Confirming Candidate C Stars (CCCStars)
Abstract:
Dr. Stefan Uttenthaler (Institute of Applied Physics, TU Wien, Austria) writes: in a recent paper, we identified 20 carbon star (C spectral type) candidates among ASAS Mira or Mira-like variables. The identification was based on 2MASS and Gaia data, but is still uncertain. With this campaign, we aim to obtain low-resolution optical spectra of the candidate C stars to verify and confirm the identification.
Justification:
In a recent paper, we identified 20 carbon stars (C spectral type) candidates among ASAS Mira or Mira-like variables (Uttenthaler & Merchan-Benitez, 2025). The identification was based on our newly defined discriminatory model of the Gaia BP/RP spectra, the WRP–WK Wesenheit index, the 2MASS J–KS colour, and the C star parameter derived from the pseudo-wavelength peak separation in the low-resolution Gaia spectra (Gaia Collaboration 2022b). Table B.2 of Uttenthaler & Merchan-Benitez, 2025, lists the candidate C stars; we add the enigmatic SC star VX Aql, bringing the total to 21 campaign targets. The only tool for reliably classifying these cool giant stars into C or M/S spectral types is optical to near-infrared spectroscopy. With this campaign, we aim to obtain low-resolution spectra of the candidate C stars to test the identification. Absorption bands of the CN and C2 molecules can identify carbon-rich stars. By contrast, oxygen-rich M- or S-type stars have molecular bands by TiO, VO, ZrO, and LaO, which appear at different wavelengths than the CN and C2 bands and have different morphologies. The most distinct bands of these molecules appear in the optical to near-infrared spectra of these cool giant stars, between about 680 to 920 nm (see Table 5 of (1) ). A single spectrum with a spectral resolution of R=600, which is achieved by the common Alpy 600 instrument, is fully sufficient to identify and distinguish the molecular bands. The campaign end date is December 31, 2027. All sample Mira stars reach a maximum in their light cycle between the beginning of the campaign and its end. We recommend observing the stars near their maximum light to maximise the signal-to-noise ratio (SNR). The date of the next maximum of each star is given in Section “Additional input for observers”. We predict the date based on the last maximum in the ASAS-SN, the KWS survey (http://kws.cetus-net.org/~maehara/Vsdata.py), or the ZTF survey and the period determined by Uttenthaler & Merchan-Benitez (2025). Note that this prediction can be uncertain in some cases. Therefore, we recommend visiting the targets a month or so before the predicted maximum. Maxima in parantheses in “Additional input for observers” fall in the annual observing gap, and observers might want to try observing just before or after the gap. The stars reach a maximum V-band brightness of between 12 and 14 mag (one is predicted to reach only 14.4 mag). However, the V magnitude is not so informative because all objects are very red and have much more flux in the spectral region of interest (680-930 nm, see public notes). Furthermore, a modest SNR of ~20-30 is sufficient to reliably identify the molecular bands in the spectra. Therefore, we encourage the observers to take spectra of the targets, even if they are relatively faint.
Requested Data Types:
Spectroscopy
Targets
| Name | Magnitude | Variability Type | Photometry Notes | Spectroscopy Notes |
|---|---|---|---|---|
| ASAS J072838-4705.2 | 13.10 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| V0790 Mon | 11.80 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J083300-2354.7 | 11.73 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J060913-2743.1 | 12.73 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| BD Pyx | 11.50 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J155913-2444.1 | 11.08 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J093523-2944.1 | 12.90 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| V0796 Mon | 11.40 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J074543-4404.3 | 11.61 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J182052-4331.9 | 12.87 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J191421-0519.1 | 15.93 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J084558-2945.1 | 11.20 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J080722-5322.4 | 13.00 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J184428-1509.3 | 12.86 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J192217-1846.1 | 12.29 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J051341+1159.2 | 13.07 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| V0379 Hya | 12.40 | SRA | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| ASAS J083432-5401.2 | 12.55 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| BI Ant | 11.80 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| EQ Nor | 10.60 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
| VX Aql | 9.80 | M | Not Requested |
Cadence: Monthly Resolution: 250 Desired SNR: 50 |
Spectroscopy Lines:
No spectroscopy lines provided
Photometry Filters:
No photometry filters provided
Comparison Stars:
Finder charts with comparison stars may be created using the AAVSO Variable Star Plotter (VSP).
Co-Authorship
Observers are eligible for co-authorship.
Additional Observer Input:
The table below lists the 21 targets, sorted by the date of the next light maximum. We only need a single spectrum per target – no continuous observation is needed! The targets are relatively faint, but you should not be discouraged by this, as their flux greatly increases towards longer wavelengths. The distinguishing molecular features are also located at long wavelengths. The further into the red the spectrum reaches, the better. If you observe at very low spectral resolution (R=200), please also observe a bright O-rich, late M-type star for comparison, e.g., omicron Ceti, R Leo, R Hya, or R Dor. In any case, please post your results in the forum so we can discuss the results! Name Per (d) Vmax (mag) Next max (JD) Next max (jjjj-mm-dd) ASAS J155913-2444.1 420.3 12.0 2461080.7 2026-02-09 V0796 Mon 399.8 12.0 2461101.8 2026-03-02 V0790 Mon 423.2 13.4 2461118.2 2026-03-18 ASAS J192217-1846.1 290.6 13.0 2461130.6 2026-03-31 ASAS J083432-5401.2 316.6 12.8 2461152.4 2026-04-21 ASAS J093523-2944.1 289.3 13.2 2461157.6 2026-04-27 ASAS J184428-1509.3 338.2 13.3 2461186.2 2026-05-25 ASAS J080722-5322.4 346.7 13.2 2461564.0 2027-06-07 ASAS J191421-0519.1 209.3 13.5 2461204.2 2026-06-12 BD Pyx 294.1 13.4 (2461577.2 2027-06-20) ASAS J074543-4404.3 382.7 12.4 (2461213.6 2026-06-22) ASAS J072838-4705.2 373.2 14.4 (2461230.2 2026-07-08) EQ Nor 297.4 11.2 2461231.4 2026-07-09 ASAS J084558-2945.1 381.4 11.4 (2461244.3 2026-07-22) ASAS J051341+1159.2 356.0 13.1 2461264.5 2026-08-11 BI Ant 322.3 12.0 (2461299.0 2026-09-15) ASAS J182052-4331.9 307.5 13.5 2461306.5 2026-09-23 ASAS J060913-2743.1 347.7 12.5: 2461309.0 2026-09-25 ASAS J083300-2354.7 342.0 12.7 2461339.9 2026-10-28 VX Aql 628.3 10.2 2461348.7 2026-11-04 V0379 Hya 287.1 12.7 2461352.6 2026-11-08
Additional Submission Location:
none
Forum URL
https://forums.aavso.org/t/observing-campaign-917-confirming-candidate-c-stars-cccstars/4419
Notes:
Molecular bands of CN, TiO, VO, and ZrO ideally between about 680 to 920 nm should be observed, but the range 600 to 870 (which also contains the Ca IR triplet) should also be fine. The further into the red the spectrum reaches, the better. More data can be found in: Uttenthaler & Merchan-Benitez, 2025, https://ui.adsabs.harvard.edu/abs/2025A%26A...700A.187U/abstract and Gaia Collaboration 2022b, https://ui.adsabs.harvard.edu/abs/2022yCat.1358....0G/abstract .