25 October 2011

NSF Budget Cuts

Title: Dire Budget Projections from NSF AST
Authors: NOAO

This morning, Julie brought to my attention a recent development in the NSF AST budget drama: Kitt Peak National Observatory (KPNO) might be closed. Due to the enormous impact this would have on the astronomical community, it was decided that this needed to be discussed at today's Astro Lunch before we got to the planned paper. The closure of KPNO hits close to home for Dartmouth astronomers as MDM Observatory is located on Kitt Peak, though about 2 miles from KPNO. Currently, much of the infrastructure at MDM is sub-leased through KPNO, so one can imagine how much strain this would put on MDM. Just imagine, if you will, renting an apartment and having your landlord walk away from the mortgage, leaving you with very few options.

It would be selfish, however, to only analyze this situation in terms of the effects on MDM. The amount of science that would be lost with the closure of KPNO is unimaginable. Small telescopes are crucial on multiple levels, even if we have a few large survey telescopes (LSST, ALMA). I won't discuss the financial details here, as they are available online. I encourage everyone to head to the website linked in the title above. NOAO gives a solid overview of the current situation and just how dire it may be. There is also an NOAO discussion forum in which people can discuss and leave their thoughts and comments on the potential closure of KPNO. We must also be careful NOT to panic, this is not set in stone and hasn't been formally approved, but clearly people are talking about it and the idea has been raised.

Civil discourse is always ideal and there is no reason to overreact. Join the discussion, follow the debates, and stay informed. While we must not panic, we must also work to save KPNO if it is endangered. Let us know your thoughts, we're interested to hear all your perspectives.

24 October 2011

An Expensive Planet

Title: Transformation of a Star into a Planet in a Millisecond Pulsar Binary
Authors: M. Bailes et al.

This past week's article comes our way via Science Magazine instead of the usual Astro-PH listing. The results made science headlines in a wide variety of popular science outlets as the "diamond planet". Normally, pop. sci. articles are filled with exaggerated claims and are mired by shaky conclusions. So, how did the science writers do on reporting these results? Quite well, actually.

After a star dies via a supernova explosion, it will form a neutron star (if it has less than ~20 solar masses), a super compact object which sustains itself against gravity via neutron degeneracy pressure. If you are familiar with electron degeneracy pressure, for instance, in the case of a white dwarf, then neutron degeneracy pressure is already known to you - except  you replace electrons by neutrons. It turns out, that when electron degeneracy pressure is not strong enough to support the gravitational collapse of matter, neutron degeneracy pressure is the next line of defense. Anyway, I digress. Post-supernova explosion, as stellar material collapses down to the core of the stellar remnant, it spins up the core due to angular momentum conservation (think figure skater pulling his/her arms in to speed up their spin). This produces a neutron star that is rapidly spinning (~1000 rotations per second) and one that has a very strong magnetic field (1012G) which spews material out in the form of bipolar jets; a (radio) millisecond pulsar.

Credit: European Space Agency & Francesco Ferraro (Bologna Astronomical Observatory)

Large magnetic fields, however, quickly help to spin-down the neutron star due to magnetic breaking - a process by which angular momentum is transferred away from the star to its surroundings via magnetic processes. The spin-down causes the pulsar to stop emitting at radio wavelengths. Interestingly, a "dead radio pulsar" can be revived if the pulsar is in a binary system with a stellar companion. Processes by which pulsars are revived are not fully understood, but we shall describe a couple of the possible scenarios later.

Now to the results of the paper. Bailes et al. have discovered one of these millisecond pulsars which has a binary companion. Not too uncommon (~70% are in binary systems), but based on fairly straight forward geometrical and dynamical arguments (think Kepler's laws), the authors were able to determine the radius and the average density of the pulsar's companion. They find that the radius is actually smaller than Jupiter's radius! Fantastic. This means there is a planet sized object orbiting the pulsar. Then, using some clever techniques, the authors are able to derive a lower limit to the average density of the object. If the object were gas planet, one would expect to find densities around 2 grams per cm3, instead, they find the lower limit is 23 g per cm3! This is a HUGE density!

