In the cold depths of space, interactions between gas and dust can produce a range of complex molecules. Everything from water and alcohol to the basic building blocks of life. While much of this chemistry occurs in cold molecular clouds, molecules can also be produced in the warm regions surrounding a young star, in what is known as a hot molecular core. A few hot molecular cores have been observed in our galaxy, but recently one was discovered in the Large Magellanic Cloud. Somewhat surprisingly, the molecular composition of this extragalactic hot molecular core is strikingly different from the ones seen in our galaxy. 

The young star in question is known as ST11, and it was recently observed by the ALMA radio telescope array. ALMA is particularly suited to observe complex molecules in space, because it is able to view them at the kind of sub-millimeter wavelengths these molecules often emit. ALMA was not only able to observe the warm dust around ST11, but also molecules such as formaldehyde and nitrous oxide. The region is lacking other molecules such as methanol, which are common in cold molecular clouds. Since the formation of different molecules depends not only on the available elements but also temperature and cloud density, the study of such hot molecular cores allows us to better understand complex molecular interactions in space, which can give us clues about how complex chemistry and even life formed on Earth.

One key difference between the Large Magellanic Cloud (LMC) and our own Milky Way galaxy is that the LMC has a much lower metallicity. This means it has a lower fraction of heavier elements such as silicon, oxygen and iron. These elements are common building blocks of interstellar dust, which is why the LMC has a lower fraction of such dust. A great deal of complex chemistry can happen on the surface of dust grains as it interacts with surrounding gases, and a lower metallicity means such interactions are less common. We see this in the types of molecules forming around ST11. Given the density of ST11’s hot molecular core, the levels of molecules such as formaldehyde and isocyanic acid are a tenth to a thousandth of levels seen in the Milky Way. This demonstrates the central role of dust surface chemistry to the formation of these molecules.

It’s a powerful insight into just how young stars can seed the cosmos with some of the complex molecules we see around us.

Paper: Takashi Shimonishi, et al. The detection of a hot molecular core in the Large Magellanic Cloud with ALMA. The Astrophysical Journal, Volume 827, Number 1 (2016)

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Cepheid variable stars are most commonly known as a standard candle for measuring galactic distances. That’s because they vary in brightness at a rate proportional to their average brightness.  But they can also tell us something about how young stars are distributed within our galaxy, and a recent study raises an interesting mystery. 

There are basically two types of Cepheid variable stars. Classical cepheids are large bright stars, typically with a mass 5 – 20 times that of our Sun. Since larger stars have shorter lifetimes, a classical Cepheid is typically no more than 100 million years old. Type II Cepheids are small, old stars, with masses much less than our Sun. They are typically around 10 billion years old. Distinguishing between these two types of Cepheid variables is straight forward, because they have very different metallicities (traces of elements other than hydrogen and helium). So we can distinguish them by looking at their spectra and the way they brighten and dim (their light curve). Because classical Cepheids are brighter, they are typically used to determine the distances to galaxies, and helped establish the Hubble law for cosmic expansion. The dimmer type II Cepheids are typically used for distances within our galaxy, such as determining the distance to the center of our galaxy.

Since the ages of these different types of Cepheids are very different we can use them as a gauge for the age of surrounding stars. For example, if a globular cluster contains type II Cepheids, we know it is billions of years old. If a star cluster contains a classical cepheid, we know that stars formed there relatively recently. A new paper in MNRAS uses this fact to look at the distribution of young stars in our galaxy, and found a rather puzzling void.

Mapping young stars in our galaxy can be a challenge, particularly in the direction of the center of our galaxy, where high amounts of gas and dust obscure most of the visible light from distant stars. Fortunately infrared light isn’t absorbed as strongly, so an infrared survey of Cepheids gives us a good view of the central region of the Milky Way. This new study found some classical Cepheids clustered very close to the center of our galaxy, but found a region about 8,000 light years in radius where there aren’t any classical Cepheids. This would seem to indicate that this region hasn’t produced stars in at least 100 million years. This is in agreement with infrared and radio surveys of the central region of our galaxy, which also find a lack of star producing regions in that area.

We don’t know why stars don’t form in this region. There is certainly plenty of matter in the region, and older stars are clearly present there. For some reason the conditions for young stars are lacking there, producing a cosmic “get of my lawn” effect.

Paper: Noriyuki Matsunaga, et al. A lack of classical Cepheids in the inner part of the Galactic disc. MNRAS 462 (1): 414-420. (2016) doi: 10.1093/mnras/stw1548

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HL Tau is a young star, only about a million years old. Despite its young age, the star is already busy at making a family. 

