Preamble

Vol. 66 No. 5

September 2025

ACTA ASTRONOMICA SINICA Vol. 66 No. 5 Sept., 2025 doi: 10.15940/j.cnki.0001-5245.2025.05.007

Search and Property Analysis of Young Stellar Objects in the Star-forming Region IC 5146*

XIAO Fan¹,² WANG Xiao-long³ WANG Fan¹,² FANG Min¹,²† ZHANG Miao-miao¹,²

(1 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023)
(2 School of Astronomy and Space Sciences, University of Science and Technology of China, Hefei 230026)
(3 Department of Physics, Hebei Normal University, Shijiazhuang 050024)

Abstract

The identification of young stellar objects in star-forming regions is essential for a comprehensive understanding of the star formation process. This study uses Gaia (Global Astrometric Interferometer for Astrophysics) astrometric data to identify and analyze young stellar objects in the IC 5146 star-forming region. First, we examine known young star candidates in this region using Gaia astrometric data, confirming 181 candidates as reliable member stars. Based on the parallax and proper motion distributions of these 181 confirmed members, combined with color-magnitude diagrams, we further conduct systematic searches across the sky area covering IC 5146. We find a total of 290 member star candidates, including 167 known members and 123 newly discovered member candidates. Compared with known members, the newly identified candidates have a comparable age (both ~1 Myr) but exhibit lower disk fractions (10% vs. 41%) and lower variability fractions (15% vs. 51%). Spatially, known members are predominantly clustered, while newly identified candidates are more dispersed, reflecting both clustered and isolated star formation modes in the IC 5146 region.

Keywords stars: formation, parallaxes, proper motions, protoplanetary disks, Hertzsprung-Russell and color-magnitude diagrams

1 Introduction

Stars form in molecular cloud cores, with the vast majority forming in clusters. Young Stellar Objects (YSOs) refer to stars that have entered the protostellar phase but have not yet evolved to the main sequence. By analyzing stellar photometric data, we can obtain the corresponding Spectral Energy Distribution (SED) and classify YSOs into four types based on SED characteristics: Class 0, I, II, and III, corresponding to different stages of early stellar evolution. Circumstellar disks are material disks surrounding stars; the gas and dust disks around young stars are called "protoplanetary disks," produced by angular momentum conservation during the collapse of molecular cloud cores into stars. As YSOs evolve, protoplanetary disks gradually dissipate.

Comprehensive searches for YSOs in star-forming regions are crucial for measuring the statistical properties of young star populations and estimating star formation rates. YSOs generally exhibit strong X-ray emission, and some show emission from circumstellar material. Previous studies often used these observational features to search for YSOs. However, without distance measurements, YSO candidates can be contaminated by non-YSO sources such as broad-line Active Galactic Nuclei (AGN), star-forming galaxies, and Asymptotic Giant Branch (AGB) stars. With the release of Gaia satellite data, a large number of YSOs in nearby star-forming regions now have precise distance and proper motion information, enabling more accurate identification of unknown YSOs when combined with these data.

The IC 5146 star-forming region consists of an HII region (also known as the Cocoon Nebula) illuminated by a B0V star BD+463474 and two parallel filamentary dark clouds extending westward. Before Gaia data release, previous studies calculated the distance to IC 5146 using various methods including UBV photometry, spectroscopy, infrared photometry, and modern calibrations of absolute magnitudes and colors, but without a unified answer. IC 5146 has been observed across multiple wavelengths, revealing numerous YSOs. Herbig et al. (2002) identified 380 pre-main-sequence candidates in the cloud region, while Harvey et al. (2008) used Spitzer's IRAC and MIPS cameras to observe the IC 5146 region, including its long filamentary dark clouds. Through color-magnitude diagrams (CMD) and color-color diagrams (CCD), they identified 202 YSO candidates, primarily clustered in two small groups: the Cocoon Nebula and the northwest region. This work builds upon these candidate samples, combining them with Gaia astrometric data for further study of YSOs in IC 5146.

