NOAO/SOAR: Where do stars end and brown dwarfs begin?

[MargaritaMc]

http://www.noao.edu/news/2013/pr1311.php

December 9, 2013
RELEASE NO: NOAO 13-11

NOAO/SOAR: Where do stars end and brown dwarfs begin?

Stars come in a tremendous size range, from many tens of times bigger than the Sun to a tiny fraction of its size. But the answer to just how small an astronomical body can be, and still be a star, has never been known. What is known is that objects below this limit are unable to ignite and sustain hydrogen fusion in their cores: these objects are referred to as brown dwarfs.

In research accepted for publication in the Astronomical Journal, the RECONS (Research Consortium On Nearby Stars) group from Georgia State University has found clear observational evidence for the theoretically predicted break between very low mass stars and brown dwarfs. The data came from the SOAR (SOuthern Astrophysical Research) 4.1-m telescope and the SMARTS (Small and Moderate Aperture Research Telescope System) 0.9-m telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile.

For most of their lives, stars obey a relationship referred to as the main sequence, a relation between luminosity and temperature – which is also a relationship between luminosity and radius. Stars behave like balloons in the sense that adding material to the star causes its radius to increase: in a star the material is the element hydrogen, rather than air which is added to a balloon. Brown dwarfs, on the other hand, are described by different physical laws (referred to as electron degeneracy pressure) than stars and have the opposite behavior. The inner layers of a brown dwarf work much like a spring mattress: adding additional weight on them causes them to shrink. Therefore brown dwarfs actually decrease in size with increasing mass.

As Dr. Sergio Dieterich, the lead author, explained, “In order to distinguish stars from brown dwarfs we measured the light from each object thought to lie close to the stellar/brown dwarf boundary. We also carefully measured the distances to each object. We could then calculate their temperatures and radii using basic physical laws, and found the location of the smallest objects we observed (see the attached illustration, based on a figure in the publication). We see that radius decreases with decreasing temperature, as expected for stars, until we reach a temperature of about 2100K. There we see a gap with no objects, and then the radius starts to increase with decreasing temperature, as we expect for brown dwarfs. “

Dr. Todd Henry, another author, said: “We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’ and we can identify a particular star (with the designation 2MASS J0513-1403) as a representative of the smallest stars.”

Aside from answering a fundamental question in stellar astrophysics about the cool end of the main sequence, the discovery has significant implications in the search for life in the universe. Because brown dwarfs cool on a time scale of only millions of years, planets around brown dwarfs are poor candidates for habitability, whereas very low mass stars provide constant warmth and a low ultraviolet radiation environment for billions of years. Knowing the temperature where the stars end and the brown dwarfs begin should help astronomers decide which objects are candidates for hosting habitable planets.

Also, because brown dwarfs cool forever, they eventually become a type of macroscopic dark matter, so it is important to know how much dark matter is trapped in the form of extremely old and cold brown dwarfs.

The research highlights the capabilities of the National Optical Astronomy Observatory system in a single project. The SOAR observations provided the missing link to a wealth of data that had previously been obtained using telescopes under the auspices of NOAO. As Dieterich explains: “We used the SOAR 4.1-m telescope to measure the visible light of faint stars and brown dwarfs, and the CTIO 0.9-m telescope to obtain precise measurements of their distances. We then combined these measurements with infrared data taken at the CTIO 1.3-m telescope and the WISE space telescope. Three out of four of these telescopes are public telescopes located at CTIO, and the fourth explores wavelengths that are only accessible from space.”

CTIO is a division of the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy Inc. (AURA) under a cooperative agreement with the National Science Foundation.

###

http://arxiv.org/abs/1312.1736

[Ann]

This is beginning to look like old news now, but I have been thinking about it, and it is really so interesting. In particular,the relation between mass and radius in planets and brown dwarfs is very interesting.

http://en.wikipedia.org/wiki/Brown_dwar ... ss_planets wrote:

A remarkable property of brown dwarfs is that they are all roughly the same radius as Jupiter.

