA catalog will not include the ~6000 bright stars. Well, it was mentionned a solution was investigated to be able to include mag 1.5-6 but it is not confirmed yet!
Then, all detected stars (or asteroid) will go through an astrometric CCD grid which will also measure the G-band photometry. but there is even more: GAIA includes two very low resolution spectrographs (I couldn't find the exact figure but it seems the resolving power is around R~25!); one for the blue portion of the spectrum, one for the red and near-IR portion. They are called Blue Photometer (BP) and Red Photometer (RP) but will actually provide small spectra. Those will be used to determine a spectral class with a certain level of probability.
Last but not least, the last columns (but covering only half of the rows) make a Radial Velocity Spectrometer (RVS). Ths maks high resolution (R~11500) in a small region (847-871nm, they lost 3nm recently!). This region include the Calcium triplet and several Paschen lines. This will provide Radial Velocities measurements but also key parameters for the astrophysics: effective Temperature, collisional broadening log(g), metallicity, chemical abundances down to mag 12, rotation v sin(i) and interstellar absorption with 8620A Diffuse Interstellar Band (DIB).
Scientific objectives of GAIA are:
– Structure, origin and history of the Milky‐Way
– Stellar physics (incl. variables, multiples, etc...)
– Interstellar medium
– Extra‐solar planets
– Solar system
– Fundamental physics
– Science alerts
GAIA and RVS were presented at pro/am 4th workshop at La Rochelle:
http://astro-proam.com/larochelle2012/p ... 2DKatz.pdf
GAIA should be ready early september, ready to be launched starting september 19th. Other satellites may be ready in that time frame and Soyouz rocket has only 4 launches per year... we will see how GAIA gets prioritized over some other commercial satellites...
Anyway, launch is soon and the program is goind very well. The satellite is packed and will be sent to the launching site for a final assembly inside the rocket...Regarding Science Alerts, the main one we looked at are the Photometric Science Alert. Basically, a new object appearing where nothing was detected before or an object rising his brightness significantly. Those are usually called transients and include supernovae (SN), novae, cataclysmic variables (CVs)... But Be stars outburst can also be detected and because those are non periodic, GAIA will provide "live" alert when such change is detected. Of course, devil is in the details and the "significantly" is very hard to define; plus some Long Term Variables are contaminating the detection system. But the goal is for GAIA to provide alerts for any suspect objects detected. And because I believe about 80 millions stars are measured every day/night, there will be a significant number of alerts if the threshholds are set too loosely... and important objects may be missed if there are set too strict!
Note that while most Science Alerts will come from photometry, there is also a Spectroscopy Science Alert team looking to provide algorythms to detect important changes (ie: alert so that ground telescope can focus observation on those objects). Also, for all Photometry Science Alert, the Spectroscopy Science Alert system will provide RVS data if available (estimation is that about 4% of PSA will get SSA data; main limitation will be magnitude of course as RVS will provide a spectrum of S/N=10 for mag 11 G-type star!).
So the GAIA Science Alert team is working on how to test the algorythms, improve them as the satellite start producing data. The first month, it will look at the pole and perform a fast multiple (240 measures per object) scan. Then, during 4 months, it will be actual observation but with a validation phase for the Science Alert team. Lot of discussion took place around this phase which will require ground observation support both photometry and spectroscopy. Lot of the workshop time was to review several ground based facilities (very interesting telescope & instrumentation overview) and how to get professional observatory telescope time for GAIA support.
I was able to talk about amateur spectroscopy and what is currently beeing done both in high resolution (Be Star was a good exemple) and low resolution (several professional discovered that amateurs were actually taking novae spectra!)... Photometry was also covered with the french AFOEV association which also attended and AAVSO was discussed (and well known to professional astronomers). In spectroscopy, I got lot of question on typical amateur telescope size and performances... so interest by professional astronomers is growing...
I also showed ARASBeAm and the ability to help amateur for the nightly operation by providing color based priorities. GAIA alert will provide lot of alerts but when filtered by magnitude, observing location, most probable spectral type... we may be able to reduce to a mangeable number of alerts for the amateur community. Anyway, an opportunity to look at specially as amateurs do not need months in advance to bok telescope time. The professional were concerned that the dead-line for several telescope time request (for 2014) is end of august...
Again, the Science Alert team goal is to provide good alerts so interesting objects get follow up with ground telescopes. But GAIA will provide a very large catalog with astrometric position but also photometry lightcurves, spectral type, astrophysical parameters... a revolution in the astronomical world!
And just for information, a key all-sky survey project is the LSST. But I'm less worried as LSST will provide billions of stars measurements continuously... but it saturates at mag 16! So we still have a playground to play in...
)Anyway, this is a quick description of what I learned there - do not hesitate to ask if you have any question!
Cordialement,
--
Olivier Thizy
http://www.shelyak.com
...vous ne verrez plus les étoiles comme avant !