by Paul Gilster | Jun 15, 2017 | Deep Sky Astronomy & Telescopes |
Interesting news this morning that begins with the Very Large Array in New Mexico, which a team of astronomers has been using to look at star formation. Their target: The Perseus molecular cloud, a stellar nursery about 600 light years from Earth. Clouds like this are sufficiently large (this one is about 50 light years in length) and dense to permit molecules to form, with molecular hydrogen (H2) being the most common, along with carbon monoxide (CO).
Although we can’t see into them in visible light (they appear as holes in the starry background because dust and gas obscure the stars forming inside, as well as background stars), such molecular clouds are ideally suited for study with radio telescopes. The VLA survey, called VANDAM (VLA Nascent Disk and Multiplicity) surveyed all the young stars in the Perseus cloud, including both single and multiple stars at separations down to < 20 AU. And now a duo of astronomers has supplemented the VLA data with observations from the James Clerk Maxwell Telescope in Hawaii, the latter helping to reveal the clumps of higher density called ‘dense cores’ — think of them as egg-shaped ‘cocoons’ — within which young stars form.
Image: Radio image of a very young binary star system, less than about 1 million years old, that formed within a dense core (oval outline) in the Perseus molecular cloud. According to the new paper, all stars likely form as binaries within dense cores. Credit: SCUBA-2 survey image by Sarah Sadavoy, CfA.
Sarah Sadavoy (Max-Planck-Institut für Astronomie) and Steven Stahler (UC-Berkeley) found 55 young stars in 24 multiple star systems (all but five of these are binary) and 45 single star systems in the Perseus cloud. All the widely separated binaries (> 500 AU) were made up of Class 0 stars (less than 500,000 years old), and were aligned with the long axis of the dense core. The older Class I binary stars (between 500,000 and 1 million years old) were closer together (separations ~ 200 AU) and showed no such alignment.
These alignments may not be random. For the authors have produced a mathematical model that explains the observations, and it’s a model with a twist. What they deduce is that stars like the Sun — all of them — are born with a companion. Says Stahler:
“We ran a series of statistical models to see if we could account for the relative populations of young single stars and binaries of all separations in the Perseus molecular cloud, and the only model that could reproduce the data was one in which all stars form initially as wide binaries. These systems then either shrink or break apart within a million years.”
A wide binary as considered here is one with a separation of more than 500 AU, which would mean that the original binary companion to our Sun, assuming it once existed, would have been about 17 times further out than Neptune is today. Nemesis? Only in that this would have been a companion to our star, though not one that, like the conjectured Nemesis once considered as a hypothesis for driving asteroid collisions with the Earth, stayed in our neighborhood.
Image: This infrared image from the Hubble Space Telescope contains a bright, fan-shaped object (lower right quadrant) thought to be a binary star that emits light pulses as the two stars interact. The primitive binary system is located in the IC 348 region of the Perseus molecular cloud and was included in the study by the Berkeley/Harvard team. Credit: NASA, ESA and J. Muzerolle, STScI.
In fact, the outcome based on Sadavoy and Stahler’s models is that the Sun’s companion escaped long ago. This would not be an uncommon result, for the early period of star formation would be turbulent. The wide binaries from the early period of star formation either break into separate stars or else shrink into tighter orbits to form tight binaries. Stahler again:
“As the egg contracts, the densest part of the egg will be toward the middle, and that forms two concentrations of density along the middle axis. These centers of higher density at some point collapse in on themselves because of their self-gravity to form Class 0 stars. Within our picture, single low-mass, sunlike stars are not primordial. They are the result of the breakup of binaries.”
An interesting result, and keep this in mind (from the paper): “If our model continues to provide a good fit to other star-forming regions, then the mass fraction of dense cores that becomes stars is double what is currently believed.”
In trying to learn how binary and multiple star systems form, we’ve begun to produce computer simulations examining the behavior of collapsing masses of gas, as well as simulating how young multiple stars behave as they emerge from their gas clouds. What this work addresses is the fact that among the population of young stars, binaries exist in higher proportion. The authors point to the need for higher resolution studies in other star-forming regions to continue the systematic analysis of the role of binaries in early stellar evolution.
