Spotlight on APOGEE: Jo Bovy and the Motion of the Sun

The spotlight this month is on Jo Bovy, a John Bahcall Fellow and Long-term Member at the Institute for Advanced Study in Princeton. He completed his PhD at New York University. Within APOGEE, he is the Science Working Group Chair for APOGEE-1, and therefore coordinates the scientific analysis of the APOGEE dataset.

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Jo uses big datasets from numerous surveys to understand how the Milky Way came to be. To do this, he studies how stars move and what are their chemical compositions. When he first tackled APOGEE’s huge database of stellar spectra in 2011, he realized that APOGEE’s spatial coverage of the Milky Way’s disk allowed the circular velocity of stars in the Milky Way to be measured with greater accuracy than had ever been done before. (The circular velocity is the speed at which a star orbits the center of the Galaxy. This number changes as a function of distance from the center, and precise measurements are required to correctly determine, for instance, the Galaxy’s mass, but also to measure peculiarities in stellar velocity, which help us determine where it might have originated.) The data analysis in his paper is complex, but he was able to draw two important and straight forward conclusions from this work:

  1. that the circular velocity near the Sun (what we call the “solar neighborhood”), at a distance of 26,000 light-years from the center of the Galaxy, is 218 ± 6 km/s; and
  2. that the Sun itself is moving 25 km/s faster than other stars at the same distance.

The first result was expected: although the circular velocity in the Sun’s neighborhood was assumed to be about 220 km/s for the last 30 years or so, Jo’s was the first precise measurement to confirm this value. The second result, however, was a surprise: previous measurements had pegged the Sun’s motion relative to nearby stars at something like 12 km/s, not 25 km/s. This result was confirmed using a sub-set of APOGEE data (a mere 19,937 stars, or about 15% of the full APOGEE dataset) known as the APOGEE Red Clump Catalog.

Why does this seemingly small difference matter? From an outsider’s perspective, going from 12 km/s to 25 km/s is still only changing from about 5% to 10% of the circular speed, so either result might seem acceptable! But is is important, and Jo explains why: We orbit the Sun, and the Sun orbits the center of the Galaxy just like every other star. Therefore, every speed that we measure for another star is relative to our own motion. If we can understand how we move in the Galaxy, then we will have a much better understanding of the dynamics of the entire Milky Way.

And understanding the Milky Way is, after all, the whole point!

Spotlight on APOGEE: Duy Nguyen and Binary Stars

We are beginning a series of spotlights on APOGEE team members, with special emphasis on their interests in APOGEE science. This month, the spotlight is on Duy Nguyen, one of APOGEE’s postdocs. He graduated from the University of Toronto with a PhD in astronomy and astrophysics, and then held postdoc positions at the University of Florida, Stockholm University, and the University of Rochester before joining the APOGEE team.

Nguyen

Duy’s research is on the subject of binary stars. A binary star is actually two stars orbiting each other. The sizes of the binary star orbits are small enough that the two stars cannot usually be distinguished in images. This can confuse the interpretation of starlight; and in a survey like APOGEE where precise velocities of stars are so important, this can be a big hindrance. As a result, a number of different methods have been employed to try to tease out whether a star is a binary or not.

But this post isn’t just about binary stars — it’s about one scientist’s research into better understanding them! And in many ways, Duy sees APOGEE as the best available experiment for binary star studies. APOGEE takes multiple spectra of most stars in its sample over months and even years, and this time sampling enables orbital periods to be measured. APOGEE’s high spectral resolution means that tiny Doppler shifts in a star’s spectral lines can be measured precisely. And most importantly, such a large sample as APOGEE has observed (more than 150,000 stars to date) means that we may be able to get a better handle on the “binary fraction” of stars in the Milky Way — a problem that has been plaguing modern astronomers for decades.

