Cost-effective Catalina

The Catalina Sky Survey is a NASA-funded effort to identify near-earth objects, or NEOs, whose orbits cross the path of the Earth and are potentially hazardous. We have talked about NASA’s NEO program in a past posting. The Catalina is a relatively small operation responsible for 70 percent of NEO discoveries over the past three years. The survey team recently announced a new $890,000 NSF grant to expand the objectives of the effort to include optical transients whose brightness changes over time. This will be known as the Catalina Real-Time Transient Survey, or CRTS. To date, the CSS has identified over 700 of these objects, including supernovae, cataclysmic variables, and blazars.

From the press release:

The Catalina Real-Time Transient Survey will be the first and only fully public synoptic sky survey, team members say. It’s a bargain-rate boon to astronomers who are trying to figure out how to manage enormous data streams to be delivered by future synoptic sky survey telescopes, such as Pan-STARRS and the LSST, they add.

And quoting from the CRTS website itself:

The Catalina Real-Time Transient Survey is a synoptic astronomical exploration that covers tens of thousands of square degrees of the sky in order discover rare in interesting transient phenomina. The survey utilizes data taken by the three dedicated telescopes of the highly successful Catalina Sky Survey (CSS) NEO project. CRTS detects and openly publishes all transients within minute of observation so that all astronomers may follow ongoing events.

From the CSS website

What is so ground-breaking about this survey is not simply its mission, but also its laudable goal of making all data freely and immediately available to the public; exactly what is necessary to foster interdisciplinary work. Furthermore, the survey will provide a testing ground for technologies being developed for larger-scale surveys such as LSST and Pan-STARRS. Congratulations to the Catalina team for their continued success!

Do Science: HST and the Amateur

The role of the amateur in astronomy is changing, with new opportunities for real scientific contributions. The American Association of Variable Star Observers (AAVSO) and its members have been on the forefront of this trend for decades. The kind letter of thanks, posted today on the AAVSO discussion groups, exemplifies the spirit of collaboration between amateur and professional that is becoming an increasingly important element to astronomical research. Moreover, I believe this trend helps to build grass-roots support for funding in astronomy – a profoundly interesting scientific endeavor whose practical applications are harder to define. My advice to anyone out there young or old interested in learning about astronomy or computers (or both!) – join in! Become a part of the process. Don’t just learn about science. Do science!

Date: Sun, 20 Sep 2009 16:00:19 -0400
From: Ed Smith
Subject: AAVSO! THANK YOU for support of the Hubble’s Cosmic Origins Spectrograph

Dear Dr. Templeton and members of the AAVSO,

I write on behalf of the Cosmic Origins Spectrograph (COS) teams at the Space Telescope Science Institute and the University of Colorado. We send our gratitude for your support of the recent successfully completed Hubble Space Telescope (HST) observations with COS of the Symbiotic Star, AG Draconis. Particular recognition must be given the dedication and many nights of observations made by Gordon Myers.

The observing programs targeting AG Dra were part of the process of verification of the instrument’s on-orbit capabilities.

The COS detectors are subject to permanent damage if exposed to a light source that is “too” bright. Various other requirements for several of our on-orbit checks led to the curious choice of AG Dra as the desired target. As you well know this object has outbursts of as much as 5 magnitudes in the U band, brightening on short time scales and with intermittent repeatability!

Without your frequent observations and rapid analysis providing assurance that the object remained quiescent in the weeks and then days leading up to the HST COS observations, we would not have risked scheduling these observations. Indeed, we remained prepared to halt the observations in the final days and hours if you had reported a sudden brightening.

Thanks to your support, successful measurements made using AG Dra are providing important characterizations of how the instrument is operating on-board Hubble. The “COS/NUV Spectral and Thermal Stability” experiment held the shutter open nearly continuously for ~8 hours with continuous readout of the spectrum. We are measuring the position of the spectra on the detector to characterize the amount and variation of drifts in that position. We expect to relate the changes to such things as the thermally induced expansion and contraction of the Hubble telescope and COS mechanisms as they undergo the day and then night portions of the spacecraft’s orbit.

Characterizations such as these not only verify the proper functioning of the instrument, but enable finely tuned interpretations of things such as temporal variation in the scientific measurements of future programs.