The star is probably most famous for an early image from the Atacama Large Millimeter/submillimeter Array (ALMA), which showed a disk of gas and dust surrounding the star. What was most striking about the image was the clear gaps in the dusty disk, which seemed to be due to young protoplanets as they began the process of formation. ALMA’s strength is the ability to observe light at millimeter wavelengths, which is the type of light emitted by cold gas and dust. Unfortunately the resolution of ALMA isn’t high enough to observe individual protoplanets, which left some doubt as to whether the gaps were caused by some other process.

Combined ALMA/VLA image of HL Tau.
Credit: Carrasco-Gonzalez, et al.; Bill Saxton, NRAO/AUI/NSF.

But new evidence from the Very Large Array (VLA) confirms the existence of at least one protoplanet. The VLA observes at longer radio wavelengths, but the VLA antennas can be spread across 36 kilometers vs ALMA’s maximum spread of 16 kilometers. This larger spread means that the VLA can make higher resolution images (though only at radio wavelengths). Recent observations of the central region of HL Tau clearly shows a clump in the inner ring of material, indicating a protoplanet in the early stages of formation.

These results show the power of combining observations from different facilities and at different wavelengths. They also demonstrate how astronomy can still surprise us. With an age of only a million years HR Tau is still in the earliest stage of its life, and yet it is already on the path toward becoming a solar system.

Paper: Carlos Carrasco-Gonzalez, et al. The VLA view of the HL Tau Disk – Disk Mass, Grain Evolution, and Early Planet Formation. arXiv:1603.03731 [astro-ph.SR] (2016)

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The image shows two colliding galaxies known as NGC 2207 and IC 2163. It’s a false-color image, where infrared is shown as dark red, visible is shown as normal, and x-ray is shown as purple. The first impression you might have is that the image looks awfully purple, and that means there are lots of x-ray sources in these two galaxies.

The reason for this is that the two galaxies are colliding. Young galaxies tend to have lots of gas and dust around to make stars, so they can produce stars at a fairly high rate, such as we see in dusty starburst galaxies. But over time the rate of stellar production goes down as the free gas and dust tends to get used up. In the Milky Way, for example, new stars form at a rate of only 1 or 2 a year.

In the x-ray only image, the ULXs are clearly seen.
Credit: NASA/CXC/SAO/S.Mineo et al

In these galaxies that rate is about 24 solar-mass stars per year. We know this because of the high number of ultraluminous x-ray sources (ULXs), seen as bright violet dots within the image. The stars associated with these sources are only about 10 million years old, and such sources don’t stay bright for long on astronomical scales.

Images such as this further support what we’ve long thought, that galaxy collisions can stir up star production in galaxies.

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When we look at a cluster of stars, we find that they are chemically similar. That is, the ratio of different elements (or metals in astronomy lingo) in various stars are basically the same. This is pretty much what we expect, since these stars all formed in the same stellar nursery, and haven’t drifted apart from each other. Just as human siblings share similarities due to their common genetic origins, sibling stars share chemical similarities due to their common origin. But what about stars with a common origin that scatter across the galaxy? Do they have a common chemical fingerprint?

The answer to this question has generally been “it depends.” All stars originating from a common stellar nursery would have some broad similarities, but just how strong those similarities are depends on the complex physics of turbulent gas and dust.  For example, we know that objects in our solar system have different chemical fingerprints due to the location of their birth within the solar system. In asteroids, for example, we see that they are clustered into chemically similar “families”, which is one indication that they aren’t the result of a destroyed planet between Mars and Jupiter.

Stellar nursery clouds form from the remnants of multiple supernovae and the like, so a similar effect could be seen in stars. If, for example, proto-stars in a stellar nursery disperse quickly, before the material within the nursery had a chance to mix together really well, then they could have rather distinct chemical fingerprints. If, on the other hand, the clouds of the stellar nursery are thoroughly mixed before proto-stars form, then they would be chemically similar.

Now a new paper in Nature shows that progenitor clouds can mix quite rapidly due to turbulent motion. The team looked at computer simulations of stellar nurseries and star formation, and found that they are highly mixed early on. Not only that, the level of mixing within a stellar nursery is basically the same regardless of the number of stars it produces. So a cloud where only about 10% of its mass forms into stars is just as mixed as one where 50% of the mass produce stars.  The greater the star production, the more widely the resulting stars tend to disperse, so this means that even widely spread stars will be chemically similar.