Duy is primarily interested in the dynamic properties of binary stars. These dynamics are primarily observed by means of the Doppler shift. As the stars in the binary pair orbit one another, each star approaches and recedes from the Earth once per orbit. Every time they approach the Earth, their spectral lines move to a slightly smaller (or “bluer” to use astronomical lingo) wavelength. And every time the star recedes, the spectral lines move to a redder wavelength. These small changes can be detected with APOGEE, and the radial velocity variations of the stars can be determined based on how large is the wavelength shift.

Duy and his collaborators are amassing radial velocity information on the stars in the APOGEE sample, looking for candidates with substantial radial velocity shifts. When they find one, they fit the data points with an orbital model to determine what the most likely stellar masses are. Here is an example fit:

Nguyen_Troup_ScreenShot

On the x-axis is the time in days, and on the y-axis is the velocity of the star relative to the Sun. This plot shows that the best fit to these data suggest that two stars, one that is at least 0.21 times the mass of the Sun and the other that is 1.6 times the mass of the Sun, are orbiting one another every 112.98 days at a distance greater than 0.065 A.U. It’s interesting to note that the less massive star in this binary is eight times smaller than its companion. Large mass discrepancies in binaries are typical, so that one star dominates the other in terms of brightness. This is one reason why binaries are so difficult to detect.

To date, about 12,000 possible stellar binaries from the APOGEE sample have been flagged based on radial velocity shifts, and 4,000 of these are of special interest because they have been visited seven or more times and exhibit significant radial velocity changes. Of these, 1,500 indicate stellar mass companions, such as the one figured above. While the 12,000 possible binaries were found automatically, the 1,500 sources with stellar mass companions have all had to be screened by hand — a process that Duy would like to fully automate.

Analyzing APOGEE’s huge repository of stellar spectra will enable the most comprehensive assessment of binary stars, including details about whether binary star characteristics are different across the Galaxy. And as an added bonus, APOGEE is sensitive enough to spot Jupiter-sized planets using these methods! How many planets are lurking in the APOGEE dataset?

Special thanks to N. Troup, D. Chojnowski, and S. Majewski for assistance preparing this post.

APOGEE’s Infrared View of the Stellar Temperature Sequence

APOGEE surveyed 156,481 stars in its first three years. And of course APOGEE-2 is going to increase this sample size significantly. But to celebrate the successful end of APOGEE and the Data Releases 11 & 12 (also see here), we’d like to share with you a slice of the kind of data it collected.

Some background: The APOGEE/APOGEE-2 instrument collects near-infrared spectra of distant stars, and the survey is aimed at studying the history of the Milky Way Galaxy. How it does that is explained here. Along the way, it has taken spectra of each known spectral type: from hot O-type stars (with surface temperatures of about 30,000 degrees, or five times the surface of our own Sun) down to M-type stars (about 3,500 degrees, or roughly half the temperature of the Sun). Each of the spectral types (O, B, A, F, G, K, M) is defined based on how many and what kind of atomic or molecular species are seen in their spectrum. For instance, O-type stars have lots of singly-ionized atomic species visible in their spectra, whereas A-type stars have very strong hydrogen lines, and M-type stars have lots of neutral molecules, especially lines of TiO when you look in the visible portion of the spectrum.

These spectral types were defined using the visible portion of the spectrum. So when we look in the near-infrared, do they appear to be different? Here we go:

apogee_tempsequence_new2

The O-type star spectrum looks pretty bland — the strongest lines due to ionized Helium in the near-infrared H-band are at 15721 and 16922 Angstroms (the line at 15271 Angstroms is due to interstellar molecules, and is therefore not from the star). The B-type star shows pretty significant absorption lines due to the Brackett series of atomic Hydrogen (those transitions beginning at the n=4 excited state), and those plus a whole bunch of smaller wiggles from other atoms can clearly be seen in the A- and G-type spectra as well. Below that and things look a lot more complicated. If you have experience with data like these, you might be tempted to think that the spectra of the G-, K-, and M-type stars are “noisy”, meaning that they weren’t observed for long enough and therefore weren’t detected well. But that’s not the case: every single spike visible in these spectra is due to an atomic or molecular transition that originates in the photosphere of the star!