The other two COS programs that targeted AG Dra are titled “COS NUV and COS FUV External Spectroscopic Performance -Part 2″. These programs characterize the spatial profile of spectral lines produced by each of the gratings used with the FUV and the NUV detectors.

The results of these programs will be reported in technical documents available via the web-site at STSCI in the coming months. Your valued contribution will be acknowledged in each relevant publication.
(My emphasis).

Thank you so much for your dedication and hard work in support of a new era of NUV/FUV spectroscopy from the Hubble Telescope with the Cosmic Origins Spectrograph.

-Ed Smith

Research and Instrument Scientist
Space Telescope Science Institute
3700 San Martin Blvd.
Baltimore, MD. 21218

Scary Asteroids! How huge sky surveys and little pushes will save the Earth.

When I was growing up in the 70’s and 80’s, the opportunities available to the amateur for making useful scientific observations  were somewhat limited.   One could observe variable stars,  record stellar occultations by the moon, planets, or asteroids, or hunt for comets and supernovae.  While such activities will continue to appeal to backyard observers, large automated survey telescopes that can repeatedly image the entire night sky to extremely faint magnitudes will certainly change the role of the amateur and the potential for novel discoveries by those of us with more modest equipment.   However, with future surveys promising to produce  petabytes of data annually and the creation of massive catalogs containing hundreds of millions of objects, new and important discoveries await the desktop-bound amateur with a penchant for data analysis.

For example, it was widely reported last year that a group of undergraduate astronomy students out of the University of Washington went looking for supernova using images obtained by the Sloan Digital Sky Survey as part of a class project.   What they found instead were 1,300 newly discovered asteroids.  From the original press release:

“We started searching for supernovae using data from the second phase of the Sloan Digital Sky Survey and all these asteroids were in the way,” said Andrew Becker, a UW research assistant professor in astronomy.  “We decided that rather than get frustrated by the asteroids we should do some science and note details about our observations. I kept asking the students what they had found and they kept saying, ‘More asteroids. No supernovae, but lots of asteroids.’”

This is a story that underscores the exciting opportunities for novel discoveries and scientific contributions available to the amateur as we enter the age of high-throughput digital astronomy.   The story got me to thinking about what we know about asteroids, and the dangers they pose to life on Earth.   Asteroids are small rocky objects revolving around the sun, mostly between the orbits of Mars and Jupiter in the so-called asteroid belt between 2.1 and 3.3 A.U from the sun.  The majority of these have shallow orbital eccentricities.   Over time, some of these asteroids have been purturbed gravitationally by Mars or more particularly Jupiter into more highly eccentric orbits.

Asteroid 243 Ida, imaged by the Galileo spacecraft

Of particular concern are the Near-Earth Asteroids (NEA’s) which cross the orbit of the Earth and thus have the potential for major devastation.

Some notable impacts from both recent and ancient history:

• Asteroid 2008 TC3 (October 7, 2008).   The first case of an asteroid’s impact time and location being predicted in advance.   The entry of the asteroid, estimated to be no more than 5 meters in diameter, generated a spectacular fireball over Northern Sudan.   The event was even capture by satellite imaging.  Energy release: 1 kiloton TNT.
• Hodges Meteorite.   (1954) The only documented case of a human getting hit from a rock from outerspace.   The 4 Kg meteorite crashed through the roof of 31-year old Ann Hodges of Sylacauga, Alabama, bouncing off her radio before striking her on the left hip.   She was badly bruised.   The radio was destroyed.   Energy: 16 lbs TNT (personal rough estimate based on size), with the roof and radio absorbing the brunt of the impact!
• The Tunguska Event (1908) flattened 2000 sq kilometers of trees in a remote region of Siberia.   It is believed to have been caused by an asteroid about 60 meters in diameter.   Energy: 10 Megatons TNT.
• Meteor crater Arizona (50,000 years ago.)    A 45-50 meter asteroid left a 1.2 km diameter crater and probably leveling everything within about 16 km (10 miles).  Energy: 2.5 megatons TNT.
• Cretaceous-Tertiary Extinction Event (65.5 million years ago).   A 10 km asteroid striking the Yucatan leading to the immediate extinction of the dinosaurs and 70% of the life on Earth.   Energy: 100 million Megatons TNT.

