Survey of $\hbox {H}{\varvec{\alpha }}$ emission-line stars in the star-forming region IC 5070

Abstract

Actively accreting young stellar objects show H$\alpha$ emission line in their spectra. We present the results of survey for H$\alpha$ emission-line stars in the star-forming region IC 5070 taken with 2-m Himalyan Chandra Telescope. Based on the H$\alpha$ slitless spectroscopy data, we identified 131 emission-line stars in $\sim$0.29 square degrees area of the IC 5070 region. Using Gaia early data release 3, we estimated the mean proper motion and parallax of the emission-line stars. We also estimated the mean distance and reddening toward the region using the emission-line stars, which are $\sim$833 pc and $\sim$2 mag, respectively. By examining the locations of these stars in the color–magnitude diagrams constructed using Gaia and PanSTARRS1 data, we found that a majority of the H$\alpha$ emitters are young low-mass (${<}1.5\, M_\odot$) stars. We also compared our catalog of emission-line stars with the available young stellar catalogs and found that most of them are class ii/flat spectrum sources with the spectral type ranging from K to M. Based on the proper motion/parallax values and locations on the color–magnitude diagrams, about 20 emission-line stars are flagged as non-members. The relative proper motion of the emission-line stars with respect to the ionizing source suggest the possibility of the ‘rocket effect’ scenario in the remnant cloud (BRC 31).

1 Introduction

Stars form from dense molecular clouds in star-forming regions (SFRs) and a majority of these emerge as groups/clusters. The spatial distribution of young stars and gas in SFRs can predict the initial conditions in which these stars are formed (e.g., Gouliermis 2018; Kuhn et al. 2020). In addition to the above, for nearby SFRs, a sample of young stars obtained with the photometric observations can provide a complete knowledge of clustered or scattered populations (Getman et al. 2018; Herczeg et al. 2019; Panwar et al. 2019; Gupta et al. 2021). H$\alpha$ emission is one of the signatures used to identify young T-Tauri stars (TTSs), which arises due to the ongoing accretion process associated with these stars. Though, objective prism surveys for H$\alpha$ emission stars were the first technique to identify large populations of young stars (e.g., Haro 1953; Herbig 1954), with the advent of CCDs combined with grisms, much deeper surveys were possible (e.g., Herbig 1998; Herbig & Dahm 2006; Pettersson & Reipurth 2019). We performed a slitless H$\alpha$ survey for the search of the H$\alpha$ emission stars in the star-forming region IC 5070.

The star-forming region IC 5070 (also known as the Pelican nebula) in Cygnus has received special attention and has become the target of many observational studies for last two decades. It is associated with the Hii region Sh2-117 or W80 (Westerhout 1958). The nebula is adjacent to the North-American Nebula (NGC 7000), which is now considered as a part of the same physical gas and dust cloud (Reipurth & Schneider 2008). The whole NGC 7000/IC 5070 cloud complex, also known as the North American and Pelican (NAP) complex, encloses many active star-forming sites with strong obscuration attributed to the high amount of gas and dust. The main exciting source of the Hii region is 2MASS J20555125$+$4352246 (O3.5f$*+$O8) spectroscopic binary (Comerón & Pasquali 2005; Maíz Apellániz et al. 2016), lying between the NGC 7000 and IC 5070 and is embedded within the highly extincted ($A_V\sim 9.6$ mag) dark dust cloud LDN 935. Though there is one more massive star, HD 199579 (O6.5 V; Sota et al. 2011), it is at larger distance and quite away from the geometrical center of the Hii region, therefore, unlikely to be a main ionizing source.

A number of H$\alpha$ emission-line stars and many Herbig-Haro objects have been found in the region (Herbig 1958; Ogura et al. 2002; Bally & Reipurth 2003; Bally et al. 2014). High sensitivity infrared (IR) and X-ray observations with the Spitzer and Chandra X-ray observatories (Guieu et al. 2009; Rebull et al. 2011; Damiani et al. 2017; Das et al. 2021) reveal several groups/clusters of young stars embedded in the whole NAP star-forming region. Presence of several protostars, cometary nebulae, bright-rimmed cloud (BRC) 31 and outflows driven by embedded young stars indicates ongoing active star formation throughout the region (Ogura et al. 2002; Bally & Reipurth 2003; Bally et al. 2014). Various studies yield different distance estimates for the NAP complex, ranging from about 500 pc to above 1 kpc (Laugalys et al. 2007; Reipurth & Schneider 2008; Guieu et al. 2009), with recent estimate based on the Gaia DR2 data placing it at $\sim$800 pc (Bhardwaj et al. 2019; Zucker et al. 2020). Though located adjacent to the massive star-forming regions of Cygnus X, NAP is reported to be nearer than Cygnus X (Kuhn et al. 2020). Recent studies on the clustering and kinematic analyses of the NAP complex show that it contains several groups of very young stars associated with the observed molecular gas, which are at different distances, spanning differences up to 150–200 pc (Fang et al. 2020; Kuhn et al. 2020; Das et al. 2021).

