Milky Way Galaxy Seeks New Dwarf Companion

It is interesting times for hunters of low-luminosity galaxies in the Local Group - our local concentration of galaxies. The low-hanging fruit has all been picked. Anything you could discover by visually examining a Schmidt plate or CCD mosaic image has been found. Is it the end of times for explorers of the Local Group?

Dr. Vasily Belokurov of Cambridge University and his collaborators reveal the answer to be no in their recent submission entitled “Leo V: A Companion of a Companion of the Milky Way Galaxy” found
here. Interestingly, the head-shot of this newly discovered dwarf galaxy shows … nothing! The foreground field stars greatly outnumber the few much more distant evolved stars in the cluster and there is no visible concentration on the image.

Seems like a hard sell, but it isn’t. Belokurov’s team has mined a vast and very influential database of object brightnesses, colors, and spectra known as the Sloan Digital Sky Survey and then obtained follow-up observations to confirm their discovery. The database provides the ability to select out stars from the only-slightly-fuzzy, much-more-distant background galaxies and also precise color information. This latter capability was key to the success of locating the new dwarf galaxy - its stars were sufficiently metal-poor that many of the evolved ones were so-called “blue horizontal branch” stars. The concentration of BHB stars on the sky does make this patch of sky stand out - it indicates a grouping of stars of similar age and metal abundance.

The final clincher was getting radial velocities for the handful of brightest, coolest stars in the galaxy - the so-called “red giant branch” (RGB) stars. Objects within the dwarf galaxy are only moving a few km/sec relative to each other. On the other hand, foreground field stars from the Milky Way have radial velocity differences of tens to hundreds of km/sec since they are orbiting the Milky Way’s much more massive center at various distances and on a variety of orbits. Belokurov’s team obtained 247 spectra on the 6.5m MMT telescope on Mount Hopkins in Arizona and found five RGB stars close to the dwarf galaxy center with near-identical velocities. Score!

The new pup - not to be confused with pope - christened “Leo V” is 180 kpc (about 600,000 light years) distant and is moving away from us at a speed of 132 km/sec. But that radial velocity includes a component of the Sun’s motion around the center of the Milky Way galaxy. When solar orbital component is removed, Leo V ends up moving only about 60 km/sec relative to the center of mass of Local Group galaxies.

Intriguingly, Leo V is found projected on the sky only three degrees away from a very similar beast with the very distinctive name Leo IV - also discovered by Dr. Belokurov and his collaborators! The researchers point out that the proximity of these objects in Local Group space may foreshadow additional discoveries along a stream of such apparently faint and intrinsically low-luminosity objects which could then inform our ideas of the formation of the Milky Way galaxy.

I must point out another very cool use of Local Group galaxy data. You may recall from high school or college physics that if you know velocities and positions of objects and the forces acting on them, you can predict where they were in the past and where they will be in the future. For galaxies, the force is gravity and you can get good estimates of their masses from their brightnesses (corrected by a dark matter fraction). You can also assume that at some time around 10 billion years ago, all of the present-day galaxies were essentially at rest with respect to each other. Given 1) their three-dimensional positions now, 2) their radial velocities now, and the assumption of zero initial velocities way back when, you have enough “boundary conditions” to solve each of their paths in the interim. But - and this is a big but - there is no simple way to do it! One has to try out many, many configurations of starting locations, run the system forward and see if the radial velocities and positions you end up with are similar to those we see now. If not, throw the galaxies back in the box, shake and try again! If you are interested in such games, check out the references and citations in this paper. Leo V can now be added to the list of objects used, so we now have an excuse to re-run these models!

Official Trailer for IYA 2009

The official trailer for the International Year of Astronomy (IYA) in 2009 is out! Check it out on YouTube by clicking here. Let’s just say that “The Dark Night” is opening in skies all over the planet!

Superhumping

You’ve maybe heard of cataclysmic variable stars. They are binary systems where one of the stars is stealing material from the other star due to their close proximity. This material forms a pancake around the star called an accretion disk. We’ve never imaged one of these systems but we have a very, very good theoretical model of how they work.