NASA’s Jet Propulsion Laboratory (JPL) has an on-going program to study Near Earth Objects.  There you can obtain orbital elements for some 5800+ catalogued NEAs.   Of these, a little over 1000 are designated as potentially hazardous, having an MOID (minimum orbit intersection distance) of less than 0.05 AU (about 5 million miles).

The site also lists upcoming close approaches.   For example, the asteroid 2008 XC1, a Tunguska-sized asteroid, will pass within about 1 million miles of the Earth, or about 4 times the distance to the moon in the next couple of days (Friday December 12, 2008).

You can also plot the orbit of any of the 437 thousand asteroids whose orbital elements have been catalogued.  The figure below depicts Earth’s upcoming close encounter with 2008 XC1.

Although only about 1000 potentially hazardous asteroids (PHAs) have been identified, it has been estimated that there may be as many as 10-20,000 such objects awaiting discovery by larger more powerful survey telescopes such as the LSST due to come online in the coming decade.

So what can we do about mitigating the risk of death-by-asteroid?   Obviously cataloging asteroids in order to assess the risk is a critical first step.   To avert an imminent collision, a number of ideas have been proposed.   Simply nuking the asteroid might actually be problematic.   One particularly interesting idea involves the use of a “tugboat” spacecraft which would rendezvous with the asteroid and by applying a small but steady force, would physically nudge the asteroid just enough to avert disaster.   In an article by Schweickart, Lu, Hut, and Chapman describing the concept, they note that with an asteroid with an orbital period of two years, a 1 cm per second velocity change would increase the period by 45 seconds, creating a delay of 225 seconds over 10 years, enough to insure a near miss, but a miss nevertheless (Scientific American, 2003).   Of course, such a scheme underscores the importance of early detection with decades of advance notice.

So ultimately it turns out the the new generation of deep-sky surveys not only leads to unprecendented insights into the nature of the universe, but may even one day save the world!

Observable long-period variables of high scientific interest

In my last post, I presented a high-level analysis looking at some of the factors leading to high observation counts in the AAVSO database.   That analysis made use of information from the General Catalog of Variable Stars (GCVS) as well as spreadsheets provided by the AAVSO containing observation counts and estimates of citations for all long-period variables (LPVs) in the AAVSO database.   We found, for example, that observation counts are highly skewed towards a relatively few LPVs, that these popular stars occur almost exclusively above zero degrees declination, and that the magnitude of the star at maximum (brightest) strongly influences the popularity (observation counts) – LPVs whose maximum falls below 10m (an approximate binocular limit – at least here in the light-polluted skies of Boston suburbia) rarely have large numbers of observations.

There are of course many other factors that observers take into account when selecting stars of interest.   Variables that are close together in the sky seem more likely to be observed because it is easier to knock them both off at once.   Stars with more interesting light-curves that offer “surprises” are I suspect going to garner greater attention, all else being equal.   A star may be difficult to observer because charts are lacking or because there are no convenient comparison stars.   And we should not forget the important influence of the AAVSO in making special requests of the user community via special bulletins, or regular articles such as “Variable of the Season.”    Indeed, one of the most important functions of the AAVSO, in my view, is to help align amateurs with the scientific goals of the professional astronomical community.

With the recent announcement that the AAVSO was forming a special section dedicated to long-period variables, an important task before the section is to choose LPVs of scientific and community interest.   Citation counts estimated by the AAVSO provide some measure of the overall scientific importance of an individual star.   We use these citation counts together with the observation counts to propose a methodology for selecting candidate LPVs for the new section.

The most obvious thing to do is to consider references per 1000 observations (RefPerKObs).   For example, Y Cas with 6,604 observerations has accumulated 100 citations giving it a RefPerKObs of 15.1.   By comparison, RV Her (Obs=10219, Refs=34) has a RefPerKObs of only 3.3.  The problem with this statistic is that it is undefined for near 3/4ths of the LPV stars in the AAVSO database having 0 observation counts, and tends to be highly skewed when the observation counts are very low.  (The difference between having 1 observation and 2 observations is a factor of two difference in RefPerKObs!)