At the high end of their mass range (60–90 Jupiter masses), the volume of a brown dwarf is governed primarily by electron-degeneracy pressure,[11] as it is in white dwarfs; at the low end of the range (10 Jupiter masses), their volume is governed primarily by Coulomb pressure, as it is in planets. The net result is that the radii of brown dwarfs vary by only 10–15% over the range of possible masses. This can make distinguishing them from planets difficult.

Gas giants have some of the characteristics of brown dwarfs. For example, Jupiter and Saturn are both made primarily of hydrogen and helium, like the Sun.

Saturn is nearly as large as Jupiter, despite having only 30% the mass.

Isn't it remarkable? Jupiter is a sort of sub-stellar/brown dwarf/gas giant standard when it comes to radius and volume. However, Jupiter appears to be larger than the smallest stars. I googled the mean radius of Jupiter (69,911±6 km), and the radius of the Sun (695,500 km), and even though the uncertainty seems to much greater when it comes to the radius of the Sun than when it comes to the radius of Jupiter, I nevertheless used these two figures to conclude that the raidus of Jupiter appears to be pretty much 10% the radius of the Sun. However,

http://www.noao.edu/news/2013/pr1311.php wrote:

Dr. Todd Henry, another author, said: “We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’ and we can identify a particular star (with the designation 2MASS J0513-1403) as a representative of the smallest stars.”

So the smallest hydrogen-fusing star may have a radius 8.7% that of the Sun, but Jupiter may have a radius 10% that of the Sun. Then again, this is without a very reasonable margin of error.

It might be more correct to say that Jupiter is, for all intents and purposes, the same size as the very smallest M-type stars.

This makes me wonder how large a gas giant planet can be. Several massive gas giants have been found among exoplanets. I remember having read about hot Jupiters whose atmospheres have been puffed up from the intense radiation of their suns, so that the planets were very large. But when it comes to gas giants at greater distances from their suns, how large can they be? Is Jupiter once again a sort of gold standard of the size of small stars, brown dwarfs and gas giant planets?

Ann

[BDanielMayfield]

That is a significant discovery Margarita, as finding the extremes of any phenomena is very important in science. Now, it’s safe to say that if an object on the red end of the HR chart is cooler than about 2075 K, it can’t be a star.

This paper was also discussed in this Sky & Telescope newsblog:
http://www.skyandtelescope.com/news/New ... 15451.html

The above article is interesting and also the comments, for the lead author of the paper, Sergio Dieterich, answers several questions, including some of mine. It’s always nice when professional astronomers answer the questions of us avid amateurs.

Bruce

[MargaritaMc]

Thank you so much for the link, Bruce. How enormously kind and helpful of Dr Dieterich to answer the queries so thoroughly.
Margarita

[BDanielMayfield]

It certainly was kind of him Margarita. Many scientists are teachers at heart I think, moved by a desire to share knowledge with others.

Findings like this one always seem to generate more questions though, don’t they? (Like Ann’s re the difference between brown dwarfs and planets.) Continuing to ask questions helps keep our minds limber. But you don’t want to go too far, becoming like a kid incessantly asking “But why?” or coming off like some kind of a groupie. But if you ask a good question and the researcher sees it, you might be delighted by an answer from the paper’s author him or herself.

Bruce

[BDanielMayfield]

In the above mentioned S&T newsblog Dr. Dieterich informed us (in his answer to our fellow forum member Anthony Barreiro) that there was a typo in this part of the press release:

Dr. Todd Henry, another author, said: “We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’ and we can identify a particular star (with the designation 2MASS J0513-1403) as a representative of the smallest stars.”

The identity of the smallest known star is 2MASS 0523-1403, not 0513-1403, for any who might wish to try photographing it.
But that might be tough since it is less than mag 21.

