Surveying Young Stars in the North America and Pelican Nebulae

Dust clouds ripple across the Pelican Nebula, creating a dramatic environment for star formation
Credit:Wikipedia user Urmymuse; CC BY-SA 4.0

Since 2018, the Zwicky Transient Facility has kept tabs on fleeting events and variable objects in the universe, scanning the entire northern sky every two days. What can this expansive survey tell us about the variability of young stellar objects?

Full sample of young stellar objects overlaid on an image of the North America Nebula (light-colored area on the left) and the Pelican Nebula (light-colored area on the right). The final sample of variable stars is indicated by the darker blue circles. Credit: Hillenbrand et al. 2022

Stellar Surveys

Stars of all ages show a broad variety of variability, from slow pulsations to quick bursts of accretion. Young stellar objects seem especially prone to interesting variations, perhaps thanks to the influence of their natal nebulae. In a new publication, Lynne Hillenbrand (California Institute of Technology) led a team of astronomers, which included four high school students, on a hunt for variability in young stellar objects.

The team surveyed stars in the North America and Pelican Nebulae, which lie 2,590 light-years away in the direction of the brightest star in the constellation Cygnus. Though they bear different names, the nebulae are actually connected, separated visually by a dark band of dust. The North America and Pelican Nebulae complex contains thousands of candidate young stellar objects, dozens of which have been monitored for signs of variability. Astronomers have previously used Zwicky Transient Facility data to study individual young stellar objects, but never a large sample — until now.

Examples of light curves with low, medium, and high (from left to right) values of the flux asymmetry parameter (M, top row) and quasiperiodicity parameter (Q, bottom row). Credit: Hillenbrand et al. 2022

Digging In to the Data

Hillenbrand and collaborators analyzed more than two years of Zwicky Transient Facility observations of 392 young stars, searching for signs of variability and classifying each star’s quasiperiodicity and flux asymmetry. As the names suggest, quasiperiodicity refers to how periodic or random an object’s variation is, while flux asymmetry quantifies how symmetrical an object’s light curve is. Both metrics are valuable ways to describe a variable star’s behavior, distinguishing those that vary smoothly and reliably from those that suddenly and haphazardly burst or flare.

The team found that 323 of the stars in their sample vary in brightness, and 15% do so with a regular period and symmetric light curves. Another 39% of the stars have symmetric light curves but vary either quasiperiodically or randomly. Roughly 14% of the stars are “bursters” and 29% are “dippers,” with abrupt increases and decreases in brightness, respectively.

The team also examined the colors of the objects, since color changes can give us clues as to why an object’s brightness is varying. The color analysis suggested that dippers might be due to changes in the dusty, light-absorbing material surrounding the young stars, while bursters may signal accretion episodes.

Categorizations of all stars in the sample according to their flux asymmetry and quasiperiodicity
Credit: Hillenbrand et al. 2022

Broad Applicability

This work not only provides information about hundreds of young stars, but also reflects on the metrics we use to study them. The quasiperiodicity index was designed for data from space telescopes, and some studies suggest that it can’t be used with less precise and lower cadence ground-based data. However, Hillenbrand and collaborators make a compelling case that applying thoughtful boundary conditions makes the quasiperiodicity index viable for ground-based applications as well.

This study marks the first time that Zwicky Transient Facility data have been used to investigate a large sample of young stellar objects, but surely not the last — be on the lookout for more exciting results from this survey!

Citation

“A Zwicky Transient Facility Look at Optical Variability of Young Stellar Objects in the North America and Pelican Nebulae Complex,” Lynne A. Hillenbrand et al 2022 AJ 163 263. doi:10.3847/1538-3881/ac62d8

By Kerry Hensley

Source: American Astronomical Society - AAS Nova

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Common Accretion across Young Stellar Objects, White Dwarfs, Black Holes and Supermassive Black Holes

The universal plane for accretion-induced variability. The figure shows that the observed characteristic break frequencies, obtained from the power spectral density of various accreting systems, agree very well with the predicted characteristic break frequencies. The predicted frequencies have been derived by inserting the observed masses, radii, and mass accretion rates into the best-fit relationship to the combined supermassive black hole, stellar-mass black hole and accreting white dwarf sample. The figure additionally shows the position of the young-stellar object V866 Sco with the filled magenta circle. This demonstrates that the variability plane of accreting systems extends from supermassive black holes all the way to young-stellar objects. Figure extracted from Scaringi, S. et al, 2015, Science Advances, 1, e1500686.