After running a few more tests to be sure of the robustness of their findings and to limit, even more, the parameters of the binary companion to the pulsar, the team discovered that, indeed, they found a pulsar with a planetary sized white dwarf companion, supported by electron degeneracy pressure mentioned earlier. What is most interesting, is that the white dwarf did not form in the typical fashion of a lower mass star evolving until its subsequent death. This is evidenced by the low mass (slightly more massive than Jupiter) and tiny radius (smaller than Jupiter) of the white dwarf. A typical evolutionary scenario would be very, very unlikely to produce a remnant of these proportions. Instead, the white dwarf must have been formed by interactions with the primary (aka: the pulsar).

Here there are a couple different scenarios which could lead to the observed configuration. The first is that the companion white dwarf, once a helium burning star, was converting helium to carbon in it's core. However, it may have transferred mass to the neutron star - a process known as accretion - which occurs when material is gravitationally removed from the star by the neutron star. This material would then be either accreted onto the neutron star or expelled in the jets which are generated by the magnetic field. Either way, this would act to spin-up the neutron star and bring it back to life (aka: emit at radio wavelengths).

This process of accretion by the neutron star can happen in a couple ways, so it's thought. The first is if the companion star to the pulsar is fairly far away from the pulsar, but is itself very large. Material can then stream away from the companion. However, this would tend to create white dwarfs of varying size depending on the distance. If, instead, the star is close to the pulsar to start with, matter can be transferred more readily, allowing more easily for the creation of a smaller white dwarf. In either case, for this particular planet to be observed, the mass transfer would have to be at a sufficient rate in order to remove enough mass from the companion BEFORE it has time to generate a typical carbon core. This close-in binary scenario also raises questions about possible interactions during the primary stars life before it became a neutron star. It presumably would have been rather large, thus possibly encompassing the secondary, or at least allowing for mass transfer at this point, as well. Complicated physics, to say the least.

Credit: An artist whose name is not associated with this image on several prominent websites.

In the end, the planet-sized object that remains in the system at hand is a very dense carbon object. Very dense carbon inevitably leads to the image of diamonds. So the popular science articles were not too far off in their analysis and spin on the results. We have here a planet that would fetch a very high price on the market and one that would make the Hope Diamond seem like pebble.

14 October 2011

The Furthest Distance Measure in the World?

Title: A New Cosmological Distance Measure Using AGN
Authors: D. Watson et al.

Measurement of actual distance to celestial bodies has always been a difficult task from the beginning of  astronomy. Every new distance measures have led to fundamental changes in our understanding of the Universe. In particular, the most recent Nobel Prize of physics was awarded to astronomers who took advantage of the powerful distance measure, type Ia supernovae, to prove that the expansion of the universe is actually increasing.

While type Ia supernovae serve as a good distance measure at high redshift, it is still limited within z~2. AGN(active galactic nuclei) reverberation, based on the argument of this paper, provides an alternative way to measure distance at high redshift.  The idea of AGN reverberation was proposed by Blandford & McKee in the early 80s : For a standard model AGN, the radiation from the broad line region(BLR) is basically the recombination lines from the gas clumps ionized by the continuum from the central accretion disk. The causal relation between the central continuum and the BLR emissions lines implies that if there's any change in the central continuum, after a time lag, the recombination lines in the broadline region should change accordingly. This time delay, can be measured using the reverberation technique, by continuously observing the AGN spectra. The distance from BLR to the AGN center, equivalent to the ionizing radius of the central continuum, can be measured from the time lag. Since the ionizing radius of the central continuum is a function of the continuum flux, there must be a direct connection between the time delay and the AGN flux measured on Earth. This relation can be used as a measurement of the luminosity distance. This paper shows that the ratio of time lag and the AGN flux  is consistent with predictions up to z~1.

