Widest known BD binary in the field - Resolved in Legacy images!


Oh that’s cool. I have two questions: what is a “Bd” binary and why are they so red?

BD stands for Brown Dwarf. Basically a ball of gas like Jupiter, but much more massive. Not massive enough to sustain hydrogen fusion in their cores, but they do burn deuterium. They’re red as they’re quite cool (< 2000 K), so their flux peaks at longer wavelengths (they’re generally brightest in the infrared). They also tend to be quite dusty/cloudy, which also shifts their emergent flux to longer wavelengths.

Quite cool….2000K…I worry about the temperature you keep your house at Tom.

Oh wow okay thank you. If I remember correctly Anton Petrov was saying that they become Brown Dwarfs at 54 times the mass of jupiter, but I am looking it up and 13 to 81 times the mass of Jupiter seems to be what wikipedia says. Do you know the name of this wide star system?

I think 13 jupiter masses is generally accepted to be the deuterium burning limit. It’s a bit up in the air still, where planets end and brown dwarfs begin. For example, the paper i’m currently writing is about some extremely red, young (< 100 Myr) L dwarf candidates. Young L dwarfs can be spectroscopically very similar to young, giant exoplanets and at very young ages, L dwarfs can even be below 13 Jupiter masses. Some of our estimates for <15 Myr L dwarfs even have lower bounds of 4-5 Mjup.

This system is called CWISE J014611.20-050850.0AB. It’s discovery was announced in a paper by a very talented undergrad at the time, Emma Softich. You can read the paper here https://arxiv.org/pdf/2202.02315.pdf

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That’s so cool you are writing a paper on such a mysterious and fascinating subject. I have so many questions still, but alas this may not be the time or place. So these must be something that James Webb will investigate.

🫣Well I hate to break it to you but deuterium also happens to be one of the main sources of fuel for covenant warships, :space_invader: and at three times the distance of pluto to the sun, I suspect this red-brown pair has been maintaining their orbit with onboard pinch reactors. We may need to send in a Pillar of Autumn… :smirk:

But yeah so many questions still, if you’re up for answering them!

Of course, i’m always happy to help people learn about brown dwarfs! Fire away!

Okay well you asked for it! Many of these questions will be basic.

The first thing I was wondering is about this deuterium burning limit. Why does 13 jupiter masses allow the fusion of deuterium?

My second question: so you are writing about a young brown dwarf, how do you measure the age of the brown dwarf? Is it a metallicity thing or do you take it from a group of stars that you know the ages of, like a group in a globular cluster, or a region of a galaxy, or do you do something else? I’m sorry I don’t really know how we age stars.

My next question is from a point you made that the young ones can burn deuterium at even smaller, even at only 4-5 masses of jupiter, which is confusing to me why would their age matter? As these objects get more massive, I’ve read they get more dense instead of disperse, usually things disperse as they get more massive due to Eddingington law, why not these? Maybe they do at a rate much less than hydrogen burning stats? Their tidal forces must be pretty intense on their moons, causing extreme vulcanism?

Do you know anything about their magnetic fields and how their magnetic fields compare to their gravitational field?

What is your favorite thing that you’ve learned about them recently? Is there a question that I should be asking?

I was thinking about them and how they radiate lots of their energy in the infrared, which isn’t as ionizing. Would this make them good, stable lightbulbs for light on exomoons, or would they not radiate enough temperature to heat up one of their moons? I could be thinking about this way off beat…

Sorry for all these questions, and if they are boring or basic my bad!!! I’ll pay you in sloan points sometime, and btw

if you want to sideline my questions, no worries, just a little paragraph about why they’re cool to you would probably help a lot more than anything I ask hahaha

Brown dwarfs are basically giant balls of contained gas. They are around the same radius of jupiter, but have >13 times more mass packed in to that area. As a result the pressure in their core is quite high. As the pressure of contained gas rises, so does its temperature. At ~13 jupiter masses, the pressure/heat is enough to trigger deuterium burning (a type of nuclear reaction that requires much less energy than the normal hydogen fusion we see in stars).

So there are definitely evolutionary models that can give you an idea of the age of a BD, and generally we can look for low surface gravity features in their spectra. But i’m using a specific method. L dwarfs tend to start their lives with very red near- and mid-infrared colors (as they haven’t fully contracted to their final equilibrium radius and so have low surface gravities, with vert dusty/cloudy atmospheres). Thats how I got my initial candidate list - a CatWISE/VHS crossmatch for red colors.
The method i’m using for telling the age is to try to link them to known young moving groups/stellar associations. These are handy as all the members have consistent ages and metallicities as they formed together. So if we can tie a BD to a young moving group, we can infer an age that way.

Oh, when I mentioned age, I was referring specifically to L dwarfs, as they have to be super young to be that low mass.
By disperse, I assume you mean increase in radius? Brown dwarfs are like white dwarfs in the sense that they are both governed fairly heavily by electron degeneracy pressure. I think that probably destroys the normal relationship between mass & radius.

They have some pretty weird magnetic fields. We’ve observed xray emission from BDs so we know their magnetic fields can be pretty strong. They can do some funky stuff. Brown dwarfs have even been observed to have auroras around their magnetic poles. Give this a quick read https://www.newscientist.com/article/2147636-brown-dwarfs-have-strong-magnetic-fields-just-like-real-stars/

Oh there’s so much cool stuff about BDs. The coolest (or weirdest i guess) thing I learned about them recently is that the noticable and statistically significant majority of them seem to have proper motion going south. We noticed this quite recently and we have no idea why!

The more massive ones could probably heat/light up a moon fairly easily. Depends on the orbital separation I guess. I can’t say i’m too familiar with exomoons around BDs so I would be speculating if I was to comment lol.

No problem! Brown dwarfs are a rather niche topic in astronomy so i’m always happy when someone takes an interest. Especially considering they’re so useful to multiple fields within astronomy. They’re useful for exoplanet people as at low mass and age, they can be extremely spectroscopically similar to young, giant exoplanets, but they don’t have to compete with the light from a host star, so they’re nice testbeds for planetary science theory. They’re also useful to stellar astronomy as they provide constraints on the lowest mass needed to be a “star”.

I find them endlessly fascinating.

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You should check out the unWISE catalog from my awesome collaborators! https://catalog.unwise.me/

Great questions and answers gents- really interesting information!

Well you’re telling me they migrate South for the winter?? That’s weird whats up with that! Flock of Brown Dwarfs, changing magnetic fields, en route to the South Pole, how rude bringing Iron rain!


Well thats really cool. I’m on my thumbs right now on a roadtrip, and actually am hoping to check out the Dragonfly telescope in New Mexico, has anyone heard anything about it, but so thanks for all this info. My deuterium question was more like is it density of cross sections of reaction space or velocity of deuterium that is the reason for that if you know.

And I was wondering how you got to study L dwarfs in the first place?