Wednesday, October 28, 2015

A Data-Driven Solution to the Stellar "Mass Mess"

Guest post by Dr. Luan Ghezzi. Luan was a postdoctoral researcher in the Harvard Exolab from 2013-2015, funded by CAPES under the Brazilian federal program Science Without Borders. This past summer he returned home to Rio de Janeiro, Brazil to continue his research into the physical properties of stars as measured from high-resolution spectroscopy. In addition to doing research at the Harvard CfA, he was also a research advisor in the 2015 Banneker Institute. 

The detection of the first extrasolar planet around a solar-type star intrigued astronomers all around the world. The newly discovered system had a planet with almost half the mass of Jupiter orbiting its star at approximately 12% the average distance between Mercury and the Sun, a configuration that is radically different from the one we observe in our own Solar System. In the following twenty years, almost 2000 other extrasolar planets were discovered and confirmed, and nearly 4000 candidates await further confirmation/validation. However, these impressive numbers don’t mean that we know everything about the planetary systems out there. New exciting results dazzle us every day, like the hot friends of Hot Jupiters or the disintegrating planet around the cosmic death star.

Devising a model that accounts for the formation and evolution of all the discovered systems so far is one of the biggest challenges in Astronomy nowadays. One key ingredient involves a little bit of cosmic "genetics." When a child is conceived, genetics is able to tell us the probabilities that the individual will have certain characteristics (for example, the color of the eyes) based on the parents’ characteristics. It seems so far that the same holds for the parent stars of extrasolar planets. 

We now know that a star with a higher abundance of metals (which means, for astronomers, every element heavier than H and He) has higher probability of hosting a giant planet (with a mass similar to that of Jupiter). The repeated confirmation of this link by independent research groups was a big step because it established that the formation of a specific class of planet is tied to one of the properties that mainly determine the evolution and fate of stars: the chemical composition. But how about the other key parameter for stellar evolution: the mass? 

The key plot from Johnson et al. 2010 showing that the
likelihood of a star hosting a Jupiter-mass planet appears
to increase with stellar mass. 
Building up on earlier contributions, John Johnson and collaborators published a paper in 2010 to further explore the role of stellar mass in the formation of giant planets. The analysis of a sample of roughly a thousand stars from the California Planet Survey revealed that the probability of hosting giant planets increases linearly with stellar mass, going from 3% for M dwarfs to 8.5% for FGK dwarfs and finally to 14% for the retired A stars. This was a striking result since there was now stronger evidence that both of the fundamental properties that mainly govern the lives of stars also affected the formation of planets around them. 

However, as often happens in science, additional analyses have muddied the waters a bit. Recently there have suggestions that the masses of the retired A stars could have been overestimated, thus creating an artificial correlation with the probability of hosting giant planets. Despite some back and forth in the literature in the following years, the controversy has remained open due to a lack of empirical results (theory can only take us so far after all). I like to think of this situation as the stellar mass mess

It doesn't do this in Rio de Janeiro!
Upon my arrival at the CfA in January 2014, during a typical really cold New England winter (seriously, it gets really cold!), John and I decided that tackling this mass mess was a good start for my post-doc there. By that time, John and collaborators were finishing a paper in which they use model-independent measurements to confirm that HD 185351 is indeed a typical retired A star (blog post here). In a perfect world, we would just simply extend this analysis to the other 243 retired A stars. But in reality, this would be demand too much time on too many telescopes, so we needed an alternative for my two-year postdoc.

The solution was to gather data from the largest database that was not fully explored: the literature! We conducted an extensive search for benchmark evolved stars which had masses determined independently from stellar evolution models. After reading about a couple of hundred papers, we were able to compile of list of 59 benchmark stars: 26 members of binary systems (in the Milky Way as well as Small and Large Magellanic Clouds) with dynamical masses and 33 isolated stars with asteroseismology-based masses. We then determined model-dependent masses for this same sample using very heterogeneous input parameters collected from the literature, the PARAM code kindly provided by Leo Girardi and PARSEC grid of evolutionary tracks.

The comparison between model-independent (y axis) and model-dependent (x axis) is shown in the figure below. We can see a very good agreement for a relatively large mass interval (~0.7 - 4.5 solar masses), even though heterogeneous data was used to derive the results. The percentage difference between the two sets of masses (in the sense Evolutionary Tracks - Reference) is -1.30 +/- 2.42% and no trends were observed in the residuals relative to the input parameters. Similar good agreements were observed in the comparisons for the radii (-4.81 +/- 1.32%) and surface gravities (0.71 +/- 0.51%). We have also found a good consistency between independently determined ages for members of the same binary systems.
I learned that this is what Americans refer to as a "money plot." It shows that
model-based masses for evolved stars match the true masses measured by
empirical methods. The masses of our evolved "benchmark" stars are not
overestimated by the models!
Put together, our results show that the determination of evolutionary parameters using the PARSEC models and the PARAM code is capable of providing reliable masses, radii, surface gravities and ages. In particular, the masses do not seem to be significantly affected by systematic errors that would end up overestimating them. This conclusion is really important because it corroborates many studies involving topics that range from extrasolar planets to Galactic evolution. If you are interested in more details, check out our ApJ Paper, Ghezzi & Johnson (2015).
The stars in Ghezzi & Johnson (2015) occupy the so-called giant
branch of the H-R diagram, illustrated above. Specifically, our
"benchmark" stars are in stages 8-10, mostly with R > 10 R☉.
It's impossible for our masses to be correct according to the
models while incorrect at and near stage 8, where the
Johnson et al. "retired A stars" are. 

