Ten-micron Emission in a Color–magnitude Diagram of M Giant Variables

Space-based infrared astronomy came of age with the mission of the IRAS satellite. In addition to broad-band photometry, this observatory obtained spectra with a Low-Resolution Spectrometer (LRS) within the 8–22 μm range of numerous late-type giants from which the presence of a silicate grain emission feature at 10 μm could be identified (IRAS IRAS Science Team 1986). Noguchi (1990) cross-matched the catalog of IRAS LRS sources with M-type variable stars that had been observed as part of the ground-based Two-Micron Sky Survey (TMSS; Neugebauer & Leighton 1969). This sample elucidated some systematics of 10 μm emission with respect to stellar variability, pulsation period, and a near-infrared color derived from photometry in the TMSS Infrared Catalog (IRC). Since Noguchi (1990) work, the arrival of data from two all-sky surveys make it worthwhile returning to this sample of stars for the purpose of charting the distribution of 10 μm dust emission on the red giant branch in an infrared color–magnitude diagram (CMD). These surveys are the 2MASS Point Source Catalog (PSC; Cutri et al. 2003) comprising JHK_s photometry, and more recently the Gaia Early Data Release 3 (EDR3) parallax data (Gaia Collaboration et al. 2021). They are used here to generate infrared CMDs for the Noguchi (1990) sample of M giants.

The M giants included in the study of Noguchi (1990) were selected by virtue of having been observed with the LRS on IRAS and also as part of the TMSS. Spectral types predominantly range from M0 to M8. These stars are pulsating variables, the great majority being semi-regular or irregular variables, plus some Miras. Periods mostly range from 20–350 days. Cross-matching Noguchi's sample with the Gaia EDR3 and 2MASS surveys gives a set of M giants for which can be constructed both a 2MASS (M_Ks , J − K_s ) CMD and a TMSS diagram showing M_K,IRC versus (I − K)_IRC. Absolute K-band magnitudes and colors were calculated without attempting to correct for interstellar absorption. The (I − K)_IRC colors were derived from the TMSS apparent magnitudes as listed in Noguchi (1990).

The two CMDs derived from this work are shown in Figure 1, with the IRAS diagram in the upper panel and the TMSS version in the middle panel. Different symbols are used to distinguish stars with 10 μm IRAS LRS emission (filled circles) from those without (open circles). Although a (V − K_s ) color provides a more sensitive correlation with spectral type than (J − K_s ) for the latest-type M giants (Houdashelt et al. 2000; Worthey & Lee 2011), the J-band photometry has been chosen here because of variability in the V-band magnitude of long-period pulsating red giants.

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Giants without 10 μm dust emission in LRS spectra are distributed throughout the populated region of the upper and middle panels of Figure 1, however, this is not the case for the 10 μm emission stars. By and large, those M giants with 10 μm emission are brighter than M_Ks  =  −6.0 in absolute magnitude and redder than (J − K_s ) = 1.15 in color. Attempts to designate a boundary within the M-giant region of the CMD, to the left of which no 10 μm dust emission occurs in the Noguchi LRS sample, except for one star, are shown as dashed lines in the upper and middle panels of Figure 1.

In terms of the Gaia photometric system the red giants in the sample have absolute RP magnitudes in the range − 5.0 < M_RP < − 2.0. A color–magnitude diagram of M_RP versus (G − G_RP) does not evince as well-defined a giant branch as those seen in Figure 1. Most of the stars with 10 μm excesses have a Gaia color of (G − G_RP) > 1.10. Effective temperatures given in the Gaia DR3 range from ∼3900 to 3280 K.

Periods are known for the semi-regular and Mira variables in the Noguchi (1990) sample, as well as some of the irregular variables. As a supplement to periods listed by Noguchi (1990), additional values were obtained from Barthes et al. (1999), Percy et al. (2001), Tabur et al. (2009), and Percy (2020). The bottom panel of Figure 1 shows the relation between period P (in days) and absolute K_s magnitude, with the same symbol designations as other panels. Occurrence of 10 μm emission is confined to variables with periods of $\mathrm{log}\ P(\mathrm{days})\gt 1.7$, i.e., P > 50 days. Although not all longer-period variables evinced 10 μm emission in IRAS LRS spectra, the results accord with a 60-day period cut-off in 22 μm dust emission found by McDonald & Zijlstra (2016).

Early indications of the distribution of circumstellar 10 μm emission came from Woolf & Ney (1969), who found that among giants it was restricted to spectral types of M5 and later. The concept of a dividing line in the Hertzsprung–Russell (H-R) diagram between giants with, and those without those mass-loss, dates to the work of Reimers (1977), who used blueshifted Ca ii K₄ circumstellar absorption lines as an indicator of mass loss. The same concept has been applied here to Figure 1, albeit with respect to a different signature of mass loss. Among giant and supergiant stars a number of observational searches have been made for various mass-loss indicators. Ultraviolet and optical circumstellar absorption lines, infrared dust emission, and millimeter and radio emission from circumstellar molecular gas, have all been surveyed (e.g., studies of circumstellar CO emission by Heske 1990; Kerschbaum & Olofsson 1999, and Kemper et al. 2003). With the arrival of Gaia DR3 parallaxes, the evolved stars observed in many of these studies could be placed in color–magnitude or H-R diagrams based on 2MASS photometry, in order to better delineate the stellar parameter space within which various mass-loss indicators are prevalent (see, for example, McDonald et al. 2012, 2017, for such efforts using Hipparcos and Gaia DR1 parallaxes).