Re: Multiplanet system imaged at HR 8799

Sirius_Alpha · by Sirius_Alpha on Fri May 29, 2009 8:40 am

Inner asteroid belt at 10 AU reported here.

A possible architecture of the planetary system HR 8799

Lazarus · by Lazarus on Fri May 29, 2009 1:12 pm

Starting to look like ye olde familiare Sol-system type planetary system architecture, but on a much larger scale: gas giants in wide, low eccentricity orbits with inner and outer planetesimal belts.

Wonder if there are any terrestrials in the inner system.

Sirius_Alpha · by Sirius_Alpha on Fri May 29, 2009 1:51 pm

Would be terribly difficult to detect terrestrial planets. For now, direct imaging of them would be out of the question, from what it seems, detected the innermost gas planet was hard enough.

Radial velocity is out of the question as well with a near face-on inclination providing a low signal, and and a heavily jittery surface to drown it out.

Could astrometry be the key?

Lazarus · by Lazarus on Fri May 29, 2009 2:28 pm

Star's quite massive and far away. Quick back-of-the-envelope calculation suggests the wobble would be far too small to measure for Earth mass planets.

Sirius_Alpha · by Sirius_Alpha on Fri May 29, 2009 4:35 pm

Far too small to measure for what? Current technology? Or SIM (and/or its derivatives)? or... ?

Lazarus · by Lazarus on Fri May 29, 2009 5:22 pm

Considering an Earth-mass planet in the HZ (at roughly 2.2 AU)

Star mass is 1.5 solar masses, so radius of the reflex orbit = m/(M+m) * d is 660 km. For reference the radius of the star is 930,000 km.

Since the star is located at a distance of 39 parsecs, the full amplitude of the astrometric wobble is 0.2 microarcseconds...

Lazarus · by Lazarus on Wed Sep 16, 2009 2:07 pm

More on the HR 8799 debris disc. Three components: an inner warm belt at 6-15 AU, an outer disc at 90-300 AU, and a halo at 300-1000 AU. This halo suggests the outer parts of the disc are dynamically active. A similar phenomenon is observed at Vega.

Two papers on the formation of the HR 8799 planets... conclusion is that it is probably gravitational instability.
The Runts of the Litter: Why planets formed through gravitational instability can only be failed binary stars
The Formation Mechanism of Gas Giants on Wide Orbits

Amusingly enough given the title of the first, the second one contains the following...

However, several lines of observational evidence show that HR 8799 and Fomalhaut are not simply binary or multiple stars with high mass ratios:

Sirius_Alpha · by Sirius_Alpha on Tue Oct 06, 2009 8:58 pm

Pre-Discovery 2007 Image of the HR 8799 Planetary System
http://arxiv.org/abs/0910.0915

Abstract wrote:We present a pre-discovery H-band image of the HR 8799 planetary system that reveals all three planets in August 2007. The data were obtained with the Keck adaptive optics system, using angular differential imaging and a coronagraph. We confirm the physical association of all three planets, including HR 8799d, which had only been detected in 2008 images taken two months apart, and whose association with HR 8799 was least secure until now. We confirm that the planets are 2-3 mag fainter than field brown dwarfs of comparable near-infrared colors. We note that similar under-luminosity is characteristic of young substellar objects at the L/T spectral type transition, and is likely due to enhanced dust content and non-equilibrium CO/CH_4 chemistry in their atmospheres. Finally, we place an upper limit of 18 mag per square arc second on the >120 AU H-band dust-scattered light from the HR 8799 debris disk. The upper limit on the integrated scattered light flux is 1e-4 times the photospheric level, 24 times fainter than the debris ring around HR 4796A.

Lazarus · by Lazarus on Wed Oct 07, 2009 3:56 pm

That's going to have implications for the stability calculations: one of the suggestions for ensuring the system is stable over the age of the star is that HR 8799d is not associated with the system/spurious detection. That doesn't work any more...

Stalker · by Stalker on Fri Oct 09, 2009 1:25 am

I assume that the hypothesis of brown dwarfs for planets c and d is moved aside. I think that such a young brown dwarf would be of spectral type M, not L/T

Lazarus · by Lazarus on Fri Oct 09, 2009 5:38 pm

Well it depends on the mass of the brown dwarf: at the low end of the mass range they cool quite quickly (especially since such objects do not fuse lithium). According to this comparison, various models allow for quite high masses.

On the other hand the configuration of the system (a non-hierarchical system with low-eccentricity orbits, with inner and outer debris belts) argues against the brown dwarf classification, even if some of these objects are in the deuterium-fusion regime. There are several discoveries which suggest the high mass tail of the planetary distribution includes objects that fuse deuterium at some point in their life cycle, while the low-mass tail of the stellar (brown dwarf) distribution includes objects which do not undergo internal fusion at all.

Sirius_Alpha · by Sirius_Alpha on Wed Nov 04, 2009 6:14 pm

Spitzer observations of the disk at HR 8799. Apparently it's "a bit of a mess."
http://www.jpl.nasa.gov/news/features.cfm?feature=2349

A large halo is observed a little over 2000 AU. Probably scattered from further in by those planets.

Lazarus · by Lazarus on Wed Nov 04, 2009 6:40 pm

More details of this are in the arXiv paper I linked upthread (0909.2687). Nice to see a press release for the architecture of a planetary system.

Stalker · by Stalker on Thu Nov 05, 2009 1:56 am

Looks like the Oort Cloud?

Sirius_Alpha · by Sirius_Alpha on Thu Nov 05, 2009 9:48 am

Probably on a much smaller and denser scale. I'm not entirely sure about this, so correct me if I'm wrong. The Oort cloud is expected to extend out several tens of thousands of AU. While HR 8799 may have such an Oort cloud, it's probably far too faint to detect, and what we are seeing is the protoplanetary disk that has been scattered outward. Perhaps given sufficient time, the planets will scatter much of what we see further out, adding to HR 8799's Oort cloud and subtracting from the visible disk.