A closer look at the young stellar group around Sh 2-295

TL;DR

The paper investigates the low-mass young stellar population around FZ CMa in Canis Major OB1/R1 to test for age mixing and the role of supernova feedback in triggering star formation. It combines Gemini/GMOS optical spectroscopy to identify T Tauri stars via Li I $6708$ Å absorption and Hα emission with Gaia DR3 photometry and PARSEC isochrones to derive ages. They identify 29 TTs (including 6 new members) and classify 3 as Classical T Tauri stars, with Li-based ages around $8.1^{+2.1}_{-3.8}$ Myr and CMD ages spanning roughly $1$–$14$ Myr, showing younger stars concentrated near Sh 2-295. The spatial-age pattern supports multiple star-formation episodes, with a younger cohort possibly triggered by the expansion of Sh 2-295, while supernova-driven feedback appears limited in shaping this region.

Abstract

Star formation is governed by multiple physical processes, making it inherently complicated. One excellent example is the Canis Major OB1/R1 Association, whose complex history of star formation is related to different episodes. Three supernova (SN) events potentially altered the environment and impacted star formation and stellar evolution. Prior investigations revealed two stellar groups of different ages associated with GU CMa and Z CMa. This work focusses on identifying the low-mass young stellar population near FZ CMa, located between these two groups and spatially related to the H II region Sh 2-295. Our main goal is to verify whether this group is age-mixed and characterise its physical properties. We analysed multi-object spectroscopic data acquired with Gemini/GMOS to search for typical features of T Tauri stars (TTs) and to determine their spectral types. Lithium absorption line ($λ$ 6708 $\mathring{A}$) was used as a youth indicator, while H$α$ emission was investigated to probe accretion activity. We also derived ages based on optical photometry from Gaia DR3 and compared the projected spatial distribution to diffuse infrared (IR) emission. We identified 29 TTs, including six new members of the association and three Classical TTs (CTTs). The equivalent width of the Li I absorption line suggests an age of $8.1^{+2.1}_{-3.8}$ Myr, while optical photometric data indicate stellar ages ranging from $\sim$1 to 14 Myr. Younger stars are concentrated around Sh 2-295, whereas the older ones are more widely dispersed. We increased the number of known TTs related to the CMa association. Our results support a scenario of multiple star-formation episodes, including a younger group that may have been triggered by the expansion of Sh 2-295. The influence of SN events appears limited in this context.

Figures (11)

• Figure 1: Top panel: optical (Digitized Sky Survey --- DSS2, F+R) image of the region studied in this work. The black squares indicate the GMOS 1-4 fields of Table \ref{['tab: GMOS_fields']}. The green dashed circle shows the Sh 2-295 H II region Sharpless1959. Bottom panel: GMOS R pre-images used to prepare GMOS masks. To facilitate the identification of the coordinate system orientation in each image, the grid lines corresponding to RA and DEC are plotted in grey as dashed and solid lines, respectively. The position of the slits are shown by open blue squares. In both panels, orange lines represent $A_V = 1$ and $A_V = 3$ mag contours (Cambrésy, priv. comm.) and open magenta circles indicate X-ray sources Santos_Silva2018. Note that not all X-ray sources were covered by the GMOS observations.
• Figure 2: Example of a GMOS spectrum highlighting H$\alpha$ emission, and Li I ($\lambda~6708$ Å) and Ca I ($\lambda$ 6718 Å) absorption features. The top-right panel shows the angular position of a TTs (green circle) compared to X-ray sources Santos_Silva2018, confirming it as an X-ray counterpart.
• Figure 3: Example of spectral typing via spectral comparison. The GMOS spectrum, normalised by the flux at 7050 Å, is shown in black in the regions used to calculated $G$ and in grey elsewhere. The pair $A_V = 0.75$ mag, and M2-type stellar spectrum (green line) represents the best fit to the object. For comparison, spectra of M1- and M-3 type stars are shown in blue and orange, respectively. The bottom panel shows the residuals of the comparison between the GMOS object and the M2-type stellar spectrum.
• Figure 4: Thresholds defining the CTT classification following whiteBasri and Barrado. Blue symbols represent our sample; grey symbols are data from Fernandes2015. These criteria were used to classify the CTTs (indicated by circles), WTTs (triangles), and possible WTTs (crosses).
• Figure 5: $W$(Li) from the 6708 Å line vs $T_{eff}$ and spectral type. Symbols and colours are the same as in Fig. \ref{['fig: CTT_class']}. Grey lines are polynomial fits for observed data in other young clusters: Sco-Cen (10–16 Myr), IC2602 (45 Myr) and Pleiades (125 Myr) — see Pecaut2016 and references therein. The orange line represents the expected curve for a population of $8.1^{+2.1}_{-3.8}$ Myr, as calculated by EAGLES v2.