In this paper, we use Gaia astrometric and photometric methods to identify candidate members in the IC 5146 molecular cloud, particularly focusing on the outer regions of previously studied clusters. We also utilize Zwicky Transient Facility (ZTF) survey data, 2MASS, and WISE data to obtain stellar properties of these members. Section 2 describes the datasets used in this work, Section 3 describes the purification of known member samples and the selection and analysis of newly searched members, and the final discussion and conclusions are presented at the end.

2 Data

Our sample search is based on the 380 candidate young stars in IC 5146 from Herbig et al. (2002) Table 1 [TABLE:1] and the 202 candidate young stars from Harvey et al. (2008) Tables 5–7 [TABLE:5]–[TABLE:7]. We use Gaia, ZTF, 2MASS, and WISE datasets to search for and identify YSOs and analyze stellar properties.

2.1 Gaia Dataset

Gaia is the European Space Agency's (ESA) second-generation space astrometry satellite. Gaia's scientific measurements include three main areas: astrometry (measuring stellar positions, parallaxes, and proper motions), photometry (measuring brightness in multiple bands and epochs), and spectroscopy (obtaining radial velocities and astrophysical parameters). Gaia's third data release (Gaia DR3) provides high-precision trigonometric parallaxes and proper motions, enabling discrimination of stars at different distances and rejection of field stars with significantly different motions. Additionally, the satellite obtains photometric data that can be used to identify young stars through isochrone fitting.

We first cross-match the coordinates of the 202 YSO candidates from Harvey et al. (2008) with Gaia DR3 using a matching radius of 2″. Data quality screening is applied to Gaia data by limiting the Renormalized Unit Weight Error (RUWE) to RUWE < 1.4, following standard practices. This initially yields high-precision parallaxes, total proper motions (pm), right ascension proper motion component (pmra), declination proper motion component (pmdec), G-band mean brightness, and other stellar parameters for 123 known members.

Similarly, cross-matching the 380 pre-main-sequence candidates from Herbig et al. (2002) with the Gaia catalog (RUWE < 1.4) provides astrometric and photometric data for 320 candidate sources. Based on the purified known YSO sample, we use Gaia DR3 astrometric and photometric data to search and select sources across the entire IC 5146 sky region (RA: 325°–329°, DEC: 46.867°–48.067°) [22]. Using the parallax and proper motion parameters of known sources, we initially screen the nearly 260,000 sample sources down to 2,321 candidates.

2.2 ZTF Dataset

The Zwicky Transient Facility (ZTF) is an ongoing time-domain survey project. Since 2018, it has conducted efficient survey observations of the northern sky using a 48-inch Schmidt telescope (P48) at Palomar Observatory. The telescope is equipped with a camera with a 47 deg² field of view containing 16 CCDs (each 6k × 6k). This work uses the 18th public ZTF data release (ZTF DR18), which adds two months of observations to DR17, with public data extending to May 7, 2023. The products include 49.6 million single-exposure images, 174,000 stacked images, a source catalog with 76.8 billion source detections extracted from these images, and 4.7 billion light curves constructed from single-exposure images. ZTF provides photometry in g, r, and i bands. We select the r-band mean magnitude and magnitude dispersion from the photometric data to determine whether sample stars are variable.

2.3 2MASS and WISE Datasets

We use 2MASS and WISE photometric data to determine the presence of circumstellar disks around YSOs. 2MASS conducted an all-sky survey in three near-infrared bands (J, H, Ks) using two 1.3 m telescopes, covering 99.998% of the sky. The wavelengths for the J, H, and Ks bands are 1.25 μm, 1.65 μm, and 2.17 μm, respectively, with limiting magnitudes of 15.8 mag, 15.1 mag, and 14.3 mag. Photometric precision for unsaturated bright sources is about 1–2%, and astrometric calibration precision is about 100 mas. The all-sky detection data released by 2MASS includes 4.1 million images and approximately 160 million extended sources.

