Paper Published!

It's been a while since I've updated this blog, but since I last wrote, my paper on the ages of stars in the DEBRIS project has been published in the Astronomical Journal! Click here to read it!

Abstract:

DEBRIS is a flux-limited survey of nearby stars (spectral types A-M) for evidence of debris disks with the Herschel Space Observatory. One goal of the survey is to determine disk incidence as a function of various stellar parameters. Understanding debris disk evolution depends on knowledge of the precise age of stars around which these debris disks are found. However, finding ages for field stars is notoriously difficult. Furthermore, in an unbiased sample like DEBRIS, one is working with stars across many spectral types. This requires a multi-method approach to age determination. In this paper, we outline several methods of age determination broken down by spectral type, including some strengths and limitations of each method. In total, we were able to calculate ages for 263 of 274 F-, G-, and K-type stars, and all 83 A-type stars in the DEBRIS sample.

Vsini ages

The paper came back from the referee with a few comments. One was that there should be a more in depth discussion of ages calculated using vsini calculations (rather than rotational period).

Lick Observing Run

With the age paper submitted to AJ, I'm focusing on my upcoming observing run at the Nickel 1 meter at Lick. The purpose of this observing run is to determine if the 1 meter telescope is powerful enough to reliably determine chromospheric emission (and therefore chromospheric ages). To do this, I am observing a group of stars which have rotational data from wright et al. 2011 and long-term chromospheric data from wright et al. 2004. I calculated ages (using the formulae in Mamajek & Hillenbrand 2008) and compared gyro to chromospheric. I'm restricting my target list to those stars for which gyro and chromo ages agree to 20% or less. This list has 36 targets. I then need to see which of these targets will actually be visible from Lick in early January. If my calculated chromospheric emission parameters match those from the literature, I can conclude that the Lick 1 meter telescope is powerful enough to use for age determination. Since the 1 meter is often undersubscribed, this could be a great opportunity to get chromospheric ages for a large number of stars with no data in the literature.

Age Paper

Working on editing the final draft of the DEBRIS age paper. New updates for r'hk ages have been added to the DEBRIS Twiki. The only thing that I changes was to preferentially take values of log(r'hk) from Wright04 and Duncan91 so that I had a longer observation baseline for the chromospheric activity. This helped cut down scatter in my plots of age vs. age.

Also, I had Billy take another look at the M star data from GALEX, and cutting out any obvious non-detections, here's what was left over:

It seems that it's not just a fluke that most of our M star ages are old. It may be that young M stars are too UV bright, and GALEX purposefully avoided imaging them. They may have even actively masked them out.

Space Motions

I calculated some UVWs for A stars to see if any of them were obviously in associations...

None of the UVWs are close enough to the published moving groups for me to change any ages, but some of my ages are similar to the ages of the moving groups. So there may be some new members in the DEBRIS sample. I need to look a little more closely.

First Paper Draft

First draft of the paper is written and edited! Second draft will be done soon. Ben asked me to compare the ages derived using activity from Wright et al. 2004 and Mamajek 2008. So here it is! The correlation breaks down around log(r'hk)=05, as Mamajek predicts.

Next step is to find UVW's for A stars and see if any of them are in known associations...

Also, my book came in from UC Irvine - An IAU symposium from 2008 called "The Ages of Stars". Sounds pretty useful!

M star ages...

So I've compiled my M star data (DEBRIS targets and field stars with known ages) and here are the results:

This would seem to imply that all of our M stars are old. I'm not comfortable with this assumption, and I want more field stars to compare. I started by trying to hunt down stars in clusters with well defined ages. The problem there is that the GALEX field for stars in clusters is extremely crowded, and plucking out one target from a cluster of targets is an unreliable selection method. I'd rather hunt down field stars in cleaner GALEX fields where I can be sure that the GALEX target I'm looking at is actually the right M star. So if anyone knows of papers with field M stars, let me know!

X ray Ages for FGK Stars

New update: I rederived my X-ray ages, because I didn't like how they turned out. Comparing the ages I get from xray data (from ROSAT) to my chromospheric ages, I don't see much of a correlation:

however, when I compare them to my gyrochonology ages (which I trust more), I see a much better agreement:

This tells me that xray ages are NOT useless, and since I have over 100 xray ages, that's good news. This plot also tells me that gyrochronology does NOT work well for slow rotators (the red dot).

M Stars at Last!

Now that I have a table of NUV data from GALEX (courtesy of our new undergrad - Billy!), we can start  making some color cuts to determine some sort of "youngness" parameter. In David's paper (Rodriguez et al. 2011), he plotted NUV-J vs J-K and made color cuts from Findeisen & Hillenbrand 2010. Here I've plotted our M stars on the same scale so we can see where they would lie on David's plot:

The purple/blue cloud in David's plot represents galaxies. What I'd really like is Figure 3 from david's plot, but scaled with stellar age. I'm going to add some of the stars from his paper which do have ages  on this plot and see where the DEBRIS M stars lie.

Another option would be to simply track down a sample of M stars with published ages which have been observed with GALEX.

Padova Isochrones

I've gotten ages for about half of my A stars using the Padova isochrones, but wanted to stop and check the progress by comparing these ages to ages determined from my other isochrones. For A star ages calculated from isochrones, I tend to trust the (recent) published literature ages. However, for consistency I'd like to have ages that were all derived using the same set of isochrones.

So which isochrone set should people trust?

According to this plot, YREC ages have the best agreement to the ages in the literature. It is interesting that the Padova ages seem to be off by an added constant, but I do not think this is significant.

Therefore, I would trust the YREC isochrones (from Pinsonneault et al. 2004) before any of other ages!