Fig. 1

Left panel: DSS2R image of the IC 5070 region. Area enclosed with the green polygon represents the coverage of the present survey of H$\alpha$ emission-line stars with the HCT ($\sim$0.29 square degrees). The red circles represent emission-line stars identified in the present work. Right panel: Color-inverted slitless H$\alpha$ image for one of the sub-regions. Stars with enhanced H$\alpha$ emission are marked with the red circle.

The Pelican Nebula is among one of the ideal site to study the influence of massive stars on subsequent star-formation activity and evolution of the parental molecular clouds. However, the optical photometric studies of PMS stars in the region are available only for a limited sample (Findeisen et al. 2013; Poljančić et al. 2014; Ibryamov et al. 2018; Bhardwaj et al. 2019 and references therein). The BVRI photometry for a sample of 17 pre-main sequence (PMS) objects was presented by Poljančić et al. (2014) in the field of the NAP complex, while Froebrich et al. (2018) found two new low-mass young stars with deep recurring eclipses in the IC 5070.

The strong radiation field and the expanding Hii region have cleared away some of the cloud surrounding the IC 5070 region, making part of the young stellar content visible at optical wavelengths. Such visible young stars are distinguishable based on H$\alpha$ emission line in their spectra, which is caused by ongoing accretion processes from a circumstellar disk through funnel flows onto the star (Hartmann et al. 2016). Since the accretion of matter on the stellar surface is episodic, the H$\alpha$ emission is thus not stationary and is therefore, expected to be variable, although the timescale of variability is poorly known (Bhardwaj et al. 2019; Das et al. 2021; Ghosh et al. 2022). An advantage of using H$\alpha$ emission to identify young stars is that even stars with little circumstellar material, which are otherwise difficult to be identified as young through infrared photometric observations, can exhibit H$\alpha$ emission. H$\alpha$ emission-line surveys and IR surveys therefore, to some extent complement each other.

Slitless spectroscopy is a very efficient method to identify H$\alpha$ emission line sources compared to slit spectroscopy or IFU surveys, which are time consuming and expensive for telescopes. In the present work, we carried out slitless H$\alpha$ spectroscopic survey of $\sim$0.29 square degrees region in the IC 5070 region to search for the H$\alpha$ emission-line stars. The organization of the paper is as follows. Section 2 present the observations, data reductions and the ancillary data obtained from published young star catalogs, Gaia early data release 3 (EDR3) and slitless spectroscopy used in this study. In Section 3, we describe the methods used to identify stellar members and characterization of the emission-line stars using photometric and kinematic information based on the Gaia EDR3 and PanSTARRS1 (PS1) data. Finally, Section 4 summarizes the main results of the present work.

2 Observations and data reduction

2.1 Optical slitless spectroscopy

We obtained slitless observations in H$\alpha$ for the IC 5070 region (see Figure 1, left panel) to detect H$\alpha$ emission line stars toward the region using the 2.0-m Himalayan Chandra Telescope (HCT) at Hanle. The observations were taken during the period of December–January 2012/2013. The details about the telescope and CCD camera can be found in Chauhan et al. (2009) and Jose et al. (2013). The data were acquired using a broadband H$\alpha$ filter (630–674 nm) in combination with Grism 5 (520–1030 nm) in slitless mode with the $2\textrm{K} \times 2\textrm{K}$ CCD. The CCD has a field-of-view (FOV) of $10 \times 10$ $\hbox {arcmin}^2$. The data were taken for about 11 overlapping regions, covering a total sky area ($\sim$0.29 square degrees), shown in Figure 1. Images with a total integration of 1200 s in slitless mode, were taken for each region followed by an image in H$\alpha$ filter. The image in H$\alpha$ was used to identify the coordinates of the emission-line sources. To clean the images, a number of bias and twilight flat-field frames were taken. We have used IRAF software for the pre-processing of the raw images. The H$\alpha$ emitting sources show sharp enhancements in their spectra around 6563 Å over the continuum. Based on this signature, we identified 131 sources having prominent H$\alpha$ emissions from these observations, as shown in Figure 1 (right panel).