Every once in a while that accretion disk gets unstable and essentially blows up in what we call an outburst. Sometimes these outbursts are extra bright and carry a signature in their light curve called “superhumps”. These are large oscillations in the light curve at a period very near, but not exactly equal to, the orbital period of the system.

On 6/30/08 one of these systems, VY Aqr, went into a superoutburst. This is a fairly infrequent event for this star. The outbursts themselves happen every few years and the superoutbursts less often than that.

I was heading out to my observatory when the word came in so I slewed my fancy 0.212m telescope (doesn’t that sound more impressive than 8.3″?) and got some data. So did a few other people and I downloaded all of their data from the AAVSO. If you click the image above a light curve will open in a new window. Let me explain it to you.

This is a phase plot using the superhump period of this star (P_sh = 92.7 minutes). So anything that happens 92.7 minutes after something else is plotted at the same phase. So the X axis is the phase of the superhump period and the Y axis is the brightness of the star. Because the star is getting dimmer, each day’s data is lower on the graph than the previous day. So each night is folded upon itself but each subsequent night is below the previous night.

You can see there are some interesting things going on! The superhump amplitude and phase change over time as the systems fades.

The latest papers on this object included Doppler tomography, new parallax measurements and spectroscopy from the Hubble Space Telescope. There is a lot of interesting physics in these systems and they are the subject of on-going study by astronomers. Including me!

Interview: Ben Wiehe on Science Cafes

We have a new podcast on the feed featuring an interview with Ben Wiehe of WGBH on how he conducts science cafes. Subscribe to the feed or click below to listen. You can also watch the video of just the interview portion.

Interview: Science Cafes (MP3, 14.5MB, 15:27, Show Notes)

A white dwarf in the making

The AAVSO has posted their Variable Star of the Season and this time around it’s FG Sagittae, a star that has given us an opportunity to watch it evolve over human timescales, something very rare in stellar evolution.

…many papers have detailed the remarkable evolution of FG Sge from a faint, hot, blue post-asymptotic giant branch star and planetary nebula in the making to a much cooler and brighter yellow supergiant. Even more exciting for variable star observers, following FG Sge’s four-magnitude brightening and several decades of relative constancy, the star now appears to exhibit the dramatic and seemingly random fluctuations and fadings of the R Coronae Borealis class of variable stars. FG Sge is clearly a star undergoing extreme changes, and we’re fortunate to be treated to its amazing show.

If you have access to a telescope you can go take a look for yourself!

Support Slacker Astronomy by supporting our advertisers with your honest and heartfelt patronage

OK, we gave in and added Google AdSense ads to the site. If you want to support Slacker Astronomy and help us do interviews, video and podcasts, please click the ads once in a while support our advertisers with your honest and heartfelt patronage. The more you click support our advertisers with your honest and heartfelt patronage, the better we can serve you.

Next up, a tip jar, coming soon.

++Carnival of Space

Here’s your weekly astronomy and space science fix at Carnival of Space #61. I could not get my sh*t together this week but I’m hoping to contribute to future carnivals. Weekly is a daunting thing for slackers.

Go read up and tell us what you think!

Slacker wins award

Slacker Astronomy founder and Tufts grad student Aaron Price has won a Chambliss Astronomy Achievement Student Award for his poster on BZ UMa.

I stopped by Aaron’s poster at AAS in St. Louis and he explained the work he presented while I grabbed it on video.

Here you go — enjoy!

Aaron Price on BZ UMa [FF] (MP4 video, 30.3MB, 04:10)

Carnival of Space No. 60

Hear ye, hear ye! Assembled here is the official Carnival of Space No. 60 wherein the written assemblage of the musings of many eminent natural philosophers are here provided for your amusement and betterment.

In order of receipt by yours truly and in the own very words of the author, notwithstanding some minor editorial discretion, here, then, are the proceedings:

From Slacker Astronomy:

Hi Michael,

Here you go - hot off the keyboard!