The Wikipedia article on this star says this:

2MASS J0523-1403 is a very low mass red dwarf star about 40 light years from Earth in the southern constellation of Lepus. With a very faint visual magnitude of 21.05 and a low effective temperature of 2074K it is visible primarily in large telescopes sensitive to infrared light. 2MASS J0523-1403 was first observed as part of the Two Micron All-Sky Survey

2MASS J0523-1403 has a luminosity of 0.000126L☉, a radius of 0.086 R☉ and an effective temperature of 2074K. These values are currently the lowest known for a main sequence star.[2] It has a stellar classification of L2.5 and a V-K color index of 9.42.[2] Observation with the Hubble Space Telescope has detected no companion beyond 0.15 arcsecond.[7] Sporadic radio emissions were detected by the VLA in 2004.[8] H-alpha (Hα) emissions have also been detected, a sign of chromospheric activity.[5]

Members of the RECONS group have recently identified 2MASS J0523-1403 as representative of the smallest possible stars.[9] Its small radius is at local minimums of the radius-luminosity and radius-temperature trends.[2] This local minimum is predicted to occur at the hydrogen burning limit due to differences in the radius-mass relationships of stars and brown dwarfs. Unlike stars, brown dwarfs decrease in radius as mass increases due to their cores being supported by degeneracy pressure. As the mass increases an increasing fraction of the brown dwarf is degenerate causing the radius to shrink as mass increases.

[Beyond]

Wait a minute, Bruce, isn't that Nitpicker's job

[BDanielMayfield]

Wait a minute, Bruce, isn't that Nitpicker's job

He probally would have found this nit in time. I certainly wouldn’t have found it if the good Dr. hadn’t pointed it out.

[Nitpicker]

Wait a minute, Bruce, isn't that Nitpicker's job

He probally would have found this nit in time. I certainly wouldn’t have found it if the good Dr. hadn’t pointed it out.

I probably wouldn't have. I've been enjoying the Summer Sun instead.

[BDanielMayfield]

I've been enjoying the Summer Sun instead.

While we freeze our ____s off! Careful, or you'll cause a massive outbreak of HLE.

[BDanielMayfield]

As a preview to something I’d like to ask about this finding, here is part of the S&T blog discussion mentioned previously:

My query:

Important Stellar Limit
Posted by Bruce Mayfield December 29, 2013 At 10:21 AM PST
Thanks for answering these questions directly Sergio Dieterich, and for the work you and your fellow professionals are reporting in this paper. Finding the limits of natural phenomena is always important, and this work will no doubt help astronomers improve stellar modeling. I enjoyed reading your paper. But no doubt like Anthony, who wanted to see the littlest star for himself, I was a bit disappointed to read that 2MA 0523-1403 is so faint. I was hoping that this star would be within the range of the Gaia mission, so that its characteristics could be nailed down even tighter. Also, I was wondering, since in general the lighter the stellar class, the more examples exist, can we expect that this trend continues all the way down to this bottom y’all have discovered? Will there be many more stars like this one?

And Dr. Dieterich’s reply:

Faintness, GAIA,and luminosity fucntion
Posted by Sergio Dieterich December 29, 2013 At 04:09 PM PST
Bruce Mayfield, Because the magnitude scale is logarithmic, sometimes we forget just how faint things can get. Increasing the magnitude by 1.0 is equivalent to making an object 2.5 times fainter, and every subsequent increase of 1 unit makes THE PREVIOUS VALUE 2.5 times fainter. It is generally accepted that under dark clear skies the human eye can just reach 6th magnitude. 2MA0523 is 15 magnitudes fainter than that. Doing the math, that means about ONE MILLION times fainter than what can be seen by the naked eye. You are right that GAIA will not reach these faint objects (the GAIA cutoff is roughly 20th magnitude), but there is really nothing that GAIA can do that cannot also be done from the ground for individual stars. GAIA wins in the fact that it can do billions of stars, and not in detailed studies of a single star. For instance, we were able to get a lot of color information necessary for determining temperature from the ground. That is something GAIA would not be able to do even if the target was brighter. Right now the consensus is that the trend of more stars with fainter luminosities peaks at slightly more massive stars, and that there is a rather sharp fall-off after that. So while we expect to find a few more nearby stars like 2ma0523, they are certainly not a lot of them. We know form the WISE survey that stars appear to outnumber brown dwarfs by about 6 to 1, and it could be that a similar ratio is also representative for very low mass stars. We don't really know the shape of the drop-off yet. Several groups, including ours, are working on it.