2.2 Ancillary data

2.2.1 Gaia EDR3

A sample of young stars obtained with the photometric observations complemented with astrometric information available with the Gaia data can be used to select the young members associated with the region. The Gaia mission has provided rich astrometric and photometric data for the stars. In the present work, we used Gaia EDR3 (Gaia Collaboration 2020) catalog to elucidate the kinematics and parallax of the emission-line stars in the region.

2.2.2 PanSTARRS data

The Panoramic Survey Telescope and Rapid Response System (PanSTARRS) has carried out a 3$\pi$ survey towards the northern sky ($\delta > -30^{\circ }$). The PS1 survey was carried out mainly at Haleakala Observatory in Hawaii in five broadband filters (g, r, i, z, y) from 2010 to 2014 (see, Chambers et al. 2016).

2.2.3 Available young star catalogs

The whole NAP complex is one of the most active star-forming sites and has been a target to study the clustering of young stellar objects and molecular gas. Recently, low-resolution spectroscopic survey of over about 3400 young stars in the whole NAP complex by Fang et al. (2020) revealed about 580 likely members. Das et al. (2021) identified and characterized several young stars in the NAP complex using Spitzer and UKIDSS near-infrared (NIR) datasets. We have also utilized their young stellar catalogs (Fang et al. 2020; Das et al. 2021) within the area covered in the present study.

3 Results and discussion

3.1 H$\alpha$ emission-line stars

Slitless grism spectroscopy enables us to directly examine the point-like H$\alpha$ features of stars over the diffuse background emission (Herbig & Dahm 2001). We identified H$\alpha$ emission-line stars by analyzing slitless spectroscopic images of the 11 overlapping sub-regions, each of $\sim$ $10\times 10$ $\hbox {arcmin}^2$ area. Those stars with enhanced H$\alpha$ emission above the background are classified as emission-line stars. H$\alpha$ emission-line appears as a bright blob superposed over the continuum spectrum seen as a bright vertical line (see Figure 1, right panel). Therefore, sources showing bright spot along the slitless spectra are designated as H$\alpha$ emission stars. In total, we identified 131 emission-line stars in the present work. The spatial distribution of these stars is shown with the red circles in Figure 1. A list of these stars is given in Table 1.

3.2 Kinematics of identified H$\alpha$ stars

We searched for the Gaia EDR3 counterparts of the H$\alpha$ emission-line sources within a match radius of $\sim$2 arcsec and obtained Gaia measurements for $\sim$110 emission-line sources. We used the proper motion in RA ($\mu _\alpha ^\star$) and proper motion in declination ($\mu _\delta$) for the young stars to generate the vector point diagram (VPD), where $\mu _\alpha ^\star \equiv {\mu }_{\alpha }\cos (\delta )$. We restricted only those sources with proper motion uncertainty <0.5 mas $\hbox {yr}^{-1}$ and G-band magnitude uncertainty <0.1 mag. The VPD for those sources is shown in Figure 2. The over-density of sources can be easily noticed in Figure 2. The proper motion of the identified H$\alpha$ emission stars in the region peaks at $\mu _\alpha ^\star$, $\mu _\delta \sim -0.95({\pm }0.28)$, $-3.57(\pm 0.58)$ mas $\hbox {yr}^{-1}$, which is comparable to the value found by Kuhn et al. (2020) for the stellar group ‘C’, in their work, which is at the same location as that of the area, we considered in this study. An area of radius about 3-$\sigma$ (about 2.3 mas $\hbox {yr}^{-1}$) around the mean proper motion values of the H$\alpha$ emission-line stars is used to select the probable members (shown with green ellipse in Figure 2, left panel), and the remaining sources in the VPD are considered as field stars. There were only six emission-line stars, which seem to be non-members based on the VPD.

Table 1 A list of identified H$\alpha$ emission-line stars.

Table 2 Continued.

Table 3 Continued.

