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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Measuring Magnetic Fields

An optical image of the Pelican Nebula, a complex region forming young stars. New observations of a similar star forming region have determined the strength of its magnetic field. Credit: Martin Pugh

Polarized light is a familiar phenomenon, as people who prefer polarized sunglasses can testify. The electric field in a beam of light can vibrate either left-right or up-down, and the scattering or reflection of light can result in the preferential absorption of one or the other of these two "polarizations." The majority of sunlight on Earth, for example, is preferentially polarized in one direction due to scattering in the atmosphere; that's what makes polarized sunglasses so effective.

Electromagnetic radiation from astrophysical sources can also be polarized. It often occurs because of selective scattering from elongated (possibly even needle-shaped) dust grains. When most of the grains in a volume have been oriented in the same direction by the region's magnetic field, the degree of polarization of scattered light will be significant. Astronomers are extremely interested in magnetic fields, which play a major - perhaps even a dominant - role in controlling the shapes and motions of interstellar gas clouds. Unfortunately, magnetic fields are very difficult to measure directly. Polarization observations, it turns out, offer a unique way to probe the magnetic fields.

SAO astronomer Paul Ho and two colleagues used the Submillimeter Array (SMA) to measure the polarization of millimeter wavelength light from a dusty region of particularly active star formation called W51e2, located about twenty two thousand light-years away from us. The SMA measures two properties of the scattered light: the angle of the vibration with respect to the cloud's contours, and the amount of polarization compared to the unpolarized light. The scientists used this information, together with known features of the region, to develop a new and potentially wide-ranging scheme to determine magnetic field strengths in interstellar clouds. With some general assumptions, they show that the field strength can derived from the angle the polarization makes with the radiation's intensity contours. In the case of W51e2, they conclude that the field's strength is relatively strong (about 65 times weaker that the Earth's magnetic field; they suspected as much - that's why they chose this object in the first place). This new technique, if corroborated by other research, can be expanded and applied to many other objects and potentially revolutionize our understanding of this key physical component of the interstellar medium.

Source: Harvard-Smithsonian Center for Astrophysics