The paper is Sadavoy and Stahler, “Embedded Binaries and Their Dense Cores,” accepted at Monthly Notices of the Royal Astronomical Society (preprint). This UC-Berkeley news release is helpful.
by Paul Gilster | Jun 14, 2017 | Astrobiology and SETI |
400 light years away in a star-forming region called Rho Ophiuchi there is an interesting stellar system in the making. Catalogued as IRAS 16293-2422, what we have here is a triple protostar system — a binary separated by 47 AU and a third star at 750 AU. All three have masses similar to the Sun, and while the system is young, it has already achieved a certain fame in that researchers working with data from the Atacama Large Millimeter Array have been able to identify a simple form of sugar called glycolaldehyde in surrounding gas.
Learning that building blocks of life can form in other systems is useful, but here we have sugar in the region where a protoplanetary disk can form, an indication that such materials are widely available in the places where planets begin to coalesce around their host star.
Then just this month we’ve learned that further ALMA work has yielded the prebiotic organic molecule methyl isocyanate (CH3NCO) in the same system. Niels Ligterink (Leiden Observatory) and Audrey Coutens (University College, London), who led one of two teams on this work, issued this statement pointing to the significance of the find:
“This star system seems to keep on giving! Following the discovery of sugars, we’ve now found methyl isocyanate. This family of organic molecules is involved in the synthesis of peptides and amino acids, which, in the form of proteins, are the biological basis for life as we know it.”
The term ‘complex organic molecules’ in this context refers to molecules consisting of six or more atoms, of which at least one is a carbon atom — such organic molecules are widely observed in regions of star formation. Prebiotics are a subset, a category of complex molecules with interesting implications for life. From the Ligterink paper, one of two scheduled to appear in Monthly Notices of the Royal Astronomical Society:
A special category of complex molecules is that of the prebiotics, molecules that can be linked via their chemical structures or reactivity to life bearing molecules, such as amino-acids and sugars. Methyl isocyanate, CH3NCO, also known as isocyanomethane, is a molecule that falls in this category, because of its structural similarity with a peptide bond… This type of bond connects amino-acids to form proteins and as such is interesting because it connects to chemistry relevant to the formation of the building blocks of life.
Image: ALMA detects methyl isocyanate around young Sun-like stars (artist’s impression). Credit: ESO.
Ligterink and Coutens were joined by another team led by Rafael Martín-Doménech (Centro de Astrobiología, Madrid) and Víctor M. Rivilla (INAF-Osservatorio Astrofisico di Arcetri, Florence). Working at several different wavelengths, the researchers found methyl isocyanate in the inner regions of gas and dust surrounding the young triple system. Their computer modeling, supplemented by laboratory experiments, included chemical models of the organic molecule in the gas-grain environment around the stars, a process explained by chemistry on the surface of dust grains in space and subsequent chemistry in the gas phase.
Moreover, the work demonstrated through cryogenic high-vacuum experiments that methyl isocyanate could form at temperatures as low as 15 Kelvin. Both teams applied spectrographic analysis of the light from the triple star system to trace the abundance of methyl isocyanate in comparison to molecular hydrogen, finding it comparable to previous detections around protostars within the Orion KL and Sagittarius B2 North star-forming regions.
Image: The Rho Ophiuchi star formation region in the constellation of Ophiuchus. Credit: ESO.
Thus we continue to home in on the possibility that part of life’s story begins in space, with basic prebiotic chemistry occurring there and transferring prebiotic molecules to star systems in formation and, eventually, the planets that emerge. As a case in point, we find complex organic molecules of the kind commonly detected in interstellar space in comets in our own system (Rosetta found evidence for such on the surface of comet 67P/Churyumov-Gerasimenko). Methyl isocyanate has itself been detected before (in the molecular cloud Sagittarius B2), but never, until now, in protostars of solar mass.