We also estimated the distance to the region using the parallax information of the H$\alpha$ emission stars having reliable parallax values ($\varpi /\sigma _\varpi >5$) and are selected as proper motion members. These sources are shown with the red star symbols in Figure 2 (right panel). Except few outliers, the emission-line stars seem to have mean parallax values around 1.2 mas. After rejecting the outliers (i.e., sources with parallax values outside ${\pm }3 \sigma$ of the mean), the mean parallax value estimated for these sources is $\sim$ $1.223 \pm 0.077$ mas. We estimated the distance after correcting the mean parallax value for the known parallax offset of ${\sim }-0.015$ (Stassun & Torres 2021). The distance estimate for the region using Gaia data of emission line sources comes out to be ${\sim }833 \pm 52$ pc, which is in agreement with that reported in previous works (e.g., Bhardwaj et al. 2019; Fang et al. 2020; Kuhn et al. 2020).

Fig. 2

Vector point diagram (left panel) and parallax vs. magnitude (right panel) for the stars in the IC 5070 region. Black dots represent the sources within the region in Figure 1 and red star symbols are the H$\alpha$ emission line sources detected in the present survey. The green ellipse show the three-$\sigma$ boundary around the mean proper motion values (see text).

Fig. 3

($G_\textrm{BP}-G_\textrm{RP})/G$ Gaia (left panel) and ($g_{\textrm{PS1}}-i_{\textrm{PS1}})/g_\textrm{PS1}$ PS1 (right panel) CMDs for the stars in the IC 5070 region. Black dots represent all the sources within the region in Figure 1 and red stars are the H$\alpha$ emission line sources detected in the present emission-line stars survey. The dashed and continuous curves are the isochrones from PARSEC models for the ages 5 and 10 Myr, respectively, which are corrected for the adopted distance and average reddening of the region.

3.3 Color–magnitude diagrams: Evolutionary stages of H$\alpha$ stars

The evolutionary stages and physical parameters of the young stars can be well constrained using the optical observations (Jose et al. 2017; Panwar et al. 2017). Locations of these stars on the optical color–magnitude diagrams (CMDs) can be utilized to confirm their evolutionary stages and formation mechanisms. We use optical photometry from Gaia EDR3 and PS1 surveys for the CMD analysis of the emission-line stars identified in the present work. Figure 3 illustrate the optical $G/G_\textrm{BP}-G_\textrm{RP}$) (left panel) and $g_\textrm{PS1}/(g_\textrm{PS1}-i_\textrm{PS1})$ (right panel) CMDs for all the stars (black dots) in the IC 5070 region.

Star symbols represent the optical counterparts of emission-line stars in the region. Dashed and continuous curves are 5 Myr and 10 Myr isochrones for the solar metallicity taken from PARSEC (Pastorelli et al. 2020). The isochrones are shifted for the adopted distance (833 pc) and visual extinction $A_V\sim 2$ mag by adopting the extinction ratios mentioned in Wang & Chen (2019). In Figure 3, about 90% of the emission-line stars appear younger (<5 Myr) and a few sources are of ages $\sim$5–10 Myr low-mass stars (${<}$1.5 $M_\odot$). The recent analysis by Pfalzner et al. (2022), shows that the median disk lifetime of low-mass stars is generally in the range of $\sim$5–10 Myr, which is in agreement with the age distribution of H$\alpha$ emission line sources within IC 5070. In Figure 3, there are a few stars having ages >10 Myr. These could be either non-members of the region or the stars formed prior to the formation of ionizing source of the region, with long-lived accretion disks, such as in Peter Pan disks (Silverberg et al. 2020). Detailed spectroscopic analysis is essential to understand the nature of these sources. The Gaia and PS1 CMDs for the emission-line sources detected in the present work, show that most of these are young members of the region with masses <1.5 $M_\odot$, i.e., TTSs. We flagged about 14 emission-line sources with age >10 Myr as non-members.

Based on the astrometric and photometric data from Gaia and PS1, we have assigned flag to the H$\alpha$ emission-line stars cataloged in Table 1. The first digit in the column ‘Flag’ represents the membership status of the star based on the kinematics (PMs and/or parallax), the second digit shows the membership status based on the Gaia CMD and the third digit based on the PS1 CMD. Further, ‘1’ represents membership status ‘yes’, ‘0’ represents ‘no’ and ‘9’ represents ‘not confirmed’. Applying above criteria, we found that there are about 20 H$\alpha$ emission-line stars that are classified as non-members based on the PM/Parallax or Gaia CMD or PS1 CMD.