Regulus - Just when you think you know a star

Cheers,
Doug

From astroENGINE:

Hi Fraser,

My entry:
Title: “No Doomsday in 2012: The Reason Why Science Will Not Win”

Just a brief discussion about the recent 2012 articles and why science is fighting a loosing battle against the scaremongers

Cheers, Ian

From 21st Century Waves:

Hi Fraser,
Here’s a post: State of the Wave, Friday 6/20/08

I hereby officially volunteer to host the Carnival.

Best regards…
Bruce Cordell

From Centauri Dreams:

Hi Fraser,

I’ll send “Alpha Centauri and the Long Haul“:

This one is a look at projects in human history that have involved lengthy time spans, with relation to interstellar concepts like the Ultimate Project, a multi-generational starship that might take 10,000 years to reach its destination. The idea of long-term thinking in a short-term culture is explored.

All best,

Paul

From Music of the Spheres:

GeoEye-1 and TMA Notes

Music of the Spheres looks at the soon-to-launch commercial Earth-imaging satellite GeoEye-1 and at some details of its high-resolution optics.

From Free Space:

humm …

how ’bout this for this week: Metaphysically Speaking

Congress may force NASA to fly a canceled dark matter experiment, but it’ll have to be without a rescue shuttle available.

Thanks!

Irene

From Start With A Bang!:

The Moon looks huge!!
Because who doesn’t love the moon, really?

From Space Feeds:

This week’s space video of the week is the 1997 sci-fi/fantasy film The Fifth Element.

Space Video of the Day - 080623

Ed

From Nextbigfuture:

Article Title: The Space elevator games and the lunar lander contest preview for 2008

Summary: The Space elevator power beaming (climber) competition is on Sept 27, 2008 and the lunar lander contest is Oct 24, 25 2008. The main focus is on the space elevator climber teams and the progress towards a tether.

Brian Wang

From Orbiting Frog:

Fraser,

Crikey, the sixtieth must be coming up!

My entry this week would have to be the ‘Font Sizes of the Planets‘

Thanks,
Rob

From Cumbrian Sky:

Hi,

I’d like to submit this Blog post for your consideration for this week’s Carnival, please.

Title of Post: “The future’s not orange, it’s ICY…”

Summary: As exciting and important as it was, contrary to what many media reports have claimed, Phoenix’s spotting of ice on Mars wasn’t actually a “discovery” - ice had been seen on Mars by other probes over the years. But while the celebrations got into full swing in Arizona, NASA quietly released another “icy image” that received almost no attention at all, yet illustrated something possibly even more profound, giving us a tantalising glimpse into the future of space exploration and Mankind…

Stuart Atkinson

From Dynamics of Cats:

Holy Vanishing Crumbs, Phoenix!
yet another entry on Phoenix lander stuff

From A Babe in the Universe:

Aloha Carnival!
Endeavour returned to Earth June 14 with some spectacular photos from the Space Station.
“Photos From STS-124”

Mission STS-124 successfully installed the Japanese Kibo module. Human figures work on the Station in the ultimate high-rise project. We see the Shuttle docked at the Station, and a view of a place an earlier Endeavour once charted.

Thank you for hosting this week’s Carnival.
LOUISE RIOFRIO

From: Tyler at The Planetary Society:

Hi there,
Here’s my latest astronomy blog posting for the Carnival of Space.

Stop 14: Bryce Canyon National Park, Utah

This one talks about light pollution and astronomy outreach within the national parks.

Cheers,
Tyler

From the weblog of Columbus State University’s Coca-Cola Space Science Center:

Would you like to swing ’round a star?

Thanks!
– Rosa Williams

From Jeff Gortatowsky:

Star Party season…Or also know as fire season here in California. However being optimistic, it is star party season in the northern hemisphere. Coming up next week are two big star parties in northern California. The Golden State Star Party (GSSP) and the Shingletown Star Party (SSP). Both are held in an area that is one of of the darkest yet still accessible areas of the state. GSSP is currently booked up. SSP however still has room and day/night passes are available at the gate.

(Editor’s Note: It appears that SSP is “postponed until at least the end of August 2008″, according to their web site.)

