What questions am I trying to answer?

The Big Questions

Are we Alone in the Universe?

The great science fiction writer Arthur C. Clarke once said "Two possibilities exist: either we are alone in the universe or we are not. Both are equally terrifying." Think about that for a moment. We do not know the answer to this question yet, but once we do know we will have to re-define what it means to be human and what it means to be alive on planet Earth. That is true regardless of whether or not we consider intelligent or unintelligent extraterrestrial life. Is life so incredibly unique and precious so as to be extremely rare or is the Galaxy teeming with life, which would make us unique in an entirely different way?

When I meet a new scientist I often ask "what is the coolest thing about your field of research?" When they inevitably ask me the question back I answer that we are very close to understanding whether our Galaxy is mostly capable of supporting other life or not. Over the last two decades we learned that planets around other stars are common. Fantastic advancements in observing technology such as NASA's soon to be launched Jame Web Space Telescope will allow us to examine these planets for the presence of atmospheric oxygen, methane, and other gasses that are indicative of life. Even if we do not find direct evidence of extraterrestrial life, we will at least know if life is a real possibility given the properties of extra-solar planets and their host stars. I do not work directly in the field of exobiology (extraterrestrial life) or even directly with exoplanets, but as I discuss in the other questions understanding stars is key to understanding the conditions in any planets that may be orbiting them.

Are There Habitable Worlds Around Low Mass Stars in the Solar Neighborhood?

We now know that planets around low mass stars are plentiful, with some estimates suggesting an average of at least two planets per star. What's more, low mass stars seem to preferentially harbor rocky planets much like Earth. Are the conditions on those planets right for life? That is a very complex question and one that we are only now beginning to be able to address. Many factors are involved, such as the planet's distance from the host star, the strength and shape of the star's magnetic field, the star's overall stability, the shape and strength of the planet's magnetic field, the planet's ability to hold on to an atmosphere, among many others. Due to their proximity to us and ease of observation star systems in the solar neighborhood provide our best chance of answering these questions. I study this problem from the point of view of trying to understand the stars, while others do the equally important job of trying to understand the planets themselves.

What Are the Properties of the Smallest Possible Stars and Where is the Line Separating Them From the Substellar Brown Dwarfs?

Answering this question was the topic of my doctoral thesis and continues to be a very active area of research for me. Stars form when interstellar gas and dust collapse under their own gravity to form dense cores. These cores eventually become so hot that nuclear fusion ignites, transforming hydrogen into helium and powering the star. But what happens if the resulting core is not hot enough to ignite fusion and if there is no more interstellar material to fall upon it and make it hotter? Then a brown dwarf is born. Brown dwarfs may shine just like stars in their youth, but because they lack an internal energy source they eventually cool off and become extremely faint. Figuring out the minimal conditions for stellar ignition is a question of intrinsic astronomical importance if we are to understand the stellar population of our Galaxy. Remember, most stars are small stars and fall somewhat close to this limit. In 2014 I published a paper that provided evidence that this limit happens at hotter temperatures and brighter luminosities than theory predicted. Recently in 2018 I published another paper indicating that the minimal mass for stars must also be higher than expected, at least 75 times the mass of Jupiter. My colleagues and I are now trying to figure out why the theoretical models do not agree with observations. A possible explanation is that these brown dwarfs lose heat more quickly than expected.

Why do Stars Appear to Vastly Outnumber Brown Dwarfs? Is This Even True?

Some estimates using data from the WISE mission indicate that stars appear to outnumber their brown dwarf cousins by a ratio of about 10:1. The reason for that is thus far a mystery. In principle a collapsing blob of interstellar material does not "care" about whether or not the resulting star (or brown dwarf) will be large enough to ignite hydrogen fusion. So why is this process producing so many more stars than brown dwarfs? A few hypotheses have been proposed, but none are (in my view) satisfactory. A particularly tough problem is reconciling the time scales of stellar formation and stellar ignition. Whereas a star (or brown dwarf) has mostly accreted all of its mass during the first 1 million years, hydrogen fusion does not become a significant source of energy until about 3 billion years. So how can hydrogen fusion affect the mass of the resulting object?

The caveat is that some very recent work by Emily Martin and others hint that extremely old and faint and cool brown dwarfs, the so-called Y dwarfs, may be more common than we originally thought. In that case the problem changes from "where are all the brown dwarfs" to "how did they all become so faint and cool". That would indicate that the cooling rate for brown dwarfs is higher than we originally thought. Some of my work points to that higher cooling rate as well, but my personal feeling at this point is that this is not enough to solve the problem.

An image from my research, showing an interstellar dust cloud obscuring stars, Swope telescope

More Specific Questions

Why do Theoretical Models of Brown Dwarf Structure and Evolution Under-predict the Mass, Temperature, and Luminosity of the Stellar-Substellar Boundary?

As I discussed earlier, a common theme of my research is that observations indicate that the smallest possible stars are hotter, brighter and more massive than the theoretical models predict. Why is that? The best explanation seems to be that very low mass stars and brown dwarfs are radiating their heat faster than expected. In other words, they lose their internal energy faster and therefore need more internal energy to keep alive.