The papers are Doménech et al., “First Detection of Methyl Isocyanate (CH3NCO) in a solar-type Protostar,” accepted at Monthly Notices of the Royal Astronomical Society (preprint) and Ligterink et al., “The ALMA-PILS survey: Detection of CH3NCO toward the low-mass protostar IRAS 16293-2422 and laboratory constraints on its formation,” also accepted at MNRAS (preprint).
by Paul Gilster | Jun 13, 2017 | Exoplanetary Science |
Small stars are fascinating because of their sheer ubiquity. Some estimates for the fraction of red dwarfs in the galaxy go as high as 80 percent, meaning the planets around such stars are going to be the most common venues for possible life. For a time, I thought brown dwarfs would be shown to be even more numerous, but the WISE [Wide-field Infrared Survey Explorer] data have indicated otherwise (see Brown Dwarfs Sparser than Expected).
Hopes for a brown dwarf closer than the Alpha Centauri stars (and thus a convenient intermediary destination for future probes) have dwindled down to nothing, but we do have interesting systems like Luhman 16 AB, the third closest system to the Sun, captured in the image below via a ‘stack’ of twelve images courtesy of the Hubble instrument. The work is from Luigi Bedin (INAF-Osservatorio Astronomico di Padova, Italy) and team, helping us with orbital parameters of the pair and demonstrating that there is no third companion.
Image: Luhman 16 AB as seen by Hubble in this stack of images. Credit: ESA/Hubble & NASA, L. Bedin et al.
We’ll keep an eye on this interesting system given its proximity. Discovered in WISE data in 2013, the brown dwarfs are separated by about 3 AU in a binary that is 6.5 light years away. Both are of about 30 Jupiter masses. Bedin and colleagues can find no exoplanets of Neptune mass or greater with a period of between one and two years. Future Hubble observations have already been approved for August of next year. From the paper:
We plan to focus on the trailed HST images already collected and those planned for August 2018, to further improve the astrometric precision and search exoplanets down to few Earth masses. Follow-up observations with the VLT/CRIRES+ instrument will provide accurate radial velocity data, an important complement to our 2-D astrometric data, as it will provide the missed component necessary for the complete tri-dimensional picture of the kinematic in the system. In addition, the Gaia DR2 dataset will provide absolute motions, positions, and distances of several stars in the field.
These observations may be able to rule out (or detect) the presence of smaller planets. If they’re there, we can expect further follow-up work with the James Webb Space Telescope as well as the Extremely Large Telescope and other assets. This system is tantalizingly close.
A Brown Dwarf Eclipsing Binary
Kepler is also giving us new information about brown dwarfs, in this case finding one orbiting a white dwarf with an orbital period of a scant 71.2 minutes. The white dwarf is WD1202-024, identified by the Sloan Digital Sky Survey and originally thought to be an isolated star. We learn instead, via data from campaign 10 of the K2 mission (the spacecraft’s extended mission following its reaction wheel problems) that we’re seeing a dramatic lightcurve from a binary. The Kepler data were followed up by five different ground-based telescopes whose combined work revealed that the white dwarf has a mass about 40 percent that of the Sun, while the brown dwarf is equivalent to about 67 Jupiter masses.
Image: K2 lightcurve (black jagged curve) folded about a period of 71.23 minutes. The red curve represents a simple geometrical model with a 5-minute long total eclipse and a 9% contribution to emulate an illumination effect on the companion star. The blue curve is the fit to the model based on the length of the K2 observations. Credit: Rappaport et al.
The work on WD1202-024 was announced by Lorne Nelson (Bishop’s University, Quebec) at the semi-annual meeting of the American Astronomical Society in Austin (TX), and you can click here for a portion of the press conference at the meeting that included discussion of the system. Saul Rappaport (MIT) and Andrew Vanderburg (Harvard Smithsonian Center for Astrophysics) found the unusual lightcurve among 28,000 K2 targets.
What we are seeing appears to be a white dwarf being eclipsed by a much cooler brown dwarf in a system that is viewed nearly edge-on from our perspective. The team’s computer models showed that the infant binary, having formed about three billion years ago, would have consisted of a 1.25 solar mass star and a brown dwarf in a 150 day orbit. The star expanded over time to become a red giant, engulfing the brown dwarf about 50 million years ago. Nelson explains the process, one which the brown dwarf survived (thus far) relatively intact:
“It is similar to an egg-beater effect. The brown dwarf spirals in towards the center of the red giant and causes most of the mass of the red giant to be lifted off of the core and to be expelled. The result is a brown dwarf in an extraordinarily tight, short-period orbit with the hot helium core of the giant. That core then cools and becomes the white dwarf that we observe today.”