This means that when we find stars with very similar chemical signatures, we have a pretty good confidence that they could have a common origin. Knowing this, we can better understand just how stars disperse from the stellar nursery after they form.

Paper: Yi Feng, Mark R. Krumholz. Early turbulent mixing as the origin of chemical homogeneity in open star clusters. Nature doi:10.1038/nature13662 (2014).

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Recently I wrote about the average color of the universe, as determined by a survey of more than 230,000 galaxies. While knowing the overall color of these galaxies is a fun little factoid, it isn’t particularly useful from a scientific standpoint. However the color was determined by the average spectrum of the galaxies, which is quite scientifically useful. This “cosmic rainbow” tells us about the history of star formation in the universe.

You can see the average spectrum in the figure below. One of the things you’ll notice is that isn’t simply a continuous spectrum. There are wavelengths that are particularly bright or dark. These are emission (bright) or absorption (dark) lines that are particularly common for the galaxies. Several of the lines are labeled by the element that causes the line. By looking at the relative brightness of these lines, we can determine the relative abundances and temperatures of typical stars. This is because young stars have hotter atmospheres than older stars, so the emission and absorption lines of a star changes over time.

Credit: Karl Glazebrook & Ivan Baldry

Given this average, you can fit it to models of historical star formation. If most stars formed earlier in the universe, then the line spectra would resemble older stars, since most of the present stars would be older. If instead stars formed at a fairly constant rate, then you would see much less bias toward older stars.

What we find is that stars haven’t been produced at a continuous rate within the universe. Instead, there was a peak of star production between 6 and 10 billion years ago, and that the rate of production has been declining ever since. Most of the stars we observe are more than 5 billion years old. New stars are still being formed, but the level of star production is nothing like what it was. The universe has shifted into middle age.

Of course this just further supports that the universe began with the big bang. A peak of star production is exactly what you’d expect in a universe that begins with raw hydrogen and helium, with each generation of stars releasing some material back into the wild via supernovae and the like, but locking part of the material into red dwarfs, neutron stars and the like.

Hints of the big bang in a cosmic rainbow.

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Usually in astronomy we study objects by the amount of light they emit.  Most regular matter gives of light in some form or another.  Even the cold interstellar medium will emit some light at infrared or radio wavelengths. But one downside of this is that the light generally comes from the surface regions of an object.  To study the interior of an object we generally have to use aspects of emitted light from the surface to determine properties of the interior.  For bright objects like stars this works pretty well, but for dim objects like dark interstellar clouds this is more of a challenge.  

Recently a team with the Spitzer space telescope has used a different method.  Spitzer is a sensitive infrared telescope, and the team has been using it to observe cold, dense interstellar clouds by the infrared light that passes through them.  There is a great deal of infrared light in the universe, and when that ambient background light passes through a dark cloud we can determine things like its density and composition from the light they absorb.  Basically it is a way to study dark clouds by the shadows they cast.

Some of the team’s results were recently published in the Astrophysical Journal Letters.  One of the things they’ve announced is the darkest and densest interstellar cloud ever discovered.  It has a mass of about 70,000 Suns, and is only 50 light years across.  This cloud is probably in the earliest stages of collapsing into a cluster of large and bright stars (O-type stars).  Gaining a better understanding of dense clouds like this one will help us understand just how such large stars form.

Paper: Michael J. Butler et al. The Darkest Shadows: Deep Mid-infrared Extinction Mapping of a Massive Protocluster. ApJ 782 L30. (2014) doi:10.1088/2041-8205/782/2/L30

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IRAS 04368+2557 is a protostar about 450 light years from us.  It is a particularly young protostar, at about 300,000 years.  Because of its age and proximity, it provides an excellent opportunity to study the early stages of stellar and planetary formation.

Structure of a protoplanetary disk. Credit Sakai et al.

Like most protostars, this one has a protoplanetary disk out of which planets are expected to form.  Surrounding that is a larger protostellar envelope.  Between the two is a transition zone.  Because of interactions within the surrounding envelope, material can be slowed so that it gradually falls inward toward the protoplanetary disk.  This can enrich the chemical composition of the disk.  Now a new paper in Nature has found that the transition zone can have a centrifuge effect that chemically filters material entering the protoplanetary disk.

The team looked at line spectra of both the disk and the surrounding envelope.  The found that while the surrounding envelope was dominated by hydrocarbons, the region of the transition zone had high concentrations of sulphur monoxide.  In other words the chemical composition of the two regions are dramatically different.

This is likely due to a centrifugal effect where some molecules can more easily penetrate the transition zone, while others have a more difficult time.   So it seems that early solar systems are not simply the product of the material surrounding a young star, but that they are more dominated by materials that can penetrate the transition zone.

