DQ Tau's hot molecular gas dims as emission shifts outward

The young binary DQ Tau produces bursts of material falling onto its stars about every 16 days. During three bursts, observations tracked carbon monoxide and water gas in the small cavity inside the surrounding disk. Their infrared emission weakened and the gas appeared to move outward over a month, unlike other signals that responded to the bursts. The pattern points to changing disk dynamics rather than rapid chemical changes.

Star system: DQ Tauri A | Distance from the Sun: 637 light years

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DQ Tau's hot molecular gas dims as emission shifts outward

Star system: DQ Tauri A | Distance from the Sun: 637 light years

In DQ Tau, the infrared signals from hot molecular gas grew fainter and narrower over about 30 to 35 days, even as the binary produced repeated bursts of material falling onto its stars. The changing signals indicate that the gas giving off the light became cooler and less massive, while its emitting region moved farther from the stars. This region lies in the disk’s inner cavity, roughly 0.2 to 0.4 astronomical units across. An astronomical unit is the average distance between Earth and the Sun.

The binary’s orbit takes about 16 days, and its bursts are linked to that cycle. Seven instruments monitored the system for 40 days, including nine observations with JWST’s MIRI instrument and nine with the ground-based IRTF iSHELL instrument. They measured the brightness and width of infrared lines from water and carbon monoxide. The width of a line shows how quickly the gas is moving; the team used these speeds to estimate how far the gas orbits from the stars. The accretion luminosity pulses, which measure the energy released as material falls onto the stars, varied by factors of about 9 to 80.

The molecular gas did not respond to each burst in the same way as atomic hydrogen or the infrared glow near five micrometers. Instead, its changing light appears to reflect movement and changing conditions in the disk more than fast chemical reactions. The pattern fits the idea that gas gathers near the disk’s edge and affects the strength of later bursts and the gas density in the cavity every four to five orbits. What causes this longer-term pattern remains unclear.

Key findings
Water and carbon monoxide emission lines became fainter and narrower over about 30 to 35 days.
The line widths indicate that the emitting gas moved to larger orbital distances as its temperature and mass decreased.
The gas changes differed from the responses of atomic hydrogen and the five-micrometer infrared glow.
The observed variations are consistent with gas building up near the disk edge every four to five orbits, though the cause is unknown.

Scientific study

Coordinated Space and Ground Monitoring of Accretion Bursts in a Protoplanetary Disk: MIRI and iSHELL observe the long-term modulation of gas within a small inner disk cavity