What should emerge out of all this, in about 250 million years or even less, is that the separation of the white and brown dwarf will dwindle until the brown dwarf will begin to be consumed by the star, making the binary into a cataclysmic variable (CV) feeding off accretion from the disk of matter surrounding the white dwarf. At the AAS meeting, Nelson referred to this system as the shortest period pre-cataclysmic variable ever discovered.
The paper is Rappaport et al., “”WD 1202-024: The Shortest-Period Pre-Cataclysmic Variable,” submitted to Monthly Notices of the Royal Astronomical Society (preprint). The Bedin paper is “Hubble Space Telescope astrometry of the closest brown dwarf binary system — I. Overview and improved orbit,” accepted at Monthly Notices of the Royal Astronomical Society (preprint).
by Paul Gilster | Jun 12, 2017 | Uncategorized |
It’s wonderful to have my friend Al Jackson back at the top of the site with a look at the career and times of JPL’s Frank Malina. Al’s service in the Apollo program came as astronaut trainer on the Lunar Module Simulator; he then spent 40 more years at Johnson Space Center, mostly for Lockheed working the Shuttle and ISS programs. His doctorate was in 1975 from the University of Texas at Austin. The author of numerous scientific papers on interstellar concepts, Al is a fixture at deep space conferences and a continuing source of inspiration on matters scientific as well as science fictional. Today Al gives us an overview of a man who played a key role in the sounding rocket era following World War II, as the infant Jet Propulsion Laboratory began its rich history of exploration and technical development.
by Al Jackson
I travel from Houston to Austin by Highway 290 fairly often, and sometimes I stop at Brenham, Texas for lunch. I skip the fast food joints on 290 and go downtown. It is a beautiful small town with a charming old downtown (founded in 1844). Only recently have I become aware that a native Texan from Brenham fulfilled a dream started by Robert Goddard, in fact doing in 10 months what Goddard had for twenty years tried to accomplish. Even more than that, he was co-founder of the Jet Propulsion Laboratory, JPL, and co-founder of Aerojet General. By 1945 he had eclipsed Goddard as the most important American rocket scientist. He was a consummate researcher in the theoretical engineering of rocketry and a master manager of several rocket and rocket vehicle projects for the U.S. Army. So who was the Texas pioneer ‘Wernher von Braun’? Dr. Frank J Malina from Brenham, Texas.
Malina was the originator and leader of a project whose anniversary is today, June 12. It is the 70th anniversary of the last launch of a Wac-Corporal sounding rocket at White Sands. We tend to forget that Robert Goddard had a solid scientific use for his development of rockets — to explore the Earth’s upper atmosphere. We all love Goddard for his inventiveness in rocket hardware and his stubborn individualism, and given time he may have realized his sounding rocket dream. However, while he struggled in the New Mexico desert in 1936, Frank Malina, still a graduate student at Caltech, had put on the wall of his office a chart of how a successful sounding rocket project might be accomplished. Unlike Goddard, he recognized the need for a team and a choice of team captains.
Malina’s dream was interrupted by World War II. Along with his mentor Theodore von Kármán (the great 20th century aerodynamicist), he directed the development of the Jet Assisted Take Off (JATO) rockets for use by the Army Air Force. This work for the U.S. Army led to the formation of JPL, and Malina became its first director. There is a straight line heritage from the solid rocket JATO motors to the intercontinental missiles in the American defense arsenal and even up to the Space Shuttle booster motors. His involvement in this project alone is enough to have made him a famous rocketeer.
Image: Dr. Theodore von Kármán (black coat) sketches out a plan on the wing of an airplane as his JATO engineering team looks on. From left to right: Dr. Clark B. Millikan, Dr. Martin Summerfield, Dr. Theodore von Kármán, Dr. Frank J. Malina and pilot, Capt. Homer Boushey. Captain Boushey would become the first American to pilot an airplane that used JATO (Jet Assisted Take-Off) solid propellant rockets. Credit: NASA/JPL.