The downside of this measure is that reverberation requires a lot of observation times. Even though it is possible to apply this method on AGNs at z>2, it would require more observing time from more powerful telescopes. The uncertainty of the AGN selection effect or the cosmological effects at high z can either hinder the accuracy of this measure, or increase the difficulties in measurement. However, the author did address some of the problems and concludes that the high-z reverberation will soon become possible.

Indeed, if the distance measure can be applied to where no one has been able to measure, it will be extremely exciting to see how it will change our understanding to the Universe.

06 October 2011

Making Non-constants Constant

Title: The call to adopt a nominal set of astrophysical parameters and constants to improve the accuracy of funametnal physical properties of stars.
Authors: Petr Harmanec & Andrej Prša

Astrophysical parameters and constants are, admittedly, not the most exciting topic for a paper. Readily available in the back of most textbooks and compiled in every edition of Allen's Astrophysical Quantities, astrophysical parameters are of little concern, right? Well, it turns out this is not necessarily the case (if it was, we wouldn't be reading a paper about it). There are systems for which observational uncertainties are pushing below 1%, especially with the instruments aboard MOST, CoRoT, and Kepler. As detector noise is reduced, the precision of observational measurements encroaches a point where systematic sources of error in data analysis models begins to play a role.

Of primary interest to the authors are the definitions (in natural units) of the solar mass, solar radius, and solar luminosity units. Better instruments, data reduction procedures, and observational techniques have lead to more refined values for the said solar parameters. Tables 1 and 2 in the paper display the evolution of the solar mass, radius, and effective temperature (re: luminosity) since 1976, where the IAU defined a set parameters. The fluctuations are on the order of about half a percent, insignificant in most practical applications. However, for systems where observational uncertainties are being quoted as less than one percent, the difference in the adopted solar parameters begins to become a substantial source of uncertainty.

How, then, do we combat this? For one, the solar mass, radius, and luminosity are not strictly constant. The solar mass may as well be considered constant; mass loss due to the solar wind is a puny 10-14 solar masses per year - not a concern. Stellar evolution theory tells us that the solar radius and luminosity change significantly over the Sun's lifetime, but the time scales for these changes is far greater than anything we need to worry about - at least for the next 100 million years or so. There is also the question of the solar cycle, which may alter the solar radius and luminosity depending on whether the Sun is in an active or quiescient period. Results of such studies are contradictory. One group finds the radius and luminosity increase during active periods while another group finds the exact opposite. Either way, if we average observations over a solar cycle or just measure the Sun when it's in the middle of a solar cycle, we can come to an acceptable solution. That is, if we are careful. Observers need to be very meticulous and perform very precise observations, especially considering that defining the "radius" is difficult when looking at an image of the Sun. The boundary of the surface is rather hazy.

Either way, assuming the solar observers have made good observations over the years (which they have), the values are still a bit different. In order to set all models on a common base, the authors suggest adopteing a nominal set of values for each of these parameters. This way, even if the actual values change as the result of more precise measurements, models (and thus other observations) will be unaffected. The authors propose a set of nominal values, although their selection process and value determination method is not mentioned. It does get the conversation started, though. Unfortunately, to make this happen on a large scale, the IAU would have to set forth a committee (or several of them) to discuss and debate which set of values should be adopted as nominal. Do we use values determined by one observer or one research group? What about an average over several observations? These issues would have to be addressed in order to define the parameters in a rigorous manner.

As a final note, how many astronomers are actually concerned about this? Would the call to adopt nominal parameters be supported strongly enough to force the IAU to invest substantial resources into sorting out this issue? If only a small sample of astronomers are truly concerned, then the discussion is moot. We can simply imagine a situation where particular groups of researchers adopt standard parameters (e.g., eclipsing binary observers adopt their own set of parameters for the sake of consistency) or researchers can simply be forced to quote in their paper which values they utilize, essentially making them define, in natural units, the results of their work.

03 October 2011


Welcome to the blog for Dartmouth's Astronomy Lunch.  We hope that this will be a way to catch up on what was talked about at astro lunch for those who can't make it, as well as a way to spur more discussion outside of our hour meetings.  If you have any comments or suggestions, please let Greg or I know.  Thanks!