So instead we consider computing two percentiles – one based on its ranking (1,2,3…) with respect to observations, the other with respect to number of citations.    Then we define a score:

(1)   RankPercentileDiff = CitationRankPercentile – ObservationRankPercentile

The resulting score varies from [-1...+1], and a high (>.5) RankPercentile suggests an under-observed star  with relatively high scientific interesest worthy perhaps of being added to the LPV program.   It turns out this score is actually quite bad because of what I noted in the last time: observation counts are highly skewed towards a relatively few popular stars.   As a result the M-type star, LX  Cyg, ranks in the 50th percentile in observations with a paltry 1700+ observations.  But over 92% of the 6.7 million LPV observations in the AAVSO database are associated with higher-ranking stars.  The RankPercentile defined above tends to underestimate the importance of a star.    LX Cyg achieve a maximum magnitude of only 11.5, so as I noted above, it’s low observation counts are to be expected.    We address these observability issues below.   Nor have we taken into account how its 23 citations stack up.

To address the above concern, we modify the Observation and Citation percentiles based on actual counts rather than a simple ranking.    Thus for each LPV:

AAVSO_Obs_Percentile = percent of observations occurring in stars with fewer observations.

NumRefs_Percentile = percent of citations associated with stars having fewer citations.

Note that stars having the same number of observations or references have the same observational or reference percentiles, respectively.

Finally:

(2)  PercentileDiff = AAVSO_Obs_Percentile – NumRefs_Percentile

Figure 1 below plots the observational percentile vs. the reference percentile, with redder stars having a higher percentile difference.   (Click on each image to see ful size).

Observational -vs- Reference percentiles

As noted in my last post, sometimes stars like CW Leo, a proto-planetary nebula surrounded by water-containing comets, generates high scientific interest outside of its variability.   But by way of example, compare RS Virginis (marked, upper left) to AF Cyg (right).   RS Vir has garnered far fewer observations (6469 -vs- 57799) but has far more citations (164 -vs- 68), a victim perhaps of the declination effect where more southerly stars tend to be neglected.   Am I suggesting that AF Cyg should be excluded from the LPV section?  Absolutely not!   But if I’m trying to get members to focus their attention, I’m focussing them more towards RS Vir – a star that seems to have high scientific interest, but has been relatively neglected by observers.

To account for observability, we filter out stars having declination south of -20 degrees, and a maximum magnitude below 10.0m    We further exclude LPVs having amplitudes less than 1.0.   We also limit ourselves to visual (V) and photographic (p) bands.   One might choose different cutoffs.  The point is simply to demonstrate that reasonable cutoffs still lead to a goodly number of focus candidates.  The above constraints still leave about 600 candidates to choose from (about 5%)

Figure 2 is a revised scatter plot with the above constraints in place.   These are stars that are more accessible to the observing community.

Observational -vs- Reference percentiles for accessible LPVs

Figure 3 below shows the distribution on PercentileDifference for the 600 or so remaining candidates.    I would argue that the 154 stars in the marked should all be included in the LPV section.

Distribution of PercentileDiff for 600 Accessible LPV Stars

The table below lists these 154 stars and their basic properties.   I emphasize that this list isn’t intended to be exclusive.    Furthermore, the final selection process should ensure that different LPV types (irregulars, semi-regulars, miras) are well represented.    Finally, it should be noted that U Herculis at the top of the list appears to be the result of an observation count error in the AAVSO spreadsheets, and that there may be a systemic error in the counts involving variable designations with Greek letters, or names that can be confused with greek letters (U = u = mu?).   This table isn’t intended to be the final word, but rather a demonstration of a methodology for narrowing down candidates.

rachlin_154_lpv_candidates.pdf

Addendum: Another thought occurred to me this afternoon. You might ask – why consider low observation counts as a factor?  Why not just look at the stars with the highest scientific interest?   It would certainly be reasonable to include such stars if they are overlooked in the table above.   What I’m focussing on here is finding stars where increases in the observation counts could potentially have the greatest scientific impact.   Adding five or ten thousand observations to stars with a few hundred in the books might provide novel scientific insights – a new dimension of understanding to a star whose scientific importance has already been established.   Adding more observations to a star already teaming with tens of thousands of observations may have less potential for impact, in my opinion.   For this reason, I consider stars currently having fewer observations as an important selection criteria.

AAVSO/RJB