One concern might be that we studied stars that are more evolved than those in the study of Johnson et al. However, since stars follow a single evolutionary sequence that varies smoothly in time and has no discontinuities, it is impossible for the model grids to be correct in the more evolved part of the giant branch while simultaneously being erroneous by as much as 50% a bit further down. (That is, unless we're willing to throw out everything we know about stellar evolution! Ed.)

Although our most recent study was a big step to solve the mass mess, much work remains to be done. That’s why myself, John and our collaborators are coordinating multiple efforts to improve the precision of observed parameters, providing better input to the stellar evolution models. For instance, John and I are revisiting the retired A star sample with new analyzes. Spoiler Alert! Preliminary results do confirm that the correlation between occurrence of giant planets and stellar mass holds. Stay tuned for these new exciting results!



Friday, October 16, 2015

Close Friends of Hot Jupiters: The WASP-47 system


Ever since a mechanical failure caused the end of the original Kepler mission in 2013, the Kepler spacecraft has been conducting a survey of new stars, searching for planets across the ecliptic plane in its new K2 mission. The K2 dataset is a goldmine of fascinating science results. One such result is the recent discovery of two new planets in the WASP-47 system.

Until a few months ago, everyone knew that hot Jupiter planets don’t have “friends”, or nearby small planets in close orbits to the host star. These other planets had been searched for extensively, through radial velocity measurements, analysis of the transit times of the hot Jupiters, and even through transits by Kepler during its original mission. All of these searches turned up nothing.

This all changed one day last July, when Hans Martin Schwengeler, a citizen scientist who enjoys poring over Kepler and K2 data searching for new transiting planets by eye, came across the telltale signatures of two extra transiting planets in the hot Jupiter system WASP-47. WASP 47b was, by all indications, a perfectly normal hot Jupiter -- in the discovery paper, Coel Hellier wrote “With an orbital period of 4.16 days, a mass of 1.14 Jupiter masses, and a radius of 1.15 Jupiter radii, WASP-47b is an entirely typical hot Jupiter”. The discovery of additional transiting planets dramatically changed the narrative.

Monday, October 12, 2015

Guest post: We must unseat Research as the sole god king of our field

This is a guest post submitted by Betsy Mills, a postdoc at the National Radio Astronomy Observatory (NRAO) in Socorro, NM. FYI, I welcome guest posts that promote social justice and advance the conversations we badly need in our field of science, and in our greater society. However, I will retain sole discretion over the content of this blog, so not all submissions of guest posts will be published. Have something you'd like to share here? Send me a proposal, outline or full draft! Y'all know how to reach me. 

I am supposed to be writing job applications right now. But it is difficult when I am having such a complicated relationship with the field in which I am trying to get permanent employment. It is not just a feeling of having lesser value as a woman in this field, seeing how poorly my female peers have been treated and disrespected for decades at Berkeley.  And I am not feeling conflicted wondering how much of this really happened (I sadly believe it all) or what sanctions for Marcy or Berkeley are appropriate (bring them on).  Rather, much of much of my internal discomfort stems from the role played in this saga by aspects of an academic culture that are not unique to Berkeley, and that allowed the behavior of a serial predator to go on for so long unchecked. 

In our profession as astronomers, research is King. And I do not wholly object to that: research is after all our primary job function: we are here to explore the universe. And I love it.  But research alone does not make our field: we cannot also keep this profession sustained in today's society without also having excellent teachers to pass on the stores of knowledge that we have built up, excellent mentors to steer new researchers in the pursuit of new knowledge, excellent outreach that conveys the value of this exploration to the public, and the inclusion and support of excellent researchers from all underrepresented backgrounds of race, gender, disability and sexuality. I believe that this truth is well recognizedbut I also believe that it is not well rewarded.  An anecdote that sticks with me is about a faculty member who received tenure decades ago at a school where tenure was based on success in 2 out of 3 aspects of academic life: research, teaching, and service. And this faculty member received tenure because even though his teaching was abysmal, and his service was nonexistent, his research was so good that it counted as a service to the department. 

Sunday, October 11, 2015

Open Letter RE: The Berkeley Boycott

The following was just forwarded to me via email. I can see this as nothing other than a man bravely and correctly leveraging his privilege to force much-needed and long-overdue changes in a very broken system. This is how you ally. No badges, no cookies. Just direct action. 