All told, these spectra allow us to study sixteen different atomic elements besides hydrogen. Which ones, you ask? Oh all right, I’ll tell you: C, N, O, Na, Mg, Al, Si, S, K, Ca, Ti, V, Cr, Mn, Fe, Co, and Ni. As you can see, this is a truly beautiful, complex dataset. We’ll keep up-to-date science results at this page.

Job Listing: Observe for the Sloan Digital Sky Surveys

SDSS would like to find a new Chief Telescope Technician to oversee SDSS observing operations at the Sloan 2.5m Telescope at Apache Point Observatory, New Mexico.

The eBOSS Collaboration on a recent visit to the Sloan Telescope at Apache Point Observatory (usually the weather is much better).

The eBOSS Collaboration on a recent visit to the Sloan Telescope at Apache Point Observatory (usually the weather is much better).

This position is advertised via New Mexico State University.  We reproduce the Job Duties and Responsibilities here:

Responsible for the maintenance and operations of  SDSS telescope facilities and plate plugging operations. Develops and maintains operating procedures and maintenance schedules. Plans and coordinates maintenance work. Supervise, direct, and evaluate work of assigned staff. Design, generate contracts for fabrication, and conduct installation and testing of various electro-, opto- mechanical systems for complex telescope systems, including interfaces and scientific instruments. Perform preventive and corrective maintenance to systems, as necessary, and ensure safety and integrity. Monitor trends and correct anomalies. Perform troubleshooting and problem analysis. Specify procedures, schedule, spare lists for maintenance and repair of telescope systems. Report activities and progress related to telescope systems engineering to Site Operations Manager. Specify, procure, maintain specialized mechanical, electronics, and optical test and maintenance equipment. Develop an annual plan and budget for telescope systems engineering activities and projects.

For full job details, and information on how to apply please visit here. Deadline 13th Feb 2014.

A 2 Billion Light Year Pie to Wish you Happy Thanksgiving

Here’s a pie (diagram) to wish everyone a Happy Thanksgiving!

orangepie

The SDSS’s map of the Universe shown as a pie diagram. Each dot is a galaxy; the colour indicates the local density (with red revealing the most dense places). This represents a slice through the Universe, with the Earth in the centre and galaxies further from the Earth plotted further from the centre (the distance is labelled here as redshift). The angle around the pie is marked by the sky co-ordinates (Right Ascension).

This pie diagram is one of the most famous images from the original phase of SDSS, which mapped the distances to 1 million galaxies out to a distance of about 2 billion light years (z=0.15, or 615 Mpc in comoving radius).

The map shows a slice through the Universe with the Earth at the centre, and each of the 1 million galaxies in the SDSS Main Galaxy Sample as a point. The points are colour coded by local density to hi-light the cosmic web  (with red points in the highest densities).

The black parts of the pie are where SDSS did not map galaxies, either because our Milky Way is blocking the view from Earth, or because those parts of the Universe are not visible from our telescope in New Mexico.

Even while the Universe is expanding, all the matter in it clumps due to gravity and the structures we see in this map are the result of that. The details of the growth of these structures over time depends on both the expansion history of the Universe and the total amount of matter in it. So by accurately mapping the locations of galaxies in this map, scientists in SDSS have been able to measure both of these things making an important contribution to our knowledge of how the Universe works.

Visit our website for more on the science results from SDSS.

SDSS Researcher Awarded for Outstanding Research

Prof. Shirley Ho, an assistant professor at the Department of Physics in Carnegie Mellon University and a member of both BOSS and eBOSS science teams has been awarded the 2014 Macronix Prize (or the Outstanding Young Researcher Award) of the International Organization of Chinese Physicists and Astronomers.

Prof. Shirley Ho, Carnegie Mellon University.

Prof. Shirley Ho, Carnegie Mellon University.

