Observing Campaign #917: Confirming Candidate C Stars (CCCStars)

Observing Campaigns
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View here: American Association of Variable Star Observers

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.

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Some comments on the use of the ALPY600 for this campaign

While an ALPY600 could potentially be used for this survey it is perhaps not as straightforward as it might appear and a number of areas would need to be considered

The ALPY is a fixed range instrument and even if the camera sensor is wide enough to include this wavelength range, the ALPY 600 is not optimised for this region in the far red/IR.

The spectrograph optics are corrected for optimum performance towards the violet end of the visible range so some refocusing of the spectrograph and some loss of sharpness across the range are to be expected.

The grism is blazed for 550nm so efficiency will be significantly lower in this wavelength range.

There is a risk of contamination from the second order, not a problem for the C star due to the low flux at the blue end but potentially significant for a hot star if used for flux calibration. This can be removed by using a long pass order filter (I have used a Baader 610nm long pass filter with success but it is best to avoid dichroic filters which can have bad ripples in the passband)

Unless the telescope optics are fully reflective, they are likely to be poorly corrected over this wavelength range and as a result the focus of these very red targets on the slit potentially significantly different from that of more typical stars. If the target is too faint to guide when on the slit and field stars are used instead, these may be out of focus when the target is in focus, potentially making guiding tricky. The potential effect on the spectrum due to any chromatism in the telescope optics across this spectral range also need to be considered.

There are a number of deep telluric bands in this region. Should these be left in or removed, for example using a telluric template, generated from the star used for flux calibration (response correction) perhaps?

Cheers

Robin

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For what it’s worth, when this paper was posted on astro-ph I examined all the carbon candidates in the list where the GAIA3 spectra were good enough to classify. All were indeed carbon stars, including some previously called M or S type or other. The star SS73 167 = ASAS J183136-1815.4, which had previously been called either a Be star and a ‘conventional Me’ star in the 1970s, looks like a possible SC star, but evidently could be some sort of composite. The GAIA3 spectrum in any case is unconventional.
There are sufficient TiO, carbon, ZrO, and other bands in the spectra between (say) the NaD lines (5900A) out to 7200A or so to separate the M, S, and carbon stars at resolution R of a few hundred.

\Brian

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It would be useful to add RA and DEC in the list of stars. It bothers me to input one star at a time to check whether I can observe it or not from my location. I really don’t understand why you have not added coordinates. It would be simpler to see their location in the sky.

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Is it useful for this campaign to submit rectified spectra?

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Robin, I have an SA-100. I’m aware of the low resolution Issue of the SA100 for this campaign, but I’d really like to read your comments about the optimized region and performance towards the IR part of the spectrum for this grating.

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Looks like some good targets for the LISA-IR…my resolution is about R=750.
These cool stars and others, like R LMi, have rich lines btw 6250A-10000A which my spectroscope can record.

James

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This is the list of stars sorted by RA and with updated periods and magnitude ranges.

VSX name Coordinates (J2000.0) Const. Type Period (d) Mag. range
ASAS J051341+1159.2 05 13 40.85 +11 59 11.8 Ori M 360 12.8 - 15.0 V
ASAS J060913-2743.1 06 09 13.13 -27 43 09.1 Col M 351 12.0 - 15.6 V
V0796 Mon 06 17 00.31 -08 36 08.5 Mon M 398 11.4 - 13.9 V
ASAS J072838-4705.2 07 28 37.52 -47 05 18.6 Pup M 370 13.1 - 17.3 V
ASAS J074543-4404.3 07 45 42.97 -44 04 13.8 Pup M 387 11.6 - 14.8 V
V0790 Mon 08 01 30.47 -10 05 41.9 Mon M 416 11.8 - 16.1 V
ASAS J080722-5322.4 08 07 21.31 -53 22 24.4 Car M 348 13.0 - 16.0 V
ASAS J083300-2354.7 08 33 00.25 -23 54 39.1 Pyx M 345 10.8 - 15.3 V
ASAS J083432-5401.2 08 34 32.29 -54 01 10.9 Vel M 316.5 12.4 - 15.6 V
BD Pyx 08 36 03.60 -19 15 08.6 Pyx M 295.2 11.5 - 15.6 V
ASAS J084558-2945.1 08 45 58.04 -29 45 07.8 Pyx M 385.3 11.0 - 14.0 V
V0379 Hya 09 06 39.02 -19 18 44.4 Hya SRA 285.3 12.4 - 14.9 V
ASAS J093523-2944.1 09 35 23.08 -29 44 05.5 Ant M 288.4 12.9 - 15.3 V
BI Ant 09 42 53.76 -40 12 54.0 Ant M 324.3 11.7 - 15.0: V
ASAS J155913-2444.1 15 59 12.77 -24 44 03.5 Sco M 438.4 11.0 - 15.1 V
EQ Nor 16 25 22.19 -58 27 49.7 Nor M 302 10.6 - 13.7 V
ASAS J182052-4331.9 18 20 52.63 -43 31 56.2 CrA M 321.8 12.6 - <15.1 V
ASAS J184428-1509.3 18 44 28.14 -15 09 18.5 Sct M 341 12.6 - 14.5 V
VX Aql 19 00 09.61 -01 34 56.7 Aql M 647 9.8 - 14.5 V
ASAS J191421-0519.1 19 14 20.95 -05 19 01.1 Aql M 210.9 13.0 - 15.6 V
ASAS J192217-1846.1 19 22 17.26 -18 46 07.8 Sgr M 289.5 11.8 - 14.3 V
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Yes it is not clear to me why the requirement is to measure so far into the IR, perhaps to work where it is brighter? though the dropping sensitivity of the ALPY600 would offset this to some extent.

