Isolated massive star candidates in NGC 4242 with GULP

TL;DR

The paper investigates whether massive stars can form in isolation by identifying 234 young, high-mass field-star candidates in NGC 4242 using HST UV–optical data. It derives ages, initial masses, and extinctions via isochrone fitting under a Milky Way–like extinction law, and analyzes environments relative to star clusters/OB associations with two distance thresholds (74 pc and 204 pc) and two methods that incorporate age information. The findings indicate that 9.8–34.6% of the sample appears isolated with respect to clusters/OB associations under positional criteria, which reduces to 3.2–11.5% when scaled to the total expected massive-star population in the galaxy; even with conservative assumptions, a non-zero isolated massive-star fraction remains. These results imply a small population of young, potentially in-situ formed massive field stars in NGC 4242, marking the first such analysis beyond the Local Group and highlighting the need for higher-resolution data to disentangle true isolation from sparse clustering or runaways.

Abstract

$\textit{Context.}$ There is considerable debate on how massive stars form, including whether a high-mass star must always form with a population of low-mass stars or whether it can also form in isolation. Massive stars found in the field are often considered to be runaways from star clusters or OB associations. However, there is evidence in the Milky Way and the Small Magellanic Cloud of high-mass stars that appear isolated in the field and cannot be related to any known star cluster or OB association. Studies of more distant galaxies have been lacking so far. $\textit{Aims.}$ In this work, we identified massive star candidates that appear isolated in the field of the nearby spiral galaxy NGC 4242 (distance: 5.3 Mpc), to explore how many candidates for isolated star formation we find in a galaxy outside the Local Group. $\textit{Methods.}$ We identified 234 massive ($M_{ini}\geq15M_{\odot}$) and young ($\leq 10$ Myr) field stars in NGC 4242 using the Hubble Space Telescope's Solar Blind Channel of the Advanced Camera for Surveys, the UVIS channel of the Wide Field Camera 3 from the Galaxy UV Legacy Project (GULP) and optical data from the Legacy ExtraGalactic UV Survey (LEGUS). We investigated the surroundings of our targets within the range of projected distances expected for runaway stars, $74$ pc and $204$ pc. $\textit{Results.}$ We find that between $9.8\%$ and $34.6\%$ of our targets have no young stellar groups or massive stars within the threshold radii, making them appear isolated. This fraction reduces to $3.2\%-11.5\%$ when we consider the total number of massive stars expected from the observed UV star formation rate. $\textit{Conclusions.}$ Our results show that there is a small population of young and massive, potentially isolated field stars in NGC 4242.

Figures (10)

• Figure 1: Optical image of NGC $4242$ using the grz filters from the Legacy Survey DR6 Dey2019 and the GULP (F$150$LP in purple, F$218$W in green) and LEGUS (F$275$W in red) mosaics.
• Figure 2: Quality of the PSF photometry for the sources detected by LEGUS in the F$438$W (B) filter. In each panel, black dots refer to the full sample, while red points indicate sources that passed the full selection process as described in Section \ref{['sec:Photometric catalog']}.
• Figure 3: Mass and age distribution of clusters/OB associations identified by the LEGUS collaboration using the theoretical models described in Section \ref{['sec:Star cluster catalog']}. The red lines correspond to the $100 M_{\odot}$ cut-off and the $15$ Myr cut-off used in Section \ref{['sec:Isolation analysis']}.
• Figure 4: Hess diagrams for different combinations of filters. Padova isochrones for different ages are overplotted, using $Z=0.02$, $E(B-V)=0.05$ mag, distance modulus $\mu=28.61$ mag Sabbi2018 and the extinction coefficients from the Gordon2016 model with $R_V=3.1$ and $f_A=1.0$. The black arrow indicates the reddening vector using $A_V=0.5$ mag.
• Figure 5: CMDs used to differentiate stars according to age. Padova isochrones for ages $1,3,5,10,15,20,25,30,40,50$ Myr, metallicity Z$=0.02$, distance modulus $\mu=28.61$ mag and intrinsic E(B-V)$=0.05$ mag are superimposed. The extinction coefficients come from Gordon2016 model with $R_V=3.1$ and $f_A = 1.0$. Stars are color-coded by the age determined by their positions between consecutive isochrones in each CMD. The black points are stars that do not lie in between consecutive isochrones. The dashed red line shows the luminosity of a main sequence star with $M_{ini}=15 M_{\odot}$ and an age of $10$ Myr. The black arrow indicates the reddening vector using $A_V=0.1$ mag.