Brown dwarf T party

By Phil Plait | September 19, 2006 10:14 pm
Spitzer Space Telescope images of the two T dwarfs: HD 3651 B on the left, and HN Peg B on the right. Those pictures may look a lot alike, but they are indeed of two different stars; compare the background stars.

Astronomers using the Spitzer Space Telescope have found a couple of brown dwarfs orbiting nearby stars. While brown dwarfs around other stars are seen fairly often, these ones are nifty for several reasons.

One of them, HN Peg B, is interesting because it’s fairly low mass, about 20 times Jupiter’s mass. This makes it what’s called a T dwarf. Those of you who’ve taken Astronomy 101 may remember that stars are classified according to their spectra, which corresponds roughly to their mass and temperature. The hottest, most massive stars are O stars, then, going lower mass and cooler, they are B, A, F, G, K, M. M stars are the lowest mass stars that can fuse hydrogen into helium into their cores. But brown dwarfs are cooler and lower mass. They get added to the end of the list above, as L and T dwarfs. T dwarfs are so cool that they can actually have methane in their atmospheres (in hotter stars, methane breaks down).

HN Peg B orbits a star about 60 light years away, relatively close by as these things go. Since it’s close, getting an image of it is easier (it appears brighter, and farther away from its parent star). The parent star is itself interesting, in that it’s a solar analogue, a star like the Sun. For those keeping track at home, it’s a G0, or slightly more massive and hotter than the Sun. No T dwarf has ever been seen orbiting a star like this. It’s probably common (G stars make up about 10% of all stars, and some are bound to have brown dwarf companions), but T dwarfs are so faint compared to their parent stars that they are really hard to detect. This one can be seen in images because T dwarfs put out most of their light in the infrared, where G stars are relatively dim. That increases the contrast and makes the T dwarf easier to see.

But the very neatest thing about the T dwarf is that it’s young: HN Peg A, as it’s called, is probably only about 300 million years old (this can be found by looking at a number of characteristics, including how fast it spins (younger stars tend to spin faster), how much lithium it has (lithium is destroyed easily in stars, so more lithium means the star is younger), magnetic activity (tied to spin, so young stars have stronger magnetic fields), and other properties).

Assuming that HN Peg B formed together with A, which is likely, that means the brown dwarf is young, too. All other T dwarfs ever seen are much older, like a billion years or more, so seeing one that’s this young is cool. Astronomers can learn more about T dwarfs!

Learning more is always good. Always.

The other T dwarf was found orbiting the star HD 3651 A, which is about 35 light years away. This one is cool because a planet has already been detected orbiting HD 3651 A. It’s a Saturn-mass planet orbiting the star about as far out as Mercury orbits our Sun. The orbit of the planet is known to be highly elliptical, which is unusual for such a close-orbiting planet — tides from the star tend to make the planets’ orbits circular. It’s been thought for a while that a massive object farther out might keep the inner planet orbit elliptical, and this is now confirmed. The T dwarf HD 3651 B orbits the star about 10 times farther out than Pluto orbits the Sun, which is a fair walk, but close enough that it can warp the inner planet’s orbit.

I studied brown dwarfs for a few years, and I’m pretty interested in them. I suppose I’d call myself an educated layman when it comes to these guys (as opposed to real experts like the ones who ran these studies), so I’m not fully up on what’s what. But it’s nice to see the field making so much progress, and as usual I’m pretty eager to find out what will happen next!

CATEGORIZED UNDER: Astronomy, Cool stuff, NASA, Science

Comments (28)

  1. Chip

    Thanks for the interesting info! I wonder if a T dwarf star could exist only 2 or 3 light years from our sun and still go undetected.

  2. One observation , the three prominent spots in this picture seem to form a right angled triangle.

    And the image on the right looks like the left image rotated clockwise about 20 degrees.

    Or am i seeing things !!!!!!

  3. SMEaton

    Sigh. I saw the BA’s original entry a few hours ago and, after his comment about the similarity of the images, considered posting a satiric blurb about how both pics would soon be conspiracy fodder. I did not do so, obviously. Meanwhile, someone has fulfilled my expectation. We are not embiggened. Thanks, Santhosh.

  4. Grand Lunar

    It’s nice to see some good science for a change!

  5. Moose

    Conspiracy fodder? Wha?

    Sincere observation: the images really do look rotated to these layperson’s eyes. I’m not finding _any_ other correspondence between the two images to suggest otherwise.

  6. KingNor

    are brown dwarfs dim enough that you could look directly at one?

  7. Of course, the first thing I think of when I read this post was, “How do we extend the old mnemonic for the spectral classes to include L and T?” You remember, Oh Be A Fine Girl, Kiss Me. . . . Lolita, Ttoday?

  8. KingNor

    Blake: might want to change that to Laura or Leela or something.

  9. Navneeth

    As BA said, take a look at the background stars; and it can be seen, without any actual measurement, that the sides (of the triangle) aren’t really equal. Also, the direction of the black band, which I don’t think is a diffraction spike, is the same in both pictures.

    All said and done, healthy skepticism is always good. :)

  10. Navneeth

    Oh be a fine girl, kiss me later tonight. ;)

  11. A solar-type star with a brown dwarf? What about Nemesis?

  12. Kaptain K

    A solar-type star with a brown dwarf? What about Nemesis?

    If we can find brown dwarfs 35 and 60 light years away, don’t you think we could find one in our own neighborhood (light days or less)?