3.4 Nature of H$\alpha$ stars based on NIR color–color diagram

Young stellar sources exhibit excess emission in NIR wavelengths. Therefore, these sources can be identified and characterized based on their location in NIR color–color (CC) space. In Figure 4, we show the NIR $(J - H)/(H - K)$ CC diagrams for the H$\alpha$ stars. To construct the CC diagram, we have used the J, H, K measurements of the YSOs from Das et al. (2021), that were obtained from UKIDSS data archive. To avoid the chances of inclusion of saturated stars, we also used the 2MASS measurements for the stars, which were brighter than 13.25 mag in J-band. The thick red curve in the CC diagram represents the main-sequence (MS) locus, and the thick blue curve is the locus of the giants (Bessell & Brett 1988). The thin green line shows the classical TTS (CTTS) locus from Meyer et al. (1997). The three parallel black dashed lines drawn from the base of MS locus, the turning point of the MS locus and the tip of CTTS locus are the reddening vectors. All the magnitudes, colors and loci of the MS, giants and CTTS are converted to the Caltech Institute of Technology system and the extinction laws of Cohen et al. (1981) are adopted, i.e., $A_J /A_V = 0.265$, $A_H/A_V = 0.155$ and $A_K /A_V = 0.090$. The sources in the NIR CC diagram are classified into ‘F’, ‘T’ and ‘P’ regions (see Ojha et al. 2004) that are generally considered as ‘MS/evolved field stars or class iii YSO candidates (weak-line TTSs)’, ‘class ii YSO candidates (CTTSs)’ and ‘class i YSO candidates with circumstellar envelopes’, respectively. However, we note that the sources in the ‘T’ region may be reddened early-type MS stars with excess emission in the K-band (Mallick et al. 2013) and there may be an overlap of Herbig Ae/Be stars with the sources in the ‘T’ and ‘P’ regions, which generally occupy the place below the CTTS locus in the NIR CC diagram (for more details, see Hernández et al. 2005).

Fig. 4

NIR color–color diagram for the emission-line stars in the IC 5070 region shown in Figure 1. NIR counterparts of H$\alpha$ emission stars taken from YSO catalog of Das et al. (2021) and 2MASS data are shown with star symbols and triangles, respectively. The thick red and blue curves represent the locus of the main-sequence (MS) stars and the giants, respectively (Bessell & Brett 1988). The thin green line shows the CTTS locus (Meyer et al. 1997). The three parallel black dashed lines drawn from the base of the MS locus, the turning point of the MS locus and the tip of the CTTS locus are the reddening vectors.

The NIR CC diagram of the H$\alpha$ stars clearly demonstrate that 100 H$\alpha$ emission line sources in our list are of classes ii and iii nature, implying that about 50% of the sources show IR excess on the NIR color–color diagram. We also estimated the visual extinction ($A_V$) towards the region by tracing the emission-line stars to the intrinsic locus for the TTSs. The average extinction for the emission-line stars in the region is $\sim$2 mag.

3.5 Comparison with previous YSO catalogs

Das et al. (2021) identified YSOs in the whole NAP complex based on the NIR and mid-IR excess using UKIDSS and Spitzer-IRAC/ MIPS data and found many clusterings of young stars including near IC 5070. They have also classified these young stars as class i, class ii, class iii and flat spectrum sources based on their NIR excess properties. To examine the evolutionary stages of emission-line stars, we also compared our emission-line star catalog with the YSOs catalog of Das et al. (2021) and found counterparts of 107 stars. Out of these 107 YSO candidates, one source is classified as class i, 88 as class ii, 3 as class iii, 14 as flat spectrum sources and one is an unclassified source. Hence, most of the emission-line sources identified in the present work belongs to either class ii or in transition from class i to class ii stage (flat spectrum), indicating that H$\alpha$ emission line surveys are more efficient to detect thick disk bearing sources, i.e., actively accreting CTTS.

To know the spectral type of these emission-line sources, we also cross-matched our catalog to the recent spectroscopically confirmed member and non-member catalogs of Fang et al. (2020). We obtained spectral information corresponding to 96 emission line sources in our list. Most of these emission-line sources are of spectral types from K to M5. One of the emission-line star (#43) is classified as a non-member (see Fang et al. 2020), that has the parallax (0.52 mas) and hence, does not match with the mean parallax value of the young stars in the region. Hence, this is most probably a field star.