From Astroblog:

G’Day

Title of Post: The Odyssey and the Celestial Clock

Brief summary: Has the date of homers Odyssey been found using the patterns of planets in the sky?

Cheers! Ian

From Emily Lakdawalla at The Planetary Society Weblog:

Hi there,

This week I’ll change things up and suggest you link to my weekly “What’s up” post, highlighting the current activities of all 20 of the active planetary space probes in and beyond the solar system. For the curious, that list includes: MESSENGER, Venus Express, Chang’e 1, Kaguya, Spirit, Opportunity, Phoenix, 2001 Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, Rosetta, Stardust, Dawn, Deep Impact, Hayabusa, Genesis, Cassini, New Horizons, and Voyager 1 and 2.

Happy Mars solstice! (And Earth, too.)

What’s up in the solar system for the week of June 23

–Emily

From Catholic Sensibility:

Hi Fraser,

If it’s not too late, here’s my entry for the carnival:

Satellite Imagination 1.6: Meet The Louisians

Cheers to all at UT

Todd

From goodSchist:

Hopefully this isn’t too late:

The importance of being Ivuna

Cheers,
-Chris

From Twisted Physics

This is a neat one with great discussion - Sean Caroll over at Cosmic Variance vetted the scientific points Jennifer discusses:

Devourer of Worlds

Best,
~Dave

From Beth Katz:

Hanny’s Voorwerp is an intriguing green blob that looks very much like “The Incredible Hulk”. You, too, can explore the Galaxy Zoo.

A little closer to home, aurora watchers have been forlornly hoping that the sun will get past its solar minimum and get some sunspots. In January 2008, NASA reported that Solar Cycle 24 had started. It seems that there has been little activity since then unless you count Tiny Tims. But the STEREO spacecraft caught stereo images of twisting solar jets. Those spacecraft have some amazing images. Maybe by the time the Solar Cycle 24 Conference rolls around in December we’ll see a few more spots.

Too many clouds? Test your knowledge of lunar phases with the lunar cycle matching phase game or these lunar phase activities.

UPDATE Oh noes! I forgot one!

Did I break some rule or offend the Gods?

I sent a blog to Fraser and one to you. You said you’d pick.
Did they both suck or what?

–
Mike Simonsen
Development Director
American Association of Variable Star Observers
www.aavso.org

So…

From Simostronomy:

Hey Michael,

You may have received an entry for me for the carnival, but you might consider this one in its place.

What Are Variable Stars?

–
Mike Simonsen
Development Director
American Association of Variable Star Observers
www.aavso.org

Many thanks to the hard work and good thinking of our submitters. It’s a lot of great reading for us to digest. I’ve never hosted a carnival before so I probably did it wrong. Please be kind to my mistakes. Authors, let me know if I made any errors or omissions with your submission and I will promptly correct.

Wanna join the Carnival of Space? Just send the URL of your entry via electronic Internet email message to carnivalofspace@gmail.com.

Cheers, beers and clear skies,
Michael

Regulus - Just when you think you know a star

Looking out at the night sky, it is easy to believe that we’ve learned everything there is to know about the brightest stars. Fortunately, they keep surprising us! A delightful paper has just appeared on the astro-ph preprint server which combines many elements of a great story.

Regulus is the 22nd brightest star in the sky to the naked-eye. Since it lies along the path followed by the Sun, Moon and planets (called the “ecliptic”), bright planets frequently pass close to the line of sight to this majestic star. In fact, it is so close to the ecliptic that the Sun passes within a half degree of it every August. (Don’t go looking for this event visually! If you want to see how close, check out the movie from the SOHO satellite here. The brightest object - besides the Sun! - is Saturn. Regulus pops out from behind the occulting disk right at the end of the MPEG.)