In 1944 Dr. Malina was sent to England and France to inspect salvaged V2s and V1 launch sites. Returning by plane over the Atlantic, he decided to ask the Army ordnance department to fund his cherished goal of building and launching a vehicle to sound the upper atmosphere in regions that could not be reached by balloons. This was December of 1944. From designs by H.S. Tsien and Malina, he and Homer Stewart submitted and got approval on a proposal to launch a sounding rocket with a 25 lb payload to 100,000 ft.
There had already been a program started at the newly founded JPL to build military rockets. Malina organized a team to use components developed from this program. It is amazing that the von Kármán-Malina program at JPL during WWII accomplished, on a smaller scale, almost the same technical objectives as von Braun’s huge V2 project. A viable liquid rocket motor using nitric acid and aniline with 1500 lbs of thrust was developed, as was the Private-series of missiles. The main difference being the V2’s much larger rocket motor and especially the guidance system, which was still being researched at JPL by the end of the war.
Once the project was approved, Malina and his JPL crew turned over several ideas for the sounding rocket. It turned out that the solid rocket motors would be too heavy for the flight. They needed a long burn light weight rocket. So a liquid motor powered vehicle boosted quickly to a high speed was designed. They needed the initial boost in order to gain a sufficient amount of stability from the vehicle fins since they had yet to developed an active onboard guidance system. The booster system used some of the solid rocket technology in the JATO units that JPL had already fashioned. The booster and 2nd stage liquid rocket were to be launched using a 60 ft tower.
In July of 1945 the flight characteristics of the booster were tested with a 1/5 scale model at Goldstone Lake, California. The tests showed the viability of the solid booster system and a three fin stabilization system rather than four fins favored by ordnance experts. One wonders: Did any copies of this ‘baby Wac Corporal’ survive to the present?
Nine months after Malina had proposed it the vehicles were taken to the new facility at White Sands Proving Grounds, New Mexico.
Four rounds of the booster called Tiny Tim were launched off the tower. Two dummy rounds of the WAC were boosted and then two with only partially filled fuel tanks were flown to get experience with the radar tracking.
These must have been counted as rounds 1 through 4 because on October 11, 1945 a fully loaded round 5 was made ready. The 16 foot long 1 foot in diameter rocket stood flight ready. It weighed 665 lbs and would be boosted by 50,000 lbs of thrust before the 1500 lb thrust liquid motor took over. In Malina’s words the flight went like this:
“11 October 1945 became our great day for the first flight of the WAC (round 5) fully charged with propellant. It was a clear day. We craned our necks to watch the WAC’s smoke trail until the engine stopped at around 80,000 ft. On the basis of radar tracking data for the 6th round of the WAC, it was estimated that the maximum altitude reached was between 230,000 and 240,000 ft. The total time of flight was about 450 sec. or 7.5 min. the velocity of the WAC at the end of the burning was about 3,100 ft per sec. The impact point of the first round was around 3,500 ft. from the launcher, which meant that the WAC had maintained a very satisfactory vertical path. Success!”
Image: Project director Frank J. Malina (a former JPL Director) poses with the fifth WAC Corporal at the White Sands Missile Range. The solid-propellant booster is not shown. Credit: NASA/JPL.
That 43 mile flight was a world record, for even the more advanced V2 had not been launched to such an altitude yet. It was an amazing achievement. In 10 months, Malina and his crew had designed and built the sounding rocket Goddard had dreamed of and made such a contribution to. Soon there followed the captured V2 flights from New Mexico and other sounding rocket programs.
Malina headed a large team of people working together just as von Braun had run a much larger team in Germany (Malina and von Braun were almost the same age). Malina remarked:
“The large number of people involved in this (WAC Corporal) program indicates why the dreams of individuals and small groups of rocket enthusiasts in the 1920’s and 1930’s to design, construct and test a high altitude sounding rocket had little chance of success. Fortunately, most pioneers do not foresee all of the practical implications of their dreams. No doubt if they were able to do so, few new wild ideas will ever be tried.”