WISE is an infrared space telescope that observed 99% of the sky in four bands: W1 (3.4 μm), W2 (4.6 μm), W3 (12 μm), and W4 (22 μm). We use the AllWISE catalog, which contains detections of over 700 million sources.

3.1 Purification of Known Member Star Candidate Sample

By cross-matching the known YSO candidate catalog from Harvey et al. (2008) [22] with the Gaia catalog, we obtain astrometric and photometric information for 123 YSO candidates, hereafter referred to as the Harvey sample. Figure 1 [FIGURE:1] illustrates the purification process for this sample.

We perform a Gaussian fit to the parallax distribution of these 123 sources, obtaining a mean parallax of π = 1.3184 mas (corresponding to a distance of 760 pc) and a standard deviation of σ = 0.1320 mas. We limit the parallax to within 3σ (0.9224 mas < π < 1.7144 mas) to reduce contamination from foreground/background stars at significantly different distances, yielding 102 YSO candidates. The parallax distribution after screening is shown in Figure 1(a).

Next, we identify reliable cluster members based on proper motion. Figure 1(b) shows the proper motion distribution of the 102 known sources. We first perform a 2D Gaussian fit to the distribution and construct a 95% confidence ellipse using the mean, standard deviations, and correlation coefficient. Four sources outside this ellipse are initially removed. A second 2D Gaussian fit is performed on the remaining sources, and 91 sources within the 95% confidence ellipse from the second fit are taken as reliable members. The mean proper motion from the second Gaussian fit is pmra = -2.9891 mas yr⁻¹ and pmde = -2.7790 mas yr⁻¹, which we adopt as the cluster's proper motion. This process excludes field stars with significantly different motions from the cluster.

Since these known YSO candidates are in the pre-main-sequence stage, we further analyze the 91 candidates using isochrones. We select theoretical isochrones from the PARSEC library (PAdova and tRieste Stellar Evolutionary Code, Version 2.0S) [31] based on Gaia DR3 data. Using the G and G_RP bands from Gaia DR3 photometry, we calculate absolute G-band magnitudes (M_G) from the sources' parallaxes and apparent magnitudes. We compare the sample with four isochrones at ages of 0.1 Myr, 0.5 Myr, 1 Myr, and 10 Myr, as shown in Figure 1(c). The distribution reveals that almost all known YSO candidates lie on or above the 10 Myr isochrone, except for four sources. Note that these data have not been corrected for extinction; after accounting for interstellar extinction, the sample will shift toward the upper-left in the CMD. We will perform extinction corrections in Section 3.2.

Cross-matching the Herbig et al. (2002) pre-main-sequence candidate catalog with the Gaia catalog yields astrometric and photometric information for 320 candidate sources, hereafter referred to as the Herbig sample. We certify members from this sample using parallax, proper motion, and photometric data to obtain more reliable IC 5146 members.

As shown in Figure 2 [FIGURE:2], the Herbig sample exhibits more dispersed proper motion, parallax, and age distributions than the Harvey sample, indicating greater contamination. The YSO candidates selected by Harvey et al. (2008) using infrared excess are thus more accurate and reliable. Therefore, using the mean parallax (π = 1.3184 mas) and standard deviation (σ = 0.1320 mas) from the Harvey sample's parallax distribution, we retain sources in the Herbig catalog within 3σ of the mean parallax (0.9224 mas < π < 1.7144 mas), reducing the sample to 170 sources. We then retain sources whose proper motions fall within the 95% confidence ellipse from the second Gaussian fit of the Harvey sample, further reducing the sample to 127 sources, as shown in Figure 2(a). The parallax distribution after screening is shown in Figure 2(b). Using G and G_RP photometry, we calculate absolute magnitudes and construct a color (G-G_RP)–magnitude (M_G) diagram, comparing with isochrones at 0.1 Myr, 0.5 Myr, 1 Myr, and 10 Myr, as shown in Figure 2(c). Again, these sources have not been dereddened.

