The Further Depletion of Carbon-Bearing Species in Comet ATLAS (2025 K1)
ATel #17518; David Schleicher (Lowell Observatory)
on 1 Dec 2025; 18:26 UT
Credential Certification: David Schleicher (dgs@lowell.edu)
Referred to by ATel #: 17538
I recently obtained an additional 3 sets of narrowband photoelectric photometry
for dynamically new Comet ATLAS (2025 K1) on 2025 November 25 (r=1.20 AU) using
the Hall 42-inch (1.1-m) telescope at Lowell Observatory, for direct comparison
with my pre-perihelion observations from August 19 (1.25 AU) as reported on in
ATel #17362. All sets utilized a 97 arcsec aperture, yielding the following
mean gas production rates using our standard methodologies and parameters
previously described:
log Q(OH; Haser) = 27.93;
log Q(H2O; vectorial) = 28.03;
log Q(CN) = null (following sky and continuum removal, 2 sets went barely negative
while one set was barely positive; sigmas were very large);
log Q(C2) = 24.06;
log Q(C3) = null (following sky and continuum removal, all 3 sets went barely
negative; sigmas were very large);
log Q(NH) = 25.55;
log Afrho = 2.2.
The carbon depletion that I reported on August 19 is now even more extreme.
Production rates of both OH and NH have approximately doubled, while C2 has
dropped by 30% and CN and C3 are now essentially undetectable (at nearly the
same distance). This is consistent with the qualitative results regarding
the depleted carbon species as compared to NH2 reported by Ganesh et al. for
Nov. 13 (ATel #17500). Also, dust production is 4x that measured pre-perihelion.
Note that on the TV guider I detected only one obvious fragment, approximately
12 arcsec down-the-tail from the main body; this may have been fragment C.
Since I could not isolate the coma of each body, these are combined results.
I assume that the higher water and dust production rates post-perihelion are due
to the ongoing fragmentation and associated increased surface area, while I
further speculate that ATLAS's interior is more depleted than its surface.