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As it happens I did try something similar to this campaign with the SA100 many years ago, something suggested by Brian Skiff, though I did not pursue it very far. Here is a poster with the details. (including the use of the order filter I mentioned, particularly useful here with slitless spectroscopy to reduce the sky background)
http://www.threehillsobservatory.co.uk/astro/Classifying_red_stars_using_a_Star_Analyser_VdS_poster.pdf
The sensitivity droped in the IR as the grating is blazed for 550nm and resolution was low as the aberrations of the converging beam setup and field curvature got more severe at the higher dispersion angle but the broad molecular bands are clear.

Cheers
Robin

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Rectifying these cool spectra with broad molecular bands consistently can be tricky because you don’t really know where the continuum goes. Perhaps flatten them along the tops of the band heads ? One advantage of working in this region of the spectrum when doing a relative flux calibration though is that atmospheric extinction is going to be relatively unaffected by air mass

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@spectroscopy This campaign is for you!

@jrfcomet During last night’s spectroscopy meeting, you showed some spectra going out to ~1,000 nm. What setup were you using and was there risk of the first and second order overlapping?

@RGN Thank you for the suggestion to add coordinates. We’ve added this to our backlog of improvements to make to the observing campaign application. At the moment, we are finishing a new application for the Solar SIG so it’ll be a while before we get back to bugfixes and minor improvements.

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I’m using this IR conversion kit for my LISA spectroscope to get R=700 from 6250A-10000A: https://www.shelyak.com/produit/se0096-kit-proche-infra-rouge/?lang=en
This changes the LISA spectroscope (to operate in the IR) in 3 ways:
a.) Replaces the grating with a version optimized (blazed) for IR
b.) Replaces the camera ring to give it an angle (which corrects chromatism).
c.) Adds an order filter (to avoid order overlap).

James

p.s. I just took IR spectra of VX Aql and ASAS J155913-2444.1 last night with my LISA at about R=720; should upload them to AVSpec in next few days.

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For anyone using a Star’Ex LR, There is also an optics kit for it optimised for work in the IR (A grating blazed in the IR, an order filter and an objective lens corrected for the IR)

Cheers
Robin

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Thanks for pointing out the issues with the ALPY600. To be honest, I don’t know the instrument myself, but it is listed as one of the commonly used instruments in the instructions to create campaigns (Observing Campaigns | aavso) and key data in agreement with the campaign’s requirements, which is why I named it. of course, I am happy with any other instrument delivering useful data.
Telluric bands (such as the O2 band at ~760 nm) do not need to be corrected.
Cheers,
Stefan

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Let me post the maxima of Mira targets in this campaign in the next few months:
ASAS J072838-4705.2 10 Jun 2026
ASAS J191421-0519.1 16 Jun 2026
ASAS J080722-5322.4 07 Jul 2026
ASAS J051341+1159.2 22 Aug 2026
ASAS J060913-2743.1 09 Sep 2026
BI Ant 10 Oct 2026

I have checked observability using the ING staralt tool (Object Visibility), assuming an ideal geographical latitude. Please check for your location if the target is observable at +/1 month of maximum (many targets are observable from a southern location).
Clear skies!

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Hi! Rectified spectra are not required. The usual flat-fielding will do. As Robin correctly points out, the spectra will be very complex, without clear continuum. Thank you!

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Excellent! I am looking forward to seeing the spectra!