Finally, we merge the 127 certified Herbig sample sources with the 91 Harvey sample sources, obtaining 181 sample sources. In summary, we have used Gaia data to purify the YSO candidates published by Harvey et al. (2008) and Herbig et al. (2002), resulting in 181 known candidate YSOs in the region.

3.2 New Member Star Search and Member Star Property Analysis

In the previous section, we cross-matched the YSO catalogs published by Herbig et al. (2002) and Harvey et al. (2008) with the Gaia catalog, obtaining new high-precision astrometric and photometric parameters to screen YSOs by parallax and proper motion, yielding a subset of reliable candidates. However, the YSO candidate catalog obtained by Harvey et al. (2008) using Spitzer data referenced many earlier studies and searched for candidates within specific regions, biasing the selection toward stars with circumstellar disks and likely missing many diskless young stars. This work uses Gaia data to systematically re-search the entire IC 5146 sky region to identify previously unrecognized YSOs.

We select the entire rectangular sky region of IC 5146 (325° < RA < 329°, 46.867° < DEC < 48.067°) [22] as our target and limit RUWE < 1.4, yielding approximately 260,000 sources from the initial Gaia DR3 search. We first constrain the parallax distribution using the range from known members. Based on the mean parallax (π = 1.3184 mas) and standard deviation (σ = 0.1320 mas) from the Gaussian fit to the Harvey sample, we limit the searched sample to within 3σ of the mean parallax (0.9224 mas < π < 1.7144 mas), reducing the candidates to 24,009 sources.

We then screen using proper motion parameters from known sources. We retain 2,321 sources whose pmra and pmde fall within the 95% confidence ellipse of the Harvey sample's mean proper motion, including 181 known member candidates. Figure 3 [FIGURE:3] illustrates this initial screening process for the newly searched sources, with proper motion distribution shown in the left panel and parallax distribution in the right panel.

Variability is also an important characteristic of YSOs. Previous studies show that YSOs have higher variability fractions than main-sequence stars, and that YSOs with disks have higher variability fractions than diskless YSOs. ZTF time-domain survey data have been widely used to study periodic variability in large samples, including YSO variability properties. We analyze the variability characteristics of stars in the IC 5146 molecular cloud using ZTF multi-epoch photometry. Since r-band photometry is significantly better than g-band and has more observations, our analysis focuses on r-band data.

We extract r-band light curves for all stars in the IC 5146 field from ZTF. Observations flagged as high-cadence or with catflags = 32768 (to exclude cloud or moonlight contamination) are removed. For sources with at least 20 measurements, we calculate the mean r-band magnitude (r̄) and standard deviation (σ_r). Cross-matching the ZTF sample with our searched sample (2,140 candidate sources and 181 known sources) yields data for 1,919 candidate sources and 171 known sources.

Figure 4 [FIGURE:4] shows the relationship between mean magnitude and magnitude dispersion in the light curves. The grayscale background indicates all ZTF sources in the IC 5146 field, red shows newly found candidates, and blue shows known sources. Solid symbols indicate variable sources, while hollow symbols indicate non-variable sources. We divide the sample into magnitude bins of 0.32 mag and calculate the mean σ_r for stars in each bin within 3σ. The cyan solid line in Figure 4 shows the trend of magnitude dispersion versus mean magnitude for non-variable sources, and the cyan dashed line shows our selection criterion (2× the trend), with sources above this line defined as variable (44 new sources, 92 known sources).

Circumstellar disks emit infrared radiation when heated by the central star, making them observable in infrared bands. This project uses WISE W1, W2 and 2MASS J, H bands to identify infrared excess emission and estimate disk fractions. We cross-match the searched sample (2,140 candidate sources and 181 known sources) with WISE and 2MASS data using a matching radius of 2″, obtaining photometry in all bands for 179 candidate sources and 100 known sources. The photometric quality requirements are: W1, W2 photometric errors < 0.2 mag, and J, H photometric errors < 0.2 mag.