3.6 Star-formation history

Expanding Hii region drives an ionization/shock fronts into the neighboring cloud, which may induce the formation of new stars due to the compression of pre-existing cores/clumps present inside the cloud in a process known as radiation driven implosion (RDI; Bertoldi 1989). As the outer part of the molecular cloud is getting ionized by the ionization front, the ionized gas from the molecular cloud flows (photo-evaporative flow) towards the ionizing source. This results the acceleration of cloud in the opposite direction, termed as ‘rocket effect’ (Oort 1954). As the remnant cloud slowly moves away from the ionizing star, an aligned distribution of the young stars formed due to the RDI, can be observed between the ionizing star and remnant cloud (Chauhan et al. 2009).

Small remnant molecular clouds, such as BRCs, globules and elephant trunks, are found to be associated with the recent star-formation activity and, in particular, BRCs are considered as ideal RDI candidates. The Pelican region consists of a bright-rimmed cloud (BRC 31) facing toward the ionizing star and contains many young stars near the bright-rim. Based on the distribution of ionized gas, bright rim, molecular gas and young stars; it is considered as one of the BRCs involved in the triggered star formation (Morgan et al. 2009).

In Figure 1 (left panel), we notice that H$\alpha$ emitters (mostly young stars) show an aligned distribution with respect to the ionizing star. As the kinematic information of most of the H$\alpha$ emission stars in the region is available, we examined the motion of these star with respect to the main ionizing source of the region, 2MASS J20555125$+$4352246. In Figure 5, vectors in red color show the direction of the proper motions of the emission line stars relative to the motion of the ionizing star. The direction of the ionizing source is shown with the blue dotted line. The cyan vector shows the mean proper motion of the emission-line stars relative to the proper motion of the ionizing star. The direction of the relative proper motion of emission-line stars is away from the ionizing source and seems almost parallel with the line joining the ionizing source to BRC 31. Our results are similar to that found by Saha et al. (2022a, 2022b). Recently, Saha et al. (2022a) investigated the BRC 18 region based on the Gaia EDR3 proper motion information and suggested possibility of rocket effect scenario. However, we note that here, we have considered only the proper motions of the stars, and because we lack the radial velocity information of these stars, the above analysis represents only the projected motions of the stars on the sky plane.

Fig. 5

Proper motions of the emission line stars. The red arrows represent the relative proper motion vectors of the $H\alpha$ stars with respect to the ionizing star, obtained from Gaia EDR3. Blue dashed line indicates the directions of the main ionizing star. The cyan vector represents the mean proper motion of the H$\alpha$ stars with respect to the ionizing star.

4 Summary

In the present work, we report the results of our survey of emission-line stars in the star-forming region IC 5070 based on the slitless spectroscopic observations taken with the 2m-HCT. The main findings of the present work are:

• We identified 131 H$\alpha$ emission-line sources in an area of $\sim$0.29 $\hbox {degree}^2$ in the IC 5070 region.

• The Gaia EDR3 kinematic information of the identified emission-line stars suggest that a majority of sources share a similar proper motion, except few outliers. We calculated a mean proper motion values of $\mu _\alpha ^\star$, $\mu _\delta \sim -0.95(\pm 0.28)$, $-3.57(\pm 0.58)$ mas $\hbox {yr}^{-1}$, which are comparable to the previous works. We also used the Gaia EDR3 parallax measurements for these stars to estimate the distance of the region. Our estimated distance value of $\sim$833 pc is similar to that found by Bhardwaj et al. (2019), Fang et al. (2020) and Kuhn et al. (2020).

• Using NIR CC diagram for these emission-line stars, we estimated the visual extinction $A_V \sim 2$ mag towards the region. Based on the CMDs constructed using Gaia and PS1 data, we found that most of the identified emission-line stars are young low-mass (mass <1.5 $M_\odot$, age <10 Myr), which is in agreement with the expected disk lifetime of low-mass mass stars.

• Based on the kinematics and photometry, we flagged the H$\alpha$ emission-line stars as members, non-members and non-confirmed sources.

• The relative proper motions of the emission stars with respect to the ionizing source suggests the possibility of the ‘rocket effect’ in BRC 31.