The OYRA (Macronix Prize) is given each year to one to two young, ethnic Chinese physicist/astronomer outside of Asia, in recognition of their outstanding achievements in physics/astronomy. The Award carries a cash prize of US $2,000 each and a certificate citing the awardee’s accomplishments in research.

The citation for Prof. Ho’s award explains:

“Much of the research accomplishment of Professor Ho has been on using SDSS-III data to measure cosmic distance scales and the growth of structure in the universe in order to get at the expansion history of the universe. She has been a leader in extracting signals of the Baryon Acoustic Oscillations, which are the tiny ripples in the density of galaxies that are an imprint left over from the quantum fluctuations in density soon after the Big Bang. She utilized these signals as a standard ruler to measure the distance scale of the universe in various epochs, and was able to achieve the most accurate measurements of cosmic distances yet with an accuracy of 1%. Her current research focuses on developing the understanding of dark energy via large-scale spectroscopy, investigating the initial conditions and contents of the universe large-scale photometry, and applying machine learning to studying non-linear cosmological problems.

Prof. Ho will collect her award at the next meeting of the American Physical Society (San Antonio, Texas, March 2-6th 2015) at which there will also be hosted a meeting of the US-China Young Physicsts Forum.

The SDSS Collaboration congratulates Shirley on both her excellent research and being recognised for it in this way.

The Future is Now: Karen Masters Wins UK Award

Dr. Karen Masters, senior lecturer at the University of Portsmouth’s Institute of Cosmology and Gravitation and Director of Public Education and Outreach for SDSS-IV, has won the Women of the Future Science award. The Women of the Future Awards acknowledge successful young women in Britain and are handed out in fields ranging from business to arts and culture to science and technology. Karen (as we like to call her) received the award for her work
on understanding how galaxies form and evolve over the history of the universe. Karen uses a diverse set of tools, including the contributions of large number of citizen scientists looking at SDSS images of galaxies at the Galaxy Zoo (www.galaxyzoo.org) and the new data coming from the MaNGA survey of SDSS-IV (https://www.sdss.org/sdss-surveys/manga/). Karen is also one of the BBC’s “100 Women of 2014”, invited to share her thoughts and experiences as part of the BBC’s pledge to represent women better in their news reporting.

 

Dr. Masters accepting the award from the Rt Hon John Bercow MP,  Speaker of the House of Commons.

Dr. Masters accepting her award from the Rt Hon John Bercow MP, Speaker of the House of Commons, and Trui Hebbelink from Shell. 

For more information, see http://www.ras.org.uk/news-and-press/2527-dr-karen-masters-wins-women-of-the-future-award and www.bbc.com/news/world-29758792

Observing the Partial Solar Eclipse with an SDSS Plate

An SDSS plate was reused to wonderful effect this week, as a pinhole camera to project 640 simultaneous images of the recent partial solar eclipse on 23rd October 2014.

Sarah Ballard (@hubbahubble) and Woody Sullivan, from SDSS member institution, the University of Washington in Seattle came up with this unique idea to observe the solar eclipse.

SDSS1-270x360

Putting an SDSS plate to use as an eclipse viewer. Credit: Sarah Ballard and Woody Sullivan (Univ. of Washington).

SDSS2-270x360

640 images of the 23rd October 2014 partial solar eclipse. Credit: Woody Sullivan and Sarah Ballard (Univ. of Washington).

For more lovely or unusual eclipse photos, see this Solar Eclipse Roundup, by Sky and Telescope, who selected Sarah and Woody’s method as their “best use of old technology” for viewing the eclipse.

SDSS member Brice Ménard Awarded Prestigious Packard Fellowship

SDSS congratulates Dr. Brice Ménard (Johns Hopkins University) on receiving a David and Lucille Packard Foundation Fellowship.  This $850,000, five-year grant is awarded to “the nation’s most promising early-career scientists and engineers” — only 18 such awards were given this year.  Dr. Ménard specializes in applying advance statistical techniques to large data sets to explore the distribution of galaxies and matter in the Universe.  Much of his work has exploited the rich data of SDSS and we look forward to seeing the future ideas and science to come out of this award.