Following Wang & Chen (2019) Table 3 [TABLE:3] [42], where A_X denotes extinction in band X, we assume an interstellar reddening law. Using intrinsic colors from Pecaut & Mamajek (2013) Table 6 [TABLE:6] [43] for M5-type stars (int denotes intrinsic color), we derive the reddening direction for M5-type YSOs. From the infrared color excess:

E(J-H) = (J-H)_obs - (J-H)_int
E(H-W1) = (H-W1)_obs - (H-W1)_int

we obtain the reddening vector. Figure 5 [FIGURE:5] shows our method for selecting sources with infrared excess. Stars to the right of the extinction line show infrared color excess, indicating substantial excess emission above photospheric levels at these wavelengths, likely from circumstellar disks. These sources are identified as disk-bearing stars. Sources satisfying both conditions in Equation (1) are classified as having disks:

[H-W1] - σ_{H-W1} > 0.82 × ([J-H] + σ_{J-H}) + 0.39
[H-W2] - σ_{H-W2} > 0.94 × ([J-H] + σ_{J-H}) + 0.86

In Figure 5, some sources in the lower right are considered diskless because they do not satisfy both conditions in Equation (1). Among the newly found candidates, 13/179 (7.3%) host circumstellar disks, while among the purified known sample, 69/100 (69%) host disks.

After calculating absolute magnitudes, we construct color (G-G_RP)–magnitude (M_G) diagrams for the sample sources. Using theoretical isochrones from PARSEC [31] based on Gaia data for different ages (0.1 Myr, 0.5 Myr, 1 Myr, and 10 Myr), Figure 6 [FIGURE:6] shows the distribution of known sources, variable sources, and disk-bearing sources in the CMD. Red circles in Figure 6(a) represent the 2,140 newly searched candidates, while in Figures 6(b) and 6(c) they represent the combined sample of 2,321 sources. Most newly found sources lie below the 10 Myr isochrone, likely being older field stars. In all three panels, 95.6% of purified known YSO candidates, 76.5% of variable sources, and 95.1% of disk-bearing sources lie above the 10 Myr isochrone.

Based on the significant differences in sample fractions above and below this isochrone, we select 10 Myr as the age boundary and retain young sources in the new candidate sample located to the upper right of the isochrone (age < 10 Myr). Since PARSEC models do not extend below the hydrogen-burning limit, we artificially extend the 10 Myr isochrone vertically to include low-mass members. Additionally, because some disk-bearing sources may appear bluer in color due to accretion activity, we retain disk-bearing sources in our CMD selection, yielding a total of 352 sources.

The sources shown in Figure 6 have not been extinction-corrected, which may introduce some bias. We therefore perform further extinction corrections on the selected sources to confirm their status as genuine YSOs. Most of these sources lack spectroscopic data, so we use multi-band photometry for extinction correction. In addition to Gaia DR3 photometry, we search for their counterparts in 2MASS, AllWISE, and Pan-STARRS DR1 catalogs. For sources missing in Pan-STARRS DR1 or lacking certain bands, we further search SDSS DR16 photometry and Gaia DR3 synthetic catalog SDSS photometry, converting SDSS magnitudes to the Pan-STARRS system using transformations from Tonry et al. (2012) [44]. Of the 352 selected sources, 317 have optical to mid-infrared photometry suitable for extinction correction.

We also generate observed colors for different extinction values using main-sequence star spectra from the Pickles stellar spectral library (intrinsic colors of main-sequence and pre-main-sequence stars are similar [45]). We use 11 color combinations: G-G_RP, J-H, H-Ks, Ks-G_RP, W1-G_RP, W2-G_RP, g-G_RP, r-G_RP, i-G_RP, z-G_RP, and y-G_RP. By comparing observed colors with calculated colors, we determine the best-fit extinction value using minimum χ² methods. For YSOs with circumstellar disks, disk infrared excess affects extinction estimates, so we use only 8 color combinations, excluding Ks-G_RP, W1-G_RP, W2-G_RP, and y-G_RP.

