Infrared SED Modeling of Velocity-Excess Maser Sources: Identifying Incipient Water-Fountain Candidates

TL;DR

This study tests velocity-excess as a diagnostic for incipient Water Fountain (WF) phases by applying one-dimensional DUSTY SED modeling to 17 circumstellar envelopes with confirmed kinematic asymmetries. The analysis finds seven objects whose outer envelopes are well described by spherical dust distributions, corresponding to an early evolutionary stage where inner jets or tori have formed but the outer CSE remains spherical; among these, five are AGB-like single-peaked SEDs and two are post-AGB-like double-peaked SEDs. Two objects, IRAS 19229+1708 and IRAS 19052+0922, emerge as plausible incipient WF candidates due to notable velocity excesses and central asymmetries within otherwise spherical envelopes. Overall, the results support a morphological sequence in which central bipolar structures develop prior to full outer-envelope disruption, and demonstrate that selecting WF candidates via velocity excess is an effective way to identify objects at the onset of jet formation and early morphological transformation.

Abstract

We investigated whether "velocity excess" in circumstellar maser lines can diagnose the earliest evolutionary phases of Water Fountains (WFs). Here we define "velocity excess" as maser emission (e.g., H$_2$O 22.235 GHz or OH 1665/1667 MHz) detected at velocities outside the velocity range of the OH 1612 MHz line, which traces the terminal expansion velocity of a spherical circumstellar envelope (CSE). Such excess velocities serve as an indicator of gas motions deviating from spherical expansion and may signify the onset of asymmetric outflows. Based on recent studies (Fan et al.2024; Xie et al.2025), we analyzed 17 sources showing velocity excess and fitted their infrared spectral energy distributions (SEDs) with the one-dimensional radiative transfer code DUSTY. Seven sources are well reproduced, implying outer CSEs that remain nearly spherical despite inner asymmetries. Among these, five exhibit single-peaked, AGB-like SEDs and two show double-peaked, post-AGB-like profiles. IRAS variability indices and NEOWISE-R light curves reveal pulsations (~600-1000 days) in three sources, supporting their AGB classification. Considering the magnitude of the velocity excess, two objects-IRAS 19229+1708 and IRAS 19052+0922-may represent the earliest or incipient WF phase, in which asymmetric outflows are beginning to emerge within otherwise spherical envelopes. These results support a morphological sequence in which bipolar jets and tori arise first in the central regions while the outer CSE remains spherical, and they show that selecting WF candidates via velocity excess effectively identifies objects at the onset of jet formation and early morphological transformation.

Figures (8)

• Figure 1: Comparison of galactic extinction curves from 1983MNRAS.203..301H (solid red line) and 2024ApJ...971..127Z (dashed green line). The curves are plotted as a function of inverse wavelength ($1/\lambda$) on the horizontal axis and normalized color excess $E(\lambda - V)/E(B-V)$ on the vertical axis.
• Figure 2: Spectral energy distributions (SEDs) of objects with velocity excess in the OH mainlines, based on 2025ApJ...978..114X. The blue circles show the observed data points without interstellar extinction correction. The red squares indicate the data points corrected for extinction using the law of 1983MNRAS.203..301H (applied only to wavelengths shorter than 8 $\mu$m), while the green triangles represent the corrections using the law of 2024ApJ...971..127Z (see Section \ref{['sec: data Used for Analysis and Interstellar Extinction Correction']} for details). The black lines represent the best-fit models obtained with the DUSTY code (see text for details). The complete dataset is provided in the electronic table accompanying this paper.
• Figure 3: Spectral energy distributions (SEDs) of Water Fountain candidates from 2024ApJS..270...13F. The notations for data points and model fits are the same as those in Figure \ref{['Fig: [spectral energy distributions of objects with velocity excess in the OH 1665/1667 MHz lines]']}; see the caption of Figure \ref{['Fig: [spectral energy distributions of objects with velocity excess in the OH 1665/1667 MHz lines]']} for details.
• Figure 4: IRAS two-color diagram using [12]$-$[25] versus [25]$-$[60] displays the good (red filled marks) and poor (open marks) DUSTY fitting cases from 2025ApJ...978..114X, 2024ApJS..270...13F, and 2017MNRAS.465.4482Y, respectively. The colour indexes are defined as [12]$-$[25]=$2.5\log(F_{\text{25$\mu m$}}/F_{\text{12$\mu m$}})$ and [25]$-$[60]=$2.5\log(F_{\text{60$\mu m$}}/F_{\text{25$\mu m$}})$. The boxes (black dotted lines) are defined by 1988AA...194..125V. The O-rich AGB (grey lines) and post-AGB (pink lines) regions are defined by 2025ApJ...978..114X. The regions LI and RI (black dashed lines) represent two post-AGB sequences 2002AJ....123.2772S2002AJ....123.2788S. Red filled and open squares represent good and poor DUSTY fitting cases from 2025ApJ...978..114X, respectively, while red filled and open circles indicate the same from 2024ApJS..270...13F. Red filled triangles mark well-fitted known WF sources, and open triangles indicate poorly-fitted ones. The open diamonds indicate two WFs (IRAS 15103$-$5754 and IRAS 17291$-$2147) not included in the results of 2017MNRAS.465.4482Y (see the texts).
• Figure 5: WISE light curves (left panel) and periodic analysis results (right panel) for the sources of 2024ApJS..270...13F for which periodicity was detected. The red curve in each WISE light curve represents the best-fit sinusoid based on the Lomb–Scargle periodogram. The corresponding fit results are: 0.65sin(2$\pi$(ft-0.25))+1.53, 1.17sin(2$\pi$(ft+0.15))+3.76, and --0.71sin(2$\pi$(ft+0.02))+1.72, where $f$ is the frequency equal to 1/period. The black vertical lines represent the error bars. In the Lomb–Scargle periodogram, the dashed brown horizontal line indicates the periodogram level corresponding to a maximum peak false alarm probability of 2%, and the red X mark denotes the selected peak.