Greetings from Palmia Observatory
Well, the Exoplanet Demographics 2020 meeting is pretty much over now. One topic I meant to listen more to this interesting discussion on photoevaporation of cold gas giant planets by white dwarfs, but unfortunately got interrupted or maybe took a nap.
The topic of photoevaporation of cold gas giants by white dwarfs is interesting and I wished I had got to listen to the rest of the presentation.
|Photoionization of cold gas giant planets (Source: Matthias Schreiber, Exoplanet Demographics 2020)|
Ok, I missed the presentation, but let's do some preliminary homework to come up to speed on this. First of all remember that white dwarfs are dead stars, that is stars that have used up their hydrogen and other elements that can be fused and so they produce no energy output and are just left to cool down. Our sun will run out of fuel in billions of years and will move off of the main sequence and end up as a white dwarf as shown on the H-R diagram below.
|Looking for White Dwarfs on Hertzsprung-Russell Diagram (Source: Wikipedia)|
It is interesting to compare the energy density of stellar objects. The density in the core of the sun is about 150,000 times as great as a given volume on Earth. Look how the density of the sun is quite low compared to a white dwarf because the sun is going through fusion and the energy released keeps the core from collapsing more. The density of a white dwarf is about 10,000 greater than the sun. Then the density of neutron stars is something like a hundred million ties greater than a neutron star. The effective density of a black hole, even though the concept of density sort of breaks down there, is much, much higher.
|Useful comparison of densities of astronomical objects (Source: Wikipedia)|
If the white dwarf does not throw off a lot of its initial mass we should find that white dwarfs are much smaller than regular stars. A typical white dwarf might be about the same diameter as the Earth. The sun is about 860,000 miles in diameter, so as a rough order of magnitude a white dwarf would be about 100 times smaller than it was when it was on the main sequence. The temperature of a white dwarf can be much hotter than the surface of the sun, let's saw about 10 times hotter. White dwarf temperatures can vary over a wide range, but once formed they just continue to cool off unless there is a lot of material that accretes onto the dwarf.
So, what is the luminosity of a white dwarf compared to its original main sequence luminosity? Well, if we use the thermodynamic relationship regarding luminosity we see that it scales up as the radius to the 2nd power and temperature to the 4th power. So, if the radius of the white dwarf is down by a factor of 100 and the temperature is higher by a factor of 10, then the luminosity is surprisingly, about the same.
|The luminosity of a an object depends on size and temperature (Source: www.astro.cornell.edu)|
So, why would we think that a white dwarf would be better able to photoevaporate any planets still in orbit. Even though the luminosity before and after could be about the same value, the higher temperature means more of the radiated energy will be at higher energy and more capable of photoionizing any gas around the planets.
So, it seems ok that a white dwarf could result in more photoevaporation of cold gas giants around the white dwarf. Now I have no idea if this is the basis of what the speaker had in mind, but at least it seems plausible. At the same time, the whole issue of what the planets will do has not considered what the effect of the evolution of a star off of the main sequence would also do to any of its planets. For instance if the star goes through a red giant phase, that certainly would affect any planets.
Anyway, with this basic homework review, I am now ready to go back and see if I can find the presentation and dig into whatever other details actually go on.
In other news at the observatory, a conic section teaching tool, that had been on order at the start of the pandemic, finally arrived. So, let's see if we can get Astronomer Assistant Willow to help out in checking out this tool. Hmm, she does not seem to be interested, especially after being awoken from her nap.
|The cone awakens Astronomer Assistant Willow from her nap (Source: Palmia Observatory)|
Now I remember studying conic sections years ago in a solid geometry class but most of the details are long gone from my memory. The conic sections have something to do with the end result of passing a plane through the cone at various angles and looking at the resulting curves. Now that Willow is a little more curious let's see what happens to the cone.
|Looking at conic sections with Astronomer Assistant Willow (Source: Palmia Observatory)|
Yep, the wooden cone is made to come apart so that the individual parts represent a circle, an ellipse, a parabola and a hyperbola.
|The wooden cone can be disassembled to show each conic section (Source: Palmia Observatory)|
If you don't recall a lot of the details, luckily we can just ask Siri or Wikipedia to remind us of the details. So all of you math wizard wannabes and physicist wannabes can check out the details. Remember it all comes down to a generating line that passes through the cone at different angles to the primary vertex of the cone.
|Conic Sections (Source: Wikipedia)|
If you want to dig deeper into the details and just watch a video lecture on conic sections then this YouTube video is pretty good and it goes into the definition and generation of conic sections. Check it out the Creative Learning video at: https://www.youtube.com/watch?v=HO2zAU3Eppo
Finally, COVID-19 risk assessment tool is now available. You can click on your location and set the number of people that are coming to your Thanksgiving dinner or other event and the tool calculates the probability that at least one person in your party is infected. I have no idea of how accurate the tool is, but it is pretty interesting. Check out the 12% probability for a dozen guest in Orange County. You can adjust for the number of people in your party and do the same for your location at the website: https://covid19risk.biosci.gatech.edu/
|COVID-19 Risk Assessment Tool (Source: https://covid19risk.biosci.gatech.edu/)|
Finally, as long as we are on the topic of COVI-19, how would you like to agree to participate in a trial of the new vaccines? This originally came to my attention during a conversation where Dilbert creator, Scott Adams, was floating an idea where volunteers would participate on a free cruise ship tour, after getting the vaccine injection. Having all of these volunteers aboard a controlled environment, like a cruise ship was thought to speed up the time needed to certify the effectiveness of the vaccine under trial. This cruise ship idea was also mentioned on this morning's KTLA 5 TV news too. Hmm, I don't know if any such trial actually exists, but would you volunteer for this free cruise? Maybe. But I would not want the risk of being one of the volunteers who received the placebo instead of the real test vaccine. How about it; what do you think about the risk of signing up for the trial? This screenshot is not about some hypothetical cruise scenario, but apparently is a real search for compensated volunteers.
|Would you volunteer for vaccine trial testing? (Source: www.ClinicalResearch.com)|
Until next time, here from our burrow, stay safe, as we recover more of our freedom,
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