Let's make a rough analogy. Imagine a camp fire that is just barely catching. If this fire were any smaller it would die off. That is analogous to the situation with the smallest possible stars: nuclear fusion is just barely sustainable. Now the wind blows and kills that fire that is just barely catching anyways. The fire dies because the blowing wind removes heat from the fire faster than the fire can generate heat. That means that only a larger fire would survive in that wind. There is some evidence that a similar process is going on with very low mass stars and brown dwarfs, but instead of a wind removing energy the general energy flow is faster because the outer layers of these objects are not as insulating as the theoretical models predict. I am currently working on a paper to explore this hypothesis based on observations of about 100 very low mass stars and brown dwarfs.

How do the Numbers of Brown Dwarfs and Very Low Mass Stars Change as a Function of Mass? (What is the Mass Function?)

We know that the stellar formation process heavily favors the formation of low mass objects, but what is the quantitative nature of this relation? If we make a histogram of number of stars versus mass what is the general shape of this histogram? This quantitative relation is called the mass function. Understanding the mass function can tell us a lot about the physics behind the stellar formation process and also the stellar population of our Galaxy. Over the last decade our understanding of the mass function for very low mass stars and brown dwarfs has improved significantly, but in a sense it is still rudimentary. The problem stems from difficulty in identifying ALL very low mass stars and brown dwarfs in a given volume and having a good handle on estimating their masses. Remember, these objects are extremely faint and therefore hard to identify. Recently the European Space Agency's Gaia mission published precise distances for over a billion stars in our Galaxy. This treasure trove of data makes it much easier to identify all stars in a given volume of space, so while there is still much work to be done determining the mass function is now within reach.

Is There a Precipitous Drop in the Mass Function Right as it Crosses the Stellar-substellar Boundary? Are These Processes Physically Related?

Still on the topic of the mass function, here is a more specific version of the "big question" of why there are more stars than brown dwarfs. Does the drop in the mass function happen EXACTLY at the stellar/substellar boundary? Some results from the Ph.D. thesis of my friend and fellow RECONSer Jen Winters indicate that this may be the case. Jen produced what is likely the most detailed mass function to date and in this mass function the bin with the lowest stellar mass (0.1 to 0.075 Solar masses) is the bin with the most stars, before a sudden drop. What we need now is to break down that bin into many sub-bins to find out where the drop happens with even more precision. But to do that we need more stars with good mass estimates. A mass match of plus or minus 30% between the mass function drop and the stellar/substellar boundary could be a coincidence, but a match of 5% would be a strong indication that these processes are somehow connected.

What is the Magnetic Field Structure of Low Mass Stars and how do we Design Observations to Answer this Question?

All stars have strong magnetic fields around them, and the structure of fields can take on many different shapes. For our Sun there is a broad global field that encompasses the star in a shape much like the skin of an apple surrounds its core (a dipolar field) as well as smaller and more localized fields that cause sun spots and surface explosions known as solar flares. Which type of structure is dominant in small stars? This is an important question if we are to understand the habitability of any planets around these stars because these magnetic fields direct the flow of the stellar wind. The stellar wind is a flow of ionized particles present around the star and it can severely harm living organisms by causing the breakdown of complex molecules and even the loss of a planet's atmosphere. Fortunately for us Earth's own magnetic field shields us from the solar wind. It re-directs the wind toward the North and South poles, and their passage through our atmosphere causes the aurora lights. But not all planets have a strong magnetic field capable of blocking stellar winds, so habitability on these planets may be more connected to how the wind is directed from the stellar magnetic fields than on how they are shielded on the planet's side.

Observing magnetic fields around low mass stars is extremely challenging and must be done indirectly through the observation os star spots and radio emissions caused by the fields. The observations require a large amount of time on very large telescopes, including radio telescopes. I am developing a technique that would shift much of the observing burden to smaller telescopes, but this technique still needs a lot of testing and refinement, if it even works at all.

How often do Low Mass Stars Flare and are Those Flares Strong and Numerous Enough to Make Nearby Planets not Habitable?

On the previous question I mentioned that the specific shape of a star's magnetic field can cause explosions on the star's surface known as flares. These flares release a substantial amount of stellar wind as well as harmful ultraviolet and X-ray radiation. Even if we don't understand the nature of the underlying magnetic fields causing these flares we may be able to get a handle on the habitability question by studying the flares themselves. Fortunately, flares are much easier to detect than magnetic fields; they cause the star to become many times brighter in blue light for a few minutes to a few hours. All we need then is to stare at a given star with a small telescope long enough and see how its brightness is changing. On Earth these observations would be broken up by the day/night cycle and also the ocasional cloudy night, but a space observatory can carry them out continuously for much longer. That is exactly what NASA's Tess satellite is doing right now as you read this! Tess stares at a given patch of sky for at least 27 days continuously and then moves on to another patch so that it will eventually observe almost the entire sky. I recently submitted a postdoctoral proposal asking NASA to fund me to do this big data analysis and determine the flare rate and flare energies of thousands of low mass stars using Tess data. I have not heard back yet, but I sure hope I get funded!