The Depletion of Carbon-Bearing Species in Comet ATLAS (2025 K1)
ATel #17362; David Schleicher (Lowell Observatory)
on 27 Aug 2025; 21:49 UT
Credential Certification: David Schleicher (dgs@lowell.edu)
I obtained 2 sets of narrowband photoelectric photometry for dynamically new
Comet ATLAS (2025 K1) on 2025 August 19 (r=1.25 AU) using the Hall 42-inch
(1.1-m) telescope at Lowell Observatory. Both sets utilized a 97 arcsec aperture,
yielding the following mean gas production rates using our standard methodologies
and parameters (cf. Schleicher & Bair 2011, AJ 141, #177):
log Q(OH; Haser) = 27.58
log Q(H2O; vectorial) = 27.66
log Q(CN) = 23.86
log Q(C2) = 24.21
log Q(C3) = 23.11 (with large uncertainty)
log Q(NH) = 25.21
log Afrho = 1.6
Note that all of the carbon-bearing species, including CN, are unusually low as
compared to OH, based on our Lowell database of comet compositions (A'Hearn et al.
1995; Schleicher and Bair 2016: AAS/DPS Meeting #48, paper #308.04).
We are aware of only two comets having a lower CN-to-OH ratio - Comet
Yanaka (1988r = 1988 Y1), for which Fink (1992) measured a depletion of
CN-to-water of more than a factor of 25, and Comet 96P/Machholz 1, for which
Schleicher (2008) measured a depletion by a factor of 72 as compared to the
mean value for typical comets. The depletions of C2 and C3 in 2025 K1 are
large when compared to OH but not when compared to CN. The NH-to-OH ratio is
normal, while the dust-to-gas ratio is low but not unusual.