I shared this here because Dr. Dieterich’s answer was so informative and because I wanted to ask about the drop-off in the numbers of these smallest stars and brown dwarfs that he mentioned. I hadn’t learned of this drop-off that was revealed by the WISE mission before reading his comment, but I’m sure long-time members of this forum must be aware of this.

As a general rule, in just about any class of things, smaller objects normally outnumber the larger. Departures from generalities are interesting. Does anyone have any ideas why this doesn’t seem to be the case with these smallest red dwarf stars and sub-stellar brown dwarfs?

I can think of practical reasons, but no physical reasons why this dearth of dwarfs occurs in nature. Any thoughts as to why this is so?

Bruce

[geckzilla]

It's probably something to do with the blurring of the line between a gas giant planet and a star at that point. Gas giants probably greatly outnumber stars but there is only a thin sliver of those objects which are just barely stars.

[BDanielMayfield]

It's probably something to do with the blurring of the line between a gas giant planet and a star at that point. Gas giants probably greatly outnumber stars but there is only a thin sliver of those objects which are just barely stars.

Well for clarity, consider the Wikipedia definition of what a brown dwarf is:

Brown dwarfs are substellar objects too low in mass to sustain hydrogen-1 fusion reactions in their cores, unlike main-sequence stars, which can. They occupy the mass range between the heaviest gas giants and the lightest stars, with an upper limit around 75[1] to 80 Jupiter masses (MJ). Brown dwarfs heavier than about 13 MJ are thought to fuse deuterium and those above ~65 MJ, fuse lithium as well.[2]
However, for some years now there has been debate concerning what criterion to use for defining the separation between a very-low-mass brown dwarf and a giant planet (~13 Jupiter masses).[3] One school of thought is based on formation, and another on interior physics.[3]
Dwarfs are categorized by spectral classification, with the major types being M, L, T, and Y.[3] Despite their name, brown dwarfs are different colours.[3] Many brown dwarfs would likely appear magenta to the human eye according to A. J. Burgasser,[3] whereas another source has noted orange/red.[4] The term brown dwarf was not chosen to indicate their colour.[3]
Another debate is whether brown dwarfs should have experienced fusion at some point in their history. Some planets are known to orbit brown dwarfs: 2M1207b, MOA-2007-BLG-192Lb, and 2MASS J044144b. Brown dwarfs may have fully convective surfaces and interiors, with no chemical differentiation by depth.[5]

Additionally, the wiki article about Brown Dwarfs adds this about distinguishing RD stars from BDs:

Distinguishing high-mass brown dwarfs from low-mass stars[edit]
• Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which achieve the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of lithium-7 and a proton producing two helium-4 nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is depleted. Therefore, the presence of the lithium line in a candidate brown dwarf's spectrum is a strong indicator that it is indeed substellar. The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test, and was pioneered by Rafael Rebolo, Eduardo Martín and Antonio Magazzu. However, lithium is also seen in very young stars, which have not yet had enough time to burn it all. Heavier stars, like the Sun, can retain lithium in their outer atmospheres, which never get hot enough for lithium depletion, but those are distinguishable from brown dwarfs by their size. Contrariwise, brown dwarfs at the high end of their mass range can be hot enough to deplete their lithium when they are young. Dwarfs of mass greater than 65 Jupiter masses can burn off their lithium by the time they are half a billion years old,[10] thus this test is not perfect.
• Unlike stars, older brown dwarfs are sometimes cool enough that, over very long periods of time, their atmospheres can gather observable quantities of methane. Dwarfs confirmed in this fashion include Gliese 229B.
• Main-sequence stars cool, but eventually reach a minimum bolometric luminosity that they can sustain through steady fusion. This varies from star to star, but is generally at least 0.01% that of the Sun.[citation needed] Brown dwarfs cool and darken steadily over their lifetimes: sufficiently old brown dwarfs will be too faint to be detectable.
• Iron rain as part of atmospheric convection processes is possible only in brown dwarfs, and not in small stars. The spectroscopy research into iron rain is still ongoing—and not all brown dwarfs will always have this atmospheric anomaly.