  13. MaDeR

    Nemezis? Well, orbits of our planets are nearly circullar…

  14. Phil Brady

    I did an off-the-cuff calculation with some dubious statistics, and it looks like any stellar companion to our Sun would need to have an absolute magnitude of 42 or so for us to have not seen it yet….

  15. Irishman

    santhosh maruthi, Moose, the three prominent stars do appear to form nearly right triangles, and the placement and size seem similar but rotated. However, look at the rest of the stars in each pattern and you will see differences. For instance, the top corner star in the right image has a dimmer companion near it that is not present in the left image. There is a very bright star in the right image just over the middle that is not present in the left image. There is a cluster of stars in the lower middle of the right image that are not present in the left image.

    What is more startling to me is the similarity of the flash pattern of the two major star images. The hexagonal shaped spikes are likely an effect of diffraction. Nevertheless, the specific pattern of brightness and the shape of the spiking is eerily similar. Not only the specific hex pattern, but the extra lumpiness at the top left, the contour on the lower right. It looks like you could cut out the bright central core of the right image along those contours and directly overlay them with the ones on the left. The midrange bright outer ring of spikes also appear similar, with some minor variations in shape that are subtle.

    I would be curious to see other star close ups from Spitzer for comparison.

  16. I thought R, N and S came right after M in stellar classification:

    O Be A Fine Girl Kiss Me Right Now Sweetie

    Where does T fit in?

  17. Ah the magic of google has explained away my dated knowledge. The internet is wonderful.

    And to think, googling “stellar classification” seems to return no porn hits. That’s amazing in and of itself.

  18. RAD

    I thought R, N and S came right after M in stellar classification:

    O Be A Fine Girl Kiss Me Right Now Sweetie

    Where does T fit in?

    ….. Thanks? right after sweetie

  19. RAD

    Its probably the word stellar that you might not find in the porn circles

  20. Irishman, the pattern of light from stars will be the same due to telescope optics. Basically, any point source will display that pattern (if it’s bright enough).

  21. So if there is an object with 20 times jupiter’s mass not fusing hydrogen that orbits a star that is fusing hydrogen, and other planets are already in the system. What defines the object as a brown dwarf, and not as a planet- the distance between the dwarf and the other star? After all of the Pluto stuff the lower limit for planets is set, but where is the upper mass limit? I’m surprised you didn’t comment on that.

  22. Tim G

    Bryan, he talked about the upper limit to a planet a couple of weeks ago.

  23. That’s why I’m surprised he didn’t bring it up again. It seems that the astronomers that discovered it are classifying it absolutely as a brown dwarf, and I’m wondering what was behind that decision.

  24. Spectral types R, N, S are rediant or supergiant ones usuallysyyubsumed under the M category.

    There’s also a W class for Wolf-Rayet stars at the opposite end of the spectral class spectrum (try saying that five tuimes quicklyafter drinking afew shots of tequila! ;-) … ) which is sometimes included as type O.

    Maybe the all-encompassing mennonic could be :

    Whoa! Oh Be A Fine Girl Kiss Me Right Now – Sex Later Tonight?! ;-)

    One point BA – I thought G type stars were 4 % of thestellar population not 10 %?

    Got this from a Ken Croswell article on Alpha Centauri in Astronomy magazine (April 1991 issue) which had a pyramid graphically illustrating the breakdown of star type /numbers as follows :

    70 % red dwarfs

    10 % white dwarfs

    15 % orange dwarfs

    4% yellow dwarfs

    1 % F class dwarfs & main-sequence white Sirian A type stars

    and finally less than 1 % all other star types…

    Now this was before the L & T type brown dwarfs were added so its a bit out of date but other than that – is it that wrong? Have we found that many more G-type stars since?? Has that idea been revised? Or did the BA (cough,cough, ahem) get it a trifle wrong? What _are_ the approximate relative percentages?

    BA? Anyone?

  25. That a brown dwarf could have such an influence on the exo-Saturn’s orbit making it eccentric from thatfaraway is indeed pretty amazing.

    So much so that it makes me rather skeptical its actually happenening. Granted the brown dwarf is quite massive with a hefty gravitational tug but from my undestanding an object at Pluto’s distance affecting something at Mercury’s so strongly just seems highly unlikely unless its a black hole or neutron star which it clearly ain’t!

    Have they ruled out other closer-in planets or possible reasons for the exo-Saturn’s orbit? Are they sure?

  26. To answer some questions that have come up before, a brown dwarf in solar orbit at the hypothesized Nemesis distance wouldn’t need to be QUITE that dim (M ~ 42) to have gone undetected. A couple of years ago a fairly detailed study came out analyzing the limits of the telescopes that would likely have found such an object by now, and all that does is put a mass limit of about 40 Jupiter masses on it, although I’m not quite sure what magnitude that corresponds to at solar age (in all likelihood it would be a T dwarf, I know people who have conducted studies ruling out most likelihoods of finding an L dwarf Nemesis), but I doubt its anywhere near 42 (probably late teens).

    The reason we might not have detected Nemesis when we can find all these other brown dwarfs is twofold. First, we’re still nowhere near finding all the T dwarfs that are right in front of our eyes. Brown dwarfs are usually found by combing through data from all-sky surveys, but T dwarfs are very faint and their colors make them look like other objects (galaxies, stars, etc.), so finding them is very much “diving into the muck.” The other problem is that Nemesis, given its proximity and its orbit around the Sun, would be moving so quickly that it could feasibly have avoided detection in the all-sky surveys.

    Oh, and try Oh, Be A Fine Girl, Kiss Me Lovingly Tonight :P


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