An international team of astronomers, led by Dr Simone Scaringi of the Max-Planck-Institute for Extraterrestrial Physics, have discovered a previously unknown link between the way young stars, white dwarfs and black holes grow feeding from their surroundings.

In a paper published in Science Advances, the authors show how the "flickering" in the brightness variations of young stellar objects (YSOs) - very young stars in the final stages of formation - is similar to the flickering seen from ancient black holes or white dwarfs as they violently pull matter from their surroundings in a process known as accretion. This observed flickering suggests that relatively cool accretion discs around young stars, whose inner edges can be several times the size of the Sun, show the same behaviour as the hot, violent accretion discs around planet-sized white dwarfs, city-sized black holes and supermassive black holes as large as the entire inner Solar system.

The researchers discuss a unified scenario for understanding brightness variations from accretion disks around different types of stars and compact objects. A previous work had unified the variability in disks around black holes of different mass ranges, but now, considering not just the mass of the object but also its size, scientists can add accreting white dwarfs and proto-stars to this unified picture. Accretion therefore seems to be a universal process operating in a similar way despite the different size, age, temperature and gravity of the accreted object.

The observations were obtained using the Kepler/K2 mission and ULTRACAM on different ground-based telescopes. Kepler/K2 is a NASA mission aimed at discovering Earth-size extrasolar planets in or near the habitable zone through the transit method from very precise, long and uniterrupted lightcurves. ULTRACAM is an ultra-fast, triple-beam CCD camera designed to study astrophysics on the fastest timescales. The ULTRACAM data were obtained at the 8.2-m Very Large Telescope (VLT) in Chile and the 4.2-m William Herschel Telescope (WHT). Both Kepler/K2 and ULTRACAM can "listen" to the seemingly random brightness variations produced by accretion discs, revealing how they all sound the same once scaled by their physical size. 

More information:

Scaringi, S., Maccarone, T. J., Koerding, E., Knigge, C., Vaughan, S., Marsh, T. R., Aranzana, E., Dhillon, V., Barros, S. C. C., 2015, "Accretion-induced variability links young stellar objects, white dwarfs, and black holes", Science Advances, 1, e1500686. Paper.

"Young stars’ flickering light reveals remarkable link with matter-eating black holes", University of Leicester press release, 13th July 2015.

"Shedding light on the growth of stars and black holes". University of Southampton press release, 14th October 2015.

"New paper shines light on little-understood process in astronomy". Texas Tech University press release, 9th October 2015.

Simone Scaringi is a Humboldt research fellow at the Max-Planck-Institute for Extraterrestrial Physics in Garching, Germany. 

Source:  Isaac Newton Group of Telescopes

Volunteer 'Disk Detectives' Top 1 Million Classifications of Possible Planetary Habitats

A NASA-sponsored website designed to crowdsource analysis of data from the agency's Wide-field Infrared Survey Explorer (WISE) mission has reached an impressive milestone. In less than a year, citizen scientists using DiskDetective.org have logged 1 million classifications of potential debris disks and disks surrounding young stellar objects (YSO). This data will help provide a crucial set of targets for future planet-hunting missions.

"This is absolutely mind-boggling," said Marc Kuchner, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the project's principal investigator. "We've already broken new ground with the data, and we are hugely grateful to everyone who has contributed to Disk Detective so far."

Volunteers using DiskDetective.org, a NASA-sponsored citizen science website to find potential planetary nurseries, have made 1 million classifications in less than a year. Goddard astrophysicist Marc Kuchner, the project's principal investigator, explains how it works.Image Credit: NASA's Goddard Space Flight Center/S. Wiessinger. Download this video in HD formats from NASA Goddard's Scientific Visualization Studio

The marked asymmetry of the debris disk around the star HD 181327 (shown here in a Hubble image) suggests it may have formed as a result of the collision of two small bodies. Disk Detective aims to discover many other stellar disks using volunteer classifications of data from NASA's WISE mission.Image Credit:  NASA, ESA, G. Schneider (U. of Arizona), HST/GO 12228 Team

Combing through objects identified by WISE during its infrared survey of the entire sky, Disk Detective aims to find two types of developing planetary environments. The first, known as a YSO disk, typically is less than 5 million years old, contains large quantities of gas, and often is found in or near young star clusters. The second planetary habitat, known as a debris disk, tends to be older than 5 million years, holds little or no gas, and possesses belts of rocky or icy debris that resemble the asteroid and Kuiper belts found in our own solar system. Vega and Fomalhaut, two of the brightest stars in the sky, host debris disks.