I first became more closely acquainted with Regulus during my postdoctoral fellowship at the Dominion Astrophysical Observatory in Victoria, British Columbia (Canada). I would frequently use the 1.2m telescope with its fantastic high-resolution spectrograph. One of the shortcomings of filament bulbs is that there is precious little light emitted at the blue end of the spectrum - if you want to calibrate the pixel-to-pixel sensitivity of your detector, you can’t get enough blue signal without saturating the red end. What to do, what to do … One fine solution is to observe a bright blue star which is rotating so quickly that all of its spectral lines are smeared out over many, many pixels. Enter Regulus! The few spectral lines in its spectrum were already broad hydrogen lines and the rotation rate of over 300 km/sec smeared them out even more. A great star for calibration.

And a very poor one for measuring the line-of-sight (”radial”) velocity using the Doppler shift! In fact, astronomers last studied it for binarity in 1912-1913 - almost a century ago! Many hot stars are far enough away that lines from interstellar gas can be used as reference points for radial velocities. Not so Regulus - it is only 24 parsecs away and there just isn’t enough gas along the line-of-sight to this neighbor of the Sun.

Regulus came back into favor when its shape and the brightness distribution could be measured by a very cool kind of optical instrument called an interferometer. Work by McAlister and collaborators using the CHARA long-baseline optical inteferometer they created on Mount Wilson found that Regulus is rotationally-flattened and it spinning at 86% of the speed at which the surface gas would cease to be bound to the star. They were able to show that it was darker along the equator of the star, too. This high rotation rate was an anomaly for a star that was as old as Regulus (apparently 150 million years - pretty old for a star of this mass) since similar stars seemed to be fast rotators only early in their lifetimes.

So Doug Gies and his collaborators embarked on a new study using modern instrumentation to see if there was any evidence of it orbiting the center-of-mass of a binary system containing it and a hitherto-unknown companion. As a bright star, there was plenty of light available to be dispersed by high-resolution spectrographs. They used several in their study including two “unusual ones” - the Kitt Peak National Observatory Coude Feed Telescope and the Multiple-Telescope Telescope!

Let me briefly describe these two instruments. A Coude room is very high-resolution spectrograph capable of tearing the light from a telescope into very fine shreds of color. It was designed to be “fed” by the 2.1m telescope at Kitt Peak. However, observatories tend to do deep imaging around the time of New Moon (i.e. when the sky is dark) and the 2.1m served a variety of such needs. It was realized that the a smaller telescope could “feed” the spectrograph during these periods and that brighter stars could be observed with that smaller telescope plus Coude spectrograph while the big telescope was busy imaging!

The Multiple-Telescope Telescope at Hard Labor Creek in Georgia is another ingenious system for bright star spectroscopy. It has nine relatively inexpensive 0.33m mirrors which focus onto nine optical fibers which then feed a stable, bench spectrograph. Since it only studies bright stars, the mirror pointings can each be individually-tweaked to center up on the bright star. It uses a cheap alt-azimuth mount and collects as much useful light as a 1.0 telescope for a tiny fraction of the cost of such a large telescope.

So - you are asking - what did Doug Gies and his collaborators find? They found that Regulus was indeed a spectroscopic binary. Once every 40.11 days, the system completes one orbit. Regulus itself has a mass of about 3.4 times that of the Sun. The companion of Regulus is much less massive - only about 0.30 solar masses. Such a small mass object is either a low-mass star or a white dwarf. The latter possibility provides an explanation for Regulus’ rapid rotation! The idea is that the companion was once the more massive member of the pair and when it finished hydrogen burning in its core, it expanded dramatically and started losing mass to Regulus in a manner which “spun it up”. A mass of 0.30 solar masses is very low for a white dwarf - such objects are found only in systems where it is clear that much mass has been transferred.

A final piece of the puzzle fell into place when spectra taken using the far-ultraviolet Spanish satellite MINISAT-01 were re-examined. When the expected contribution from Regulus was removed, light remained in the ultraviolet region of interest - consistent with a white dwarf but not a cool low-mass star. So Regulus joins the list of bright stars in the sky (which includes Sirius and Procyon) having white dwarf companions and proves once again that “three out of every two stars is a binary”!

Their paper has been accepted for publication in the prestigious Astrophysical Journal Letters.

A Spectroscopic Orbit for Regulus
Doug Gies (GSU) et al