It is good to remember a fellow Texan, Dr. Frank Malina, a man not as well-known as Dr. Goddard, or Dr. von Braun, but a rocketeer who had profound and lasting impact on the American development of rocket vehicles, astronautics and spaceflight.
by Paul Gilster | Jun 9, 2017 | Exoplanetary Science |
Just how did the seven planets around TRAPPIST-1 form? This is a system with seven worlds each more or less the size of the Earth orbiting a small red dwarf. If these planets formed in situ, an unusually dense disk would have been required, making planet migration the more likely model. But if the planets migrated from beyond the snowline, how do we explain their predominantly rocky composition? And what mechanisms are at work in this system to produce seven planets all of approximately the same size?
New work out of the University of Amsterdam attempts to resolve the question through a different take on planet formation, one that involves the migration not of planets but planetary building blocks in the form of millimeter to centimeter-sized particles. Chris Ormel (University of Amsterdam) and team note that thermal emission from pebbles like these has been observed around other low-mass stars and even brown dwarfs. The researchers believe these migrating particles become planetary embryos as they reach the snowline, which at TRAPPIST-1 occurs at about 0.1 AU.
Once within the snowline, the embryos would grow by the accretion of rocky pebbles from the inner circumstellar disk, with inward migration eventually stopping at the inner edge of the disk. A key assumption here is that the planets of TRAPPIST-1 formed sequentially rather than simultaneously, a novel concept indeed. So let me go to the paper at this point:
In our model we assume that the H2O iceline is the location where the midplane solids-to-gas ratio exceeds unity, triggering streaming instabilities and spawning the formation of planetesimals. These planetesimals merge into a planetary embryo, whose growth is aided by icy pebble accretion. Once its mass becomes sufficiently large, it migrates interior to the H2O iceline by type I migration, where it continues to accrete (now dry) pebbles until it reaches the pebble isolation mass.
The process then begins again for a second planet:
After some time, a second embryo forms at the snowline, which follows a similar evolutionary path as its predecessor. Even though the inner planet’s growth could be reduced by its younger siblings’ appetite for pebbles, it always remains ahead in terms of mass. Planet migration stalls at the inner disk edge, where the planets are trapped in resonance.
Image: Astronomers from the University of Amsterdam (the Netherlands) present a new model for how seven earth-sized planets could have been formed in the planetary system Trappist-1. The crux is at the line where ice changes to water. Credit and copyright: NASA/R. Hurt/T. Pyle. And please note this JPL news release on the artists who produced this image. All too often, artists like Tim Pyle and Robert Hurt receive scant attention in the stories that run their work. It’s excellent to see their background and methods explained.
The TRAPPIST-1 planets, indeed, form what the authors call ‘a resonant convoy,’ with the outer planets ‘pushing’ on the inner ones. The paper’s numerical simulations produce the observed planetary system with the exception that a 3:2 mean motion resonance emerges among planets b and c, as well as among c and d. Although neither pair is presently at the 3:2 MMR, the authors argue that during the disk dispersion phase of the system’s formation, the 3:2 MMRs of these pairs were broken, leaving us with the overall architecture we see today.
The paper’s most radical contention is that planets have assembled at a specific location, the snowline, as opposed to forming in situ or migrating from their formation regions beyond the snowline. Clearly, many questions remain, including how the streaming instabilities induced at the snowline operate in the presence of planetary embryos. The paper does, however, make a prediction: If a giant planet forms rapidly at the snowline, it should end the flux of pebbles to the inner disk, depriving it of planet-building material. From the paper:
Hence, we expect a dichotomy: when giant planet formation fails, pebbles can drift across the iceline to aid the growth of super-Earths and mini-Neptunes. Conversely, when a giant planet forms at the iceline we expect a dearth of planetary building blocks in the inner disk. Therefore, the close-in super-Earth population found by Kepler and the cold Jupiter populations found chiefly by radial velocity surveys should be anti-correlated – a prediction that could be tested with future exoplanet surveys.
The paper is Ormel et al., “Formation of Trappist-1 and other compact systems,” accepted at Astronomy & Astrophysics (abstract). A preprint is available, but be aware that a number of internal references are not yet filled in, another reason not to assume that preprints necessarily mirror the final paper.