Planets form and grow within disks of gas, dust and icy grains surrounding young stars. The particles absorb the star's light and reradiate it as heat, which makes the stars brighter at infrared wavelengths -- in this case, 22 microns -- than they would be without a disk.

Computer searches already have identified some objects seen by the WISE survey as potential dust-rich disks. But software can't distinguish them from other infrared-bright sources, such as galaxies, interstellar dust clouds and asteroids. There may be thousands of potential planetary systems in the WISE data, but the only way to know for sure is to inspect each source by eye.

Kuchner recognized that searching the WISE database for dusty disks was a perfect opportunity for crowdsourcing. He worked with NASA to team up with the Zooniverse, a collaboration of scientists, software developers and educators who collectively develop and manage citizen science projects on the Internet.

At DiskDetective.org, volunteers watch a 10-second "flip book" of a disk candidate shown at several different wavelengths as observed from three different telescopes, including WISE. They then click one or more buttons that best describe the object's appearance. Each classification helps astronomers decide which images may be contaminated by background galaxies, interstellar matter or image artifacts, and which may be real disks that should be studied in more detail.

In March 2014, just two months after Disk Detective launched, Kuchner was amazed to find just how invested in the project some users had become. Volunteers complained about seeing the same object over and over. "We thought at first it was a bug in the system," Kuchner explained, "but it turned out they were seeing repeats because they had already classified every single object that was online at the time."

Some 28,000 visitors around the world have participated in the project to date. What's more, volunteers have translated the site into eight foreign languages, including Romanian, Mandarin and Bahasa, and have produced their own video tutorials on using it.

Many of the project's most active volunteers are now joining in science team discussions, and the researchers encourage all users who have performed more than 300 classifications to contact them and take part.

One of these volunteers is Tadeáš Černohous, a postgraduate student in geodesy and cartography at Brno University of Technology in the Czech Republic. "I barely understood what scientists were looking for when I started participating in Disk Detective, but over the past year I have developed a basic sense of which stars are worthy of further exploration," he said.

Alissa Bans, a postdoctoral fellow at Adler Planetarium in Chicago and a member of the Disk Detective science team, recalls mentioning that she was searching for candidate YSOs and presented examples of what they might look like on Disk Detective. "In less than 24 hours," she said, "Tadeáš had compiled a list of nearly 100 objects he thought could be YSOs, and he even included notes on each one."

Speaking at a press conference at the American Astronomical Society meeting in Seattle on Tuesday, Kuchner said the project has so far netted 478 objects of interest, which the team is investigating with a variety of ground-based telescopes in Arizona, California, New Mexico, Argentina and Chile. "We now have at least 37 solid new disk candidates, and we haven't even looked at all the new telescope data yet," he said.

Disk Detective currently includes about 278,000 WISE sources. The team expects to wrap up the current project sometime in 2018, with a total of about 3 million classifications and perhaps 1,000 disk candidates. The researchers then plan to add an additional 140,000 targets to the site.

“We’ve come a long way, but there’s still lots and lots more work to do -- so please drop by the site and do a little science with us!” added Kuchner.

WISE has made infrared measurements of more than 745 million objects, compiling the most comprehensive survey of the sky at mid-infrared wavelengths currently available. With its primary mission complete, the satellite was placed in hibernation in 2011. WISE was awoken in September 2013, renamed the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), and given a new mission to assist NASA's efforts in identifying the population of potentially hazardous near-Earth objects (NEOs).

