Is this the oldest known brown dwarf?

I’ve written about Justin Crepp’s TRENDS survey before:  Justin is looking at stars that exhibit radial velocity “trends” in our planet search programs at Lick and Keck.  The stars show acceleration from distant orbiting companions.  When we see around 10 years of constant acceleration, we can be pretty sure that the unseen companion is a star or brown dwarf.  Justin uses adaptive optics to image these companions and see what they are.

These companions are useful because we think they have the same age, distance, and composition as their companions.  It is often easier to find the age of a bright G dwarf than of a late type star or degenerate object, so these fainter companions can serve as benchmarks for similar star found alone.

I wrote about his discovery of a benchmark M dwarf here, and a couple more from his follow up paper here.
I wrote about his discovery of a white dwarf companion here, which is cool because its colors are similar to a brown dwarf.
 
Now Justin has just published this discovery of a brown dwarf companion to a metal poor star
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This is especially cool because brown dwarfs are very hard to pin down in terms of age, mass, and composition.  If you see one in the field, these parameters are all degenerate so it is difficult to know anything about it.  In a benchmark system like this, we can determine all of those parameters from the host star, HD 19467, making this a touchstone for similar objects in the field.

We don’t yet know the brown dwarf’s mass, but the fact that we can measure its position and acceleration on its companion means that it is just a matter of time before we can get that.

The age is tricky, too, but we have an interesting constraint:  the primary star is metal poor, but despite that it sits above the Main Sequence.  Metal-poor stars usually sit below the Main Sequence (so, hotter and dimmer than stars of the same composition of the Sun), so the fact that it is a quarter-magnitude above means that it must be near the end of its hydrogen-burning life, and beginning its transition to the subgiant branch towards giant-hood. 

Although we don’t have a firm age yet, this means the star is probably 8-10 billion years old.

This means that the brown dwarf, too, is 8-10 billion years old, making it, as far as I can tell, the oldest brown dwarf for which we have a good handle on the age (that is, older brown dwarfs might have been discovered, but since we don’t know their ages we can’t be sure that they’re older).

Such an old object provides a lot of leverage for studies of how brown dwarfs cool and should but tough constraints on models of these fascinating objects.

You can find Justin’s latest on the arXiv here.

5 thoughts on “Is this the oldest known brown dwarf?

  1. Justin

    We are going to measure the radius of HD 19467 A directly using interferometry which should place a very strong constraint on the system age.

    -Justin

  2. Robert Filter

    Wow, really astonishing that such old objects can be observed. It would be interesting to know if one could infer any change of the laws of physics during that time closely examining the brown dwarf (in the sense of Smolin).

  3. KEVIN LUHMAN

    Crepp et al. present 2 discrepant ages, and adopt the younger one. So it’s unclear to me how the paper can be cited as presenting an object with an age that is either well-defined or old.

    “Free-floating BDs have neither independently measured masses nor ages”: not all brown dwarfs are single and free-floating. Some known brown dwarfs are in binaries (with stars or others BDs) and thus have masses and ages.

  4. Jason T Wright

    Hi, Kevin. I agree it’s not very metal poor. My argument isn’t that “metal-poor implies old”, is that since it’s both slightly metal poor AND above the MS it must be slightly evolved, and therefore old.

    The paper’s age of 4.3 Gyr is based on activity/gyrochronology, but clearly that age cannot be right because if its spot on the CMD (as the paper says). The isochrone fits don’t converge, but best guess is 9+/-1 Gyr.

    I think that age, if we can pin it down, is interesting because we’ll get small error bars on it along with a mass. Free-floating BDs have neither independently measured masses nor ages, so can’t really serve as benchmarks, even if we have very good reason to suspect that they are halo/thick-disk objects, and therefore very likely to be older.

  5. KEVIN LUHMAN

    [Fe/H]=-0.15 is only slightly metal poor. See Figure 9 from Nordstrom 2004.

    “the star is probably 8-10 billion years old”: But that’s not the age adopted in the paper in question. It adopted 4.3 Gyr instead.

    There are a number of known brown dwarfs that are much more metal poor than this object, both companions to stars with measured metallicities and free-floating subdwarfs. This object is not especially interesting from the standpoint of old/metal-poor brown dwarfs.

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