For more details see the JHU press release at

http://hub.jhu.edu/2014/10/15/brice-menard-packard-fellowship

 

 

 

SDSS hits the Big time

SDSS has made it big! How big? The Big 12! To explain a little more, especially for those who are not American college football fans, the Big 12 is a group of universities* that form a league in American college football. During broadcasts of college football games, which are very popular, there are a couple of advertisements that highlight the universities’ educational and research prowess. Usually these involve good-looking students with colorful liquids in test tubes or surrounding a professor in a lab coat at a computer terminal. But that’s not good enough for TCU, home to SDSS members Kat Barger and SDSS-IV Survey Coordinator Peter Frinchaboy. Their contribution to the Big 12 ad, on a broadcast seen by over 2 million people, features a shot of the Sloan Foundation telescope opening up for a night’s observing. TCU also has its own ad for these games, which focuses entirely on its involvement in the Sloan Digital Sky Survey, including more beautiful shots of the Sloan Foundation Telescope in New Mexico and a “starring” role for Peter. Take a look at www.big12makingadifference.com/university/tcu

* 10 universities are part of the Big 12. Don’t ask.

SDSS Collaboration Meetings in Park City, Utah, USA

Over 150 scientists from institutions in 13 countries in Europe, Asia, North America and South America recently traveled to Park City, Utah for the SDSS Collaboration meetings. First SDSS-IV got underway. The start of SDSS-IV observations on July 1, 2014 meant that this meeting was much less anticipatory and much more participatory than the SDSS-IV meeting last year. For the second half of the week, the SDSS-III collaboration, data all taken, was focused on the interesting science results coming out of this very successful 6-year survey. The overlap between the membership of the SDSS-IV and SDSS-III collaborations is quite large, so expect to see many of the faces in the photo from the SDSS-III half of the meeting in the future as well! Our enthusiastic thanks to the University of Utah for playing host to such a fabulous set of meetings.

SDSS-III collaboration meeting picture from the wonderful setting of Park City, Utah

SDSS-III collaboration meeting picture from the wonderful setting of Park City, Utah

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Sloan Research Fellowships Open for Nominations

The Alfred P. Sloan Foundation is now accepting nominations for Sloan Research Fellowships in eight fields: chemistry, computational and evolutionary molecular biology, computer science, economics, mathematics, neuroscience, ocean sciences, and physics. These two-year, $50,000 fellowships are awarded annually to 126 early-career faculty in recognition of their distinguished performance and exceptional potential as researchers. Candidates must be nominated by a department head or other senior researcher. For more information, please visit this site:

http://www.sloan.org/sloan-research-fellowships

The Sloan Digital Sky Survey Expands Its Reach

With the start of SDSS-IV this July, the Sloan Digital Sky Survey is entering a new and exciting phase of exploring the Universe. We’ve imaged 1/3 of the sky and taken over 3 million spectra, but we haven’t explored beyond the centers of nearby galaxies, haven’t mapped the Universe between 3 and 7 billion years after the Big Bang, and haven’t studied the part of the Milky Way that is only visible from the Southern Hemisphere. Well, that all changes starting now! We have a press release today featuring the science of SDSS-IV and including a fantastic video by John Parejko illustrating how SDSS takes all that data (hint: it starts with a lot of work in the daytime and continues with a lot of work in the nighttime).

 

Revamp of SDSS.org

As part of the transition from SDSS-III to SDSS-IV we have just launched a revamped version of the sdss.org website.

The site is redesigned to represent the entire SDSS, from the beginning through today. We hope that it provides a good balance between presenting our amazing results so far and our exciting future.

The original SDSS website is still available at classic.sdss.org, and the SDSS-III website is still available at www.sdss3.org.

Congratulations to the web team on the successful transition of the sites.