Figure 7 [FIGURE:7] shows the CMD after extinction correction. Some sources move below the 10 Myr isochrone after correction. Some of these host circumstellar disks, and their photometry may be strongly affected by variability, with inconsistent observation times between catalogs causing color variations that bias extinction estimates. Additionally, when disk inclinations are large, observed emission primarily comes from disk scattering, making colors bluer and luminosities lower. We retain these sources but exclude 27 diskless sources.

Through these steps, we obtain a final sample of 290 sources, including 123 newly found candidate members (red circles) and 167 known YSO candidates (blue circles). The astrometric (coordinates, parallax, proper motion) and photometric (dereddened magnitudes and G-G_RP colors) properties of these candidate YSOs are listed in Appendix Table A1, with indications of known YSOs and sources with variability or circumstellar disks.

4 Discussion

This work presents a detailed study of IC 5146 based on the Gaia point source catalog. Through membership assessment, we screened nearly 260,000 sources from the Gaia search of the entire IC 5146 sky region, ultimately identifying 290 young star candidates, including 123 new members and 167 known members.

Comparing the positions of member stars in the CMD with theoretical isochrones, Figure 7 shows that most members have ages between 0.1–10 Myr. Figure 8 [FIGURE:8] fits the mean age of both samples. Using PARSEC isochrones (green solid lines), we estimate average ages of ~1 Myr for both new and known candidates, with similar mean ages. Visual inspection of Figure 8 shows that the new YSO candidates have a more concentrated age distribution, strictly between 0.5–10 Myr, while known members show more dispersion. Among the new candidates, 10% (13/123) host circumstellar disks and 12% (15/123) show ZTF variability. For known candidates, these fractions are 41% (69/167) and 51% (85/167), respectively. YSOs with circumstellar disks are more likely to be variable [34–38], consistent with the disk and variability fractions in both samples. The earlier member searches were clearly biased toward disk-bearing YSOs, missing many diskless young stars, which our work remedies. Note that while we certified 181 known YSO candidates earlier, the final sample of 290 YSOs includes only 167 known sources because we excluded 14 sources older than 10 Myr without disks after extinction correction.

Figure 9 [FIGURE:9] shows the spatial distribution of the 290 YSOs, overlaid on the Planck 353 GHz intensity map (tracing material distribution in the star-forming region). Blue circles indicate newly identified members, orange shows purified known YSOs, red plus signs mark variable sources, and cyan boxes indicate disk-bearing sources. YSOs on the left side of the image are relatively concentrated, forming a dense cluster in regions where background material is aggregated. YSOs on the right side are more dispersed, forming spiral arm-like structures in the periphery where gas and dust are sparse. In addition to the three previously identified clusters (green boxes in Figure 9), there exists a dense core region (dark red box) with no current active star formation. Compared with previous work, our newly identified YSOs are mostly located in the outer molecular cloud regions beyond the three clusters, indicating primarily isolated star formation.

We calculate the stellar surface density Σ for individual YSOs following Casertano et al. (1985) [46]:

Σ = (n - 1) / (π r_n²)

where r_n is the distance to the nth nearest star. We adopt n = 6 as the surface density reference, consistent with Gutermuth et al. (2009) [47]. The resulting surface density distributions for both YSO samples are shown in Figure 10 [FIGURE:10]. The median densities are 53.42 pc⁻² for known members and 6.28 pc⁻² for new candidates. Known members have higher surface densities than newly found members, and combined with Figure 9, we see that the new YSOs are more dispersed and uniformly distributed throughout the molecular cloud, while previous searches focused primarily on clusters.