Paper:  Sakai N, Sakai T, Hirota T, et al. Change in the chemical composition of infalling gas forming a disk around a protostar. Nature. (2014)

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After the big bang, the only elements in the universe were hydrogen, helium and trace amounts of lithium.  There was no carbon, oxygen or iron, because these elements are only formed when stars undergo fusion in their cores.  About 100 to 300 million years after the big bang, the first stars began to appear.  These first generation stars were likely very large, about a hundred times the mass of our Sun.  Because of their size, they had short lives which ended as supernovae.  From the remnants of those stars, a new generation of stars would form.  These second generation stars would have traces of elements such as carbon, but still lack heavier elements such as iron.  Now a new paper in Nature (arxiv version) has announced the discovery of just such a second generation star.

In astronomy, all elements other than hydrogen and helium are referred to as “metals.”  For this reason, a measure of the amount of other elements a star contains is known as its metallicity. One way to define the metallicity of a star is simply as the fraction of a star’s mass which is not hydrogen or helium.  For the Sun, this number is Z = 0.02, which means that about 2% of the Sun’s mass is “metal”.  Another way to express the metallicity of a star is by its ratio of Iron to Hydrogen, known as [Fe/H].  This is given on a logarithmic scale relative to the ratio of our Sun.  So the [Fe/H] of our Sun is zero.  Stars with lower metallicity will have negative [Fe/H] values, and ones with higher metallicity have positive values.

Spectrum of a low metallicity (Fe/H = -0.8) star. Credit: Anna Frebel.

Stars are often categorized by their metallicity.  For example, Population I stars have an [Fe/H] of at least -1, meaning they have 10% of the Sun’s iron ratio or more.  Population II stars have an [Fe/H] of less than -1.  There is a third category, known as Population III.  These would be the first stars of the universe, with essentially no “metals” in them.

We have yet to observe a Population III star, but this new discovery comes very close.  This new star, known as SM0313 contains no measurable traces of iron.  From its spectrum you can see carbon, calcium and magnesium, but nothing else beyond hydrogen.  In comparison to the spectrum of a typical low metallicity star the difference is striking.

Spectrum of SM0313. Credit: Anna Frebel.

The complete lack of measurable iron in the spectrum is surprising.  Based on the limits of their observations, the authors calculate the metallicity of SM0313 to be no more than Fe/H = -7.1.  This means it is likely a second generation star, and its first generation progenitor ended in a supernova that wasn’t powerful enough to cast out significant quantities of iron.

This changes our understanding of first generation stars. It has been thought that the large size of first generation stars meant their supernova would be a particularly powerful kind known as a pair-instability supernova. Such supernovae would cast out large quantities of material, and as a result the remnants of gas and dust would tend to be enriched and mixed on a relatively short cosmic time scale.  As a result, even second generation stars such as SM0313 would have some measurable levels of iron.  The discovery of SM0313 means that low energy first-generation supernovae were more common.  Thus the mixing and enrichment of gas and dust happened more gradually than originally expected.

Paper:  Keller SC, Bessell MS, Frebel A, et al. A single low-energy, iron-poor supernova as the source of metals in the star SMSS J031300.36-670839.3. Nature. 2014;

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The Orion Nebula has been in the news recently due to a new set of pictures such as this one from Astronomy Picture of the Day.  It is an image of high velocity “bullets.”

It can be a bit difficult to wrap your head around what is really going on here.   The small blue dots at the tip of these orange streams are the “bullets,” and each one is about 10x larger than our solar system.  These bullets are not solid objects but rather dense lumps of iron-rich gas.  They’ve probably been pushed away from the central region of the nebula by intense stellar winds, and they now create shock waves as they stream through the surrounding nebular gas.  They are moving at about 400 km/s, ripping through the gas as they go.

We tend to think of nebula as static clouds of gas floating peacefully in space, but many nebulae are actually violent and complex interactions of gas, dust and stars.  Within the Orion Nebula, there is a cluster of bright central stars, there are dust masses colliding, and new stars are being born with planetary systems forming around them. There’s intense stellar wind colliding with all the gas and dust, pushing lumps like these bullets outward at supersonic speeds.

The Orion Nebula is not a particularly unusual nebula, but it is only about 1300 light years away, so it gives us a front row seat to the type of nebula that gives birth to stars.  Our own Sun likely formed in a similar stellar nursery billions of years ago.  So in a way this is like looking at baby pictures of your cousins.

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