--------------

Dear recent recipients of NASA's prize fellowships (Einstein/Hubble/Sagan),

I have written a letter requesting a boycott of UC Berkeley's open faculty position in response to UCB's complete refusal to remove Geoff Marcy for at least a decade of repeated (and admitted) sexual harassment and assault. You can read the full text of the letter at this address:


Until UCB acts, I think it is in the best interest of all postdocs to refuse considering a position at their university. I believe that together we can convince them to make the right choice and to remove Geoff Marcy.

Thanks for reading, and sorry to bother everyone on a Sunday!


- James Guillochon

Saturday, October 10, 2015

On Sexual Harassment and Our Culture of Denial

Sexual harassers, their apologists, the silence of their colleagues, and those who doubt their victims all work together to strengthen systemic/institutional sexism. These are not independent phenomena, but interworking pieces of a powerful machinery that systematically devalues women and their work, and artificially inflates the value of men and their intellectual contributions. You don't even have to think to get caught up in it. That's the nature of systemic *isms. Doing nothing enforces them. Only by consciously working against them can their effects be mitigated.

The toxic cycle of abuse, fear, denial and silence around sexual harassment in academia is described in detail by Janet Stemwedel (@DocFreeride) in her excellent (as usual) Forbes piece about the Marcy scandal. 

h/t Renée for this very illustrative cartoon:


Friday, October 9, 2015

The Long Con

Hiding in Plain Sight

ESPN has a series of sports documentaries called 30 For 30. One of my favorites is called Broke which is about how professional athletes often make tens of millions of dollars in their careers yet retire with nothing. One of the major "leaks" turns out to be con artists, who lure athletes into elaborate real estate schemes or business ventures. This naturally raises the question: In a tightly-knit social structure that is a sports team, how can con artists operate so effectively and extensively? The answer is quite simple: very few people taken in by con artists ever tell anyone what happened. Thus, con artists can operate out in the open with little fear of consequences because they are shielded by the collective silence of their victims.

I can empathize with this. I've lost money in two different con schemes. One was when I was in college, and I received a phone call that I had won an all-expenses-paid trip to the Bahamas. All I needed to do was put a deposit down and I'd receive my vacation package in the mail. When it never came, and when no one answered the phone at the "company," it slowly sank in that I had been conned. Here I was, a highly intelligent college student studying physics, and I was too "stupid" to not see what was happening to me. Thinking back, I still get a sick feeling in my stomach. The shame is still palpable, 17 years later. I never told anyone in authority. In fact, this is the first time I've spoken about it publicly. 

Being conned feels the way it does because your trust is a valuable commodity. As social creatures, we don't give our trust to everyone. Others must earn it. But once they do, they have access to other valuable aspects of yourself. In my case, the person on the other end of the phone gained access to my debit card (fortunately, they only made a single charge). The other time I was conned, the person worked for months to gain access to my time and effort, in addition to my money. But things like money can be recouped. Other things cannot.

Sexual Harassment as a Con Game

Something that people rarely think of as a con game is sexual harassment, but after listening to the lived experiences of women who have been sexually harassed and/or assaulted, I feel the analogy is apt. Like a con artist, the sexual harasser usually knows their victim well and uses their authority or "friendship" to gain trust. With that trust, the harasser then works to gain access to something far more valuable than money. They gain access to the victim's body, their sexuality, their most private selves. In addition to anger and frustration, the common theme in the stories I've heard is shame and guilt. These feelings are why sexual offenses are so infrequently reported. 

Tuesday, October 6, 2015

Thoughts, Take 2: On The Struggle

This is the second part of a haphazardly organized series of responses to Ta-Nahesi Coates' book Between the World and Me. The first part can be found here

I was recently at a discussion about racism held in one of Harvard's dorms, Currier House, as part of their Currier Conversation series. As the honored guest and moderator of the discussion, I decided to have the participants break into groups of 2-4 and define some seemingly simple words before we had a proper dialog:

Race
Racism
Power
Social Justice

After the audience discussed for 5-10 minutes, I had them speak out about their definitions, thoughts and questions while I recorded their responses on a large paper pad mounted on a tripod. The self-selected group of attendees were well versed in the notions of race as a social construct; racism as the melding of prejudice and power; power as the ability of a social group to shape the choices and outcomes of other groups; and social justice as the simple notion that every human should have the same social, economic and political opportunities, even while they currently do not in our country. I was speaking to the choir, but it was a good discussion nonetheless.

It was the sort of conversation that would no doubt cause Harvard's founders, those well-rounded captains of industry who harnessed the power of the American slave economy to amass great wealth, pass it to their children's children, and have buildings named after them, to squirm in their well-appointed graves. It was the sort of evening that also likely made many of the white attendees feel like the moral arc of the universe was, indeed, bending toward justice. However, this participant was under no such illusions, nor were the sisters and brothers with whom I spoke after the room emptied out.

The final question of the evening was offered by the house "master" (a title that needs to be placed on the trash heap of history) who asked me and another guest, "Here you are, two successful African Americans from different generations speaking to the same issues. Do you see progress toward and hope for the future with regards to race in this country?" My fellow guest said, and I paraphrase, "No." My answer was not dissimilar, even if I remember it in greater detail.