The Gaia DR3 catalog is complete to G ≈ 19–20 mag [48–49]. We adopt G = 19 mag as our sample's completeness limit. At the molecular cloud distance of ~760 pc, this corresponds to an absolute magnitude of M_G = 9.6 mag. Using PARSEC stellar evolutionary models [31] and considering that YSOs in IC 5146 are primarily distributed between 0.1–10 Myr with a concentration around 1 Myr (see CMD in Figure 7), this absolute magnitude corresponds to a stellar mass of ~0.1 M⊙. Using the Chabrier (2003) initial mass function (IMF) [50] with lower and upper limits of 0.1 M⊙ and 80 M⊙, and considering that 211 sources in our sample are brighter than 19 mag, we obtain a total stellar mass M_star ≈ 178 M⊙. Johnstone et al. (2017) [51] measured the IC 5146 molecular cloud mass as M_cloud = 16,000 M⊙ using extinction methods. We thus derive a star formation efficiency (SFE) of SFE = M_star / (M_star + M_cloud) ≈ 1.1%. This value is about twice that of Harvey et al. (2008) [22], primarily because we have identified numerous diffusely distributed YSOs, substantially increasing the YSO sample in IC 5146.

5 Summary

Using Gaia DR3 astrometric data, ZTF time-domain survey data, and AllWISE infrared observations, we have searched for YSOs in the IC 5146 molecular cloud and calculated variability and circumstellar disk fractions for the young star sample. We also investigated age characteristics and spatial distribution differences between known and newly identified YSO members. Our main results are summarized as follows:

1. Based on the YSO member samples summarized by Harvey et al. (2008) and Herbig et al. (2002), we used Gaia astrometry to certify known YSO candidates in the IC 5146 molecular cloud, confirming 181 reliable member stars with mean parallax π = 1.3184 mas and mean proper motion pmra = -2.9891 mas yr⁻¹, pmde = -2.7790 mas yr⁻¹.

2. Using Gaia DR3 high-precision parallax, proper motion, and photometric data, we re-searched the entire IC 5146 cloud region for member certification, obtaining 290 young star candidates (167 known, 123 new) sharing common distances, motions, and ages.

3. Using ZTF data, we identified 100 variable sources from the relationship between mean magnitude and magnitude dispersion. Combining with the AllWISE point source catalog, we identified 82 sources with circumstellar disks via infrared excess. The new sample contains more diskless and non-variable sources.

4. Fitting isochrones to the distance- and extinction-corrected color-magnitude distribution shows both samples have average ages of ~1 Myr, with the new sample having a more concentrated age distribution.

5. Examination of the spatial distribution reveals that previous searches focused primarily on clusters in this region. Combined with stellar surface density calculations, known members have higher surface densities than newly found members, with the new YSOs being more dispersed and uniformly distributed throughout the molecular cloud.

6. Using the Chabrier (2003) IMF, we calculate a total stellar mass M_star ≈ 178 M⊙ in the IC 5146 region. Based on Johnstone et al. (2017) extinction measurements giving a molecular cloud mass of 16,000 M⊙, we derive a star formation efficiency of ~1.1%, twice the previous result.

Our study of IC 5146 suggests that many more candidate members may exist beyond the few previously known YSOs. Statistically, compared with the candidates published by Harvey et al. (2008) and Herbig et al. (2002) (primarily reliable members in three cluster cores), our newly identified YSOs are mostly located in the outer molecular cloud regions beyond the clusters.

Previous YSO searches primarily selected candidates based on infrared excess, which was always contaminated by non-YSO sources and missed many young stars without infrared excess. The Gaia catalog serves as a powerful tool for searching for YSOs and exploring their evolutionary states, advancing our understanding of star formation.

Acknowledgments

We thank the referee for valuable suggestions that significantly improved the quality of this paper. We also thank the Gaia database for its assistance.

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Appendix

Table A1 Properties of YSO candidates in the IC 5146 molecular cloud

The full table continues with 290 entries listing the astrometric and photometric properties of all candidate YSOs identified in this study.