[BDanielMayfield]

And this about the BD/Planet boundary:

Distinguishing low-mass brown dwarfs from high-mass planets[edit]

A remarkable property of brown dwarfs is that they are all roughly the same radius as Jupiter. At the high end of their mass range (60–90 Jupiter masses), the volume of a brown dwarf is governed primarily by electron-degeneracy pressure,[11] as it is in white dwarfs; at the low end of the range (10 Jupiter masses), their volume is governed primarily by Coulomb pressure, as it is in planets. The net result is that the radii of brown dwarfs vary by only 10–15% over the range of possible masses. This can make distinguishing them from planets difficult.

In addition, many brown dwarfs undergo no fusion; those at the low end of the mass range (under 13 Jupiter masses) are never hot enough to fuse even deuterium, and even those at the high end of the mass range (over 60 Jupiter masses) cool quickly enough that they no longer undergo fusion after a period of time on the order of 10 million years. However, there are ways to distinguish brown dwarfs from planets:

X-ray and infrared spectra are telltale signs. Some brown dwarfs emit X-rays; and all "warm" dwarfs continue to glow tellingly in the red and infrared spectra until they cool to planetlike temperatures (under 1000 K).

Gas giants have some of the characteristics of brown dwarfs. For example, Jupiter and Saturn are both made primarily of hydrogen and helium, like the Sun. Saturn is nearly as large as Jupiter, despite having only 30% the mass. Three of the giant planets in the Solar System (Jupiter, Saturn, and Neptune) emit much more heat than they receive from the Sun.[12] And all four giant planets have their own "planetary systems"—their moons. Brown dwarfs form independently, like stars, but lack sufficient mass to "ignite" as stars do. Like all stars, they can occur singly or in close proximity to other stars. Some orbit stars and can, like planets, have eccentric orbits.

Currently, the International Astronomical Union considers an object with a mass above the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) to be a brown dwarf, whereas an object under that mass (and orbiting a star or stellar remnant) is considered a planet.[13]

The 13 Jupiter-mass cutoff is a rule of thumb rather than something of precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 Jupiter mass value is somewhere in between. The amount of deuterium burnt also depends to some extent on the composition of the object, specifically on the amount of helium and deuterium present and on the fraction of heavier elements, which determines the atmospheric opacity and thus the radiative cooling rate.[14]

The Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter masses, and the Exoplanet Data Explorer up to 24 Jupiter masses. Objects below 13 Jupiter-mass are sometimes studied under the label "sub-brown dwarf".

[geckzilla]

However you choose to define brown dwarfs that definition will still occupy thin slice of, say, the area under a parabolic curve. So it doesn't take much for the rest of the things filling the area under that curve to outnumber that slice. Obviously that's an extremely simplistic example but what you are saying is drawing the line at one point and then counting everything to the right and saying it should outnumber the things to the left, which is true. But brown dwarfs should represent just a slice. I probably need to draw a picture... I'm pretty awful at explaining this.

[neufer]

[b][color=#0000FF]A direct image of a brown dwarf companion (arrowed) taken at the Keck Observatory. (Credit: Crepp et al. 2014 APJ).[/color][/b]

---

Nearby Brown Dwarf Captured in a Direct Image
by David Dickinson, Universe Today, January 24, 2014

<<A recent find announced by astronomers may go a long ways towards understanding a crucial “missing link” between planets and stars. The team, led by Friemann Assistant Professor of Physics at the University of Notre Dame’s Justin R. Crepp, recently released an image of a brown dwarf companion to a star 98 light years or 30 parsecs distant. This discovery marks the first time that a T-dwarf orbiting a Sun-like star with known radial velocity acceleration measurement has been directly imaged.

