An international team of astronomers, led by Dr.
Tae-Soo Pyo (Subaru Telescope, NAOJ), has revealed a complicated outflow
structure in the binary UY Aur (Aurigae). The team observed the binary
using the Gemini North's NIFS (Near-Infrared Integral Field
Spectrometer) with the Altair adaptive optics system (Note 1). The team found that the primary star has a wide, open outflow, while the secondary star has a well-collimated jet.
Because many stars form together as companions in
binary or multiple systems, investigating these systems is essential for
understanding star and planet formation. Although jets (i.e., narrow
bright streams of gas) and outflows (i.e., less collimated flows of gas)
from single young stars are ubiquitous, only a few observations have
shown jets or outflows from multiple, low-mass young stars. Therefore,
the current team chose to examine the outflow structure of binary UY
Aur, which is a close binary system composed of young stars separated by
less than an arcsecond (0".89).
To better understand this system, the team began by
trying to identify the driving source of the receding jets. To separate
the binary stars and distinguish their driving sources, they used Gemini
North's NIFS with its adaptive optics system to observe this close
binary system in the 1-micrometer infrared wavelength region. Since
ionized iron gas ([Fe II]) traces shocked gas in jets and outflows very
well, the team used iron gas emissions to examine the emission gas
distribution. They found that [Fe II] is associated with both the
primary and the secondary stars (Figure 1).
Figure 1: Emission images of UY Aur. The large plus
marks indicate the primary star, located in the upper portion of each
panel and designated as "A" in the continuum image. The smaller plus
marks indicate the secondary star, located below the primary star and
labeled as "B" in the continuum panel. Large tick marks correspond to
measures of 1 arcsecond (140 Astronomical Unit). The filled circles at
the bottom right corners show the spatial resolution (=0.12"). (Credit:
NAOJ)
In addition, they found that the shape of the gas
distribution conformed to simulations of gas streaming between the
primary and secondary stars. However, the high velocity of the gas (100
km/s or > 20,000 mile/h) indicated that it emanated from the close
vicinity of stars rather than arose in the disk gas around the two
stars.
Further investigation of the emission structure involved separation of the receding and approaching emissions (Figure 1).
The team found that the distribution of gas was different for each of
the stars. While the approaching gas was widely spread in an outflow
from the primary star and slightly connected with the secondary star,
the receding gas was spread widely toward the secondary star and flowing
beyond it (Figure 1).
Figure 2: Schematic drawings of the UY Aur binary.
Left:
Redshifted and blueshifted outflows. The primary star (A) shows wide,
open, redshifted and blueshifted outflows as well as a redshifted jet
(in solid red), while the secondary star (B) shows a blueshifted jet (in
solid blue). Redshifted jets from B might be within the solid red
outflow area from A. (Credit NAOJ).Right: Model of the UY Aur binary and its outflows and jets, showing the wide redshifted and blueshifted outflows of the primary star (A) and the blueshifted jet from the secondary star (B). The dotted red lines indicate where the redshifted jets from the secondary star might be. (Credit: NAOJ)
What explains this difference? The team analyzed the
system in terms of bipolar outflows, i.e., each star has a disk and
ejects both blueshifted (approaching) and redshifted (receding) outflows
or jets. The primary ejects wide,open bipolar outflows. Its redshifted
(receding) outflow overlaps with the secondary. In contrast, the
approaching gas from the secondary is distributed in a well-collimated
bipolar jet, with its blueshifted flow tilted toward the wide, open wind
from the primary (Figure 2).
It is known from mid-infrared (wavelength of ~10 micrometer)
observations that the circumstellar disk of the secondary is not aligned
with the plane of the circumbinary disk. This misalignment is
consistent with the jet from the secondary tilted toward the wide, open
outflow from the primary star.
Two jets from a binary system can be explained if the
jets emanate from each of the star-disk systems. Some binaries show
only one jet or outflow. A larger sample of [Fe II] gas distribution
toward binary and multiple young-star systems can clarify how typical
the outflow structure of the UY Aur system is.
Figure 3: Artist's rendition of UY Aur's probable outflow system (Credit: NAOJ)
Notes:
- The observations were conducted as part of the Subaru/Gemini Time Exchange Program, during which astronomers from each telescope's community can mutually access some of each telescope's unique instruments.
- The first circumbinary disk to be resolved and imaged was around the GG Tau A system. To view this image obtained with Subaru Telescope's Coronagraphic Imager with Adaptive Optics (CIAO), go to: http://subarutelescope.org/Introduction/instrument/CIAO.html.
Reference:
Pyo, T.-S., Hayashi, M., Beck, T. L., Davis, C. J.,
and Takami, M. 2014 "[Fe II] Emissions Associated with the Young
Interacting Binary UY Aurigae", Astrophysical Journal, Volume 786, 63.
Source: Subaru Telescope