Facilities involved in follow-up studies of objects found with Disk Detective include Apache Point Observatory in Sunspot, New Mexico; Palomar Observatory on Palomar Mountain, California; the Fred Lawrence Whipple Observatory on Mount Hopkins, Arizona; the Leoncito Astronomical Complex in El Leoncito National Park, Argentina; and Las Campanas Observatory, located in the Atacama Desert of Chile.

NASA is exploring our solar system and beyond to understand the universe and our place in it. We seek to unravel the secrets of our universe, its origins and evolution, and search for life among the stars. Today’s announcement shares the discovery of our ever-changing cosmos, and brings us closer to learning whether we are alone in the universe.

Related Links

• Download HD video from NASA Goddard's Scientific Visualization Studio
• Disk Detective website
• NASA-Sponsored 'Disk Detective' Lets Public Search for New Planetary Nurseries (01.30.2014)
• WISE mission

Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Maryland

Source: NASA's Goddard Space Flight Center

Violent birth announcement from an infant star

IRAS 14568-6304

Credit: ESA/Hubble & NASA
Acknowledgements: R. Sahai (Jet Propulsion Laboratory), Serge Meunier

This new Hubble image shows IRAS 14568-6304, a young star that is cloaked in a haze of golden gas and dust. It appears to be embedded within an intriguing swoosh of dark sky, which curves through the image and obscures the sky behind.

This dark region is known as the Circinus molecular cloud. This cloud has a mass around 250 000 times that of the Sun, and it is filled with gas, dust and young stars. Within this cloud lie two prominent and enormous regions known colloquially to astronomers as Circinus-West and Circinus-East. Each of these clumps has a mass of around 5000 times that of the Sun, making them the most prominent star-forming sites in the Circinus cloud. The clumps are associated with a number of young stellar objects, and IRAS 14568-6304, featured here under a blurry fog of gas within Circinus-West, is one of them.

IRAS 14568-6304 is special because it is driving a protostellar jet, which appears here as the "tail" below the star. This jet is the leftover gas and dust that the star took from its parent cloud in order to form. While most of this material forms the star and its accretion disc — the disc of material surrounding the star, which may one day form planets — at some point in the formation process the star began to eject some of the material at supersonic speeds through space. This phenomenon is not only beautiful, but can also provide us with valuable clues about the process of star formation.

IRAS 14568-6304 is one of several outflow sources in the Circinus-West clump. Together, these sources make up one of the brightest, most massive, and most energetic outflows ever reported. Scientists have even suggested calling Circinus-West the "nest of molecular outflows" in tribute to this activity.

A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Serge Meunier.

Source:  ESA/Hubble - Space Telescope

A nursery for unruly young stars

Chamaeleon Cloud (Cha I) and HH90A

Credit: NASA & ESA.Acknowledgements: Kevin Luhman (Pennsylvania State University),  and Judy Schmidt

This striking new image, captured by the NASA/ESA Hubble Space Telescope, reveals a star in the process of forming within the Chamaeleon cloud. This young star is throwing off narrow streams of gas from its poles — creating this ethereal object known as HH 909A. These speedy outflows collide with the slower surrounding gas, lighting up the region.

When new stars form, they gather material hungrily from the space around them. A young star will continue to feed its huge appetite until it becomes massive enough to trigger nuclear fusion reactions in its core, which light the star up brightly.

Before this happens, new stars undergo a phase during which they violently throw bursts of material out into space. This material is ejected as narrow jets that streak away into space at breakneck speeds of hundreds of kilometres per second, colliding with nearby gas and dust and lighting up the region. The resulting narrow, patchy regions of faintly glowing nebulosity are known as Herbig-Haro objects. They are very short-lived structures, and can be seen to visibly change and evolve over a matter of years (heic1113) — just the blink of an eye on astronomical timescales.

These structures are very common within star-forming regions like the Orion Nebula, or the Chameleon I molecular cloud — home to the subject of this image. The Chameleon cloud is located in the southern constellation of Chameleon, just over 500 light-years from Earth. Astronomers have found numerous Herbig-Haro objects embedded in this stellar nursery, most of them emanating from stars with masses similar to that of the Sun. A few are thought to be tied to less massive objects such as brown dwarfs, which are "failed" stars that did not hit the critical mass to spark reactions in their centres.

A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Judy Schmidt.

Source: ESA/ HUBBLE - Space Telescope