Located in the constellation Eridanus, the object weighs in at about 52 Jupiter masses, and orbits a 0.95 Sol mass star 51 Astronomical Units (AUs) distant once every 320-1900 years. Note that this wide discrepancy stems from the fact that even though we’ve been following the object for some 17 years since 1996, we’ve yet to ascertain whether we’ve caught it near apastron or periastron yet: we just haven’t been watching it long enough. The T-dwarf, known as HD 19467 B, may become a benchmark in the study of sub-stellar mass objects that span the often murky bridge between true stars shining via nuclear fusion and ordinary high mass planets.

Brown dwarfs are classified as spectral classes M, L, T, and Y and are generally quoted as having a mass of 13 to 80 Jupiters, and utilize the first steps in the proton-proton chain reaction of nuclear fusion to fuse two hydrogen nuclei into deuterium. Low mass red dwarf stars have a mass range of 80 to 628 Jupiters or 0.75% to 60% the mass of our Sun. The Sun has just over 1,000 times Jupiter’s mass.

Researchers used data from the TaRgeting bENchmark-objects with Doppler Spectroscopy (TRENDS) high-contrast imaging survey, and backed it up with more precise measurements courtesy of the Keck observatory’s High-Resolution Echelle Spectrometer or HIRES instrument. TRENDS uses adaptive optics, which relies on precise flexing the telescope mirror several thousands of times a second to compensate for the blurring effects of the atmosphere. Brown dwarfs shine mainly in the infrared, and objects such as HD 19467 B are hard to discern due to their close proximity to their host star. In this particular instance, for example, HD 19467 B was over 10,000 times fainter than its primary star, and located only a little over an arc second away. “This object is old and cold and will ultimately garner much attention as one of the most well-studied and scrutinized brown dwarfs detected to date,” Crepp said in a recent Keck observatory press release. “With continued follow-up observations, we can use it as a laboratory to test theoretical atmospheric models. Eventually we want to directly image and acquire the spectrum of Earth-like planets. Then, from the spectrum, we should be able to tell what the planet is made of, what its mass is, radius, age, etc… basically all of its relevant properties.

Another key player in the discovery was the Near-Infrared Camera (second generation) or NIRC2. This camera works in concert with the adaptive optics system on the Keck II telescope to achieve images in the near infrared with a better resolution than Hubble at optical wavelengths, perfect for brown dwarf hunting. NIRC2 is most well known for its analysis of stellar regions near the supermassive black hole at the core of our galaxy, and has obtained some outstanding images of objects in our solar system as well.

What is the significance of the find? Free floating “rogue” brown dwarfs have been directly imaged before, such as the pair named WISE J104915.57-531906 which are 6.5 light years distant and were spotted last year. A lone 6.5 Jupiter mass exoplanet PSO J318.5-22 was also found last year by the PanSTARRS survey searching for brown dwarfs. “This is the first directly imaged T-dwarf (very cold brown dwarf) for which we have dynamical information independent of its brightness and spectrum,” team lead researcher Justin Crepp told Universe Today.
Analysis of brown dwarfs is significant to exoplanet science as well. “They serve as an essential link between our understanding of stars and planets,” Mr. Crepp said. “The colder, the better.”

And just as there has been a controversy over the past decade concerning “planethood” at the low end of the mass scale, we could easily see the debate applied to the higher end range, as objects are discovered that blur the line… perhaps, by the 23rd century, we’ll finally have a Star Trek-esque classifications scheme in place so that we can make statements such as “Captain, we’ve entered orbit around an M-class planet…”

Something that’s always been fascinating in terms of red and brown dwarf stars is also the possibility that a solitary brown dwarf closer to our solar system than Alpha Centauri could have thus far escaped detection. And no, Nibiru conspiracy theorists need not apply. Mr. Crepp notes that while possible, such an object is unlikely to have escaped detection by infrared surveys such as WISE. But what a discovery that’d be!>>