Gas content and evolution of a sample of YSO associations at 𝑑 ≲ 3.5 kpc from the Sun

Young Stellar Objects (YSO) are newly formed stars from molecular clouds. They stay close to where they were born and serve as good tracers to study gas and star formation. During cloud evolution, young massive stars can disrupt the surrounding gas through stellar feedback, changing the gas distribution. We study the distribution of the gas around a sample of YSO associations located at 𝑑 ≲ 3 . 5 kpc from the Sun by comparing the location and morphology between 12 CO (J = 1 − 0) emission, Planck 870 𝜇 m maps and YSO associations. Based on the spatial distribution of the gas compared to that of the YSOs, we classify the YSO associations into six types: direct, close, bubble, complex, diffuse, and clean associations. The complex associations are large structures consisting of both gas-rich and gas-poor segments. We study the velocity dispersion-size relation toward different association types. From the ratio between different types, we estimate a feedback time of ≈ 1.7 Myr in the solar neighborhood. The sample sets a solid foundation to explore the relationship between interstellar medium evolution, star formation, and Galaxy structure.


INTRODUCTION
Stars form in molecular clouds, whose collapse is driven by gravity (McKee & Ostriker 2007) yet affected by other processes like turbulence (Larson 1981;Vazquez-Semadeni et al. 2000;Mac Low & Klessen 2004), magnetic field (Li et al. 2014) and stellar feedback (Krause et al. 2013;Krumholz et al. 2014).Instead of being alone, stars form in a clustered way (Larson 1995;Abt 1983;Lada & Lada 2003).Stars born in the same cloud would retain some properties from the parent molecular clouds.At a smaller scale, the spatial distribution of young stars appears to be structured, with clusters being associated with the high-density regions in clouds (Lada et al. 1993;Zinnecker et al. 1993).
In recent years, accurate parallax measurements and photometric measurements of young stars provided by Gaia satellite (Gaia Collaboration et al. 2018, 2023), have revealed a picture of a dynamically evolving galactic disk.Zari et al. (2018) provided a study of the star formation region in the solar neighborhood using the young stars from Gaia DR2.Zhang (2023)derived the distance of 63 molecular clouds by matching them with over 3000 YSOCs (YSO cluster).Besides the location study, Großschedl et al. (2021) studied the relation between the mean velocity of YSOs and that of the gas in the Orion molecular cloud, and found a good consistency, proving that YSOs are robust tracers of the cloud kinematics.Ha et al. (2022) use the 6D measurements of locations and velocities of young stars in Orion, Ophiuchus, Perseus, and Taurus star-forming region and studied the turbulence by building the velocity structure function.Using kinematic information obtained towards young stars, Zucker et al. (2022) studied the structure and expansion kinematics and star formation of gas surrounding the Local Bubble.As the young stars evolve in the association, they start to disrupt the ISM in nearby clouds, which might lead to a complete removal of the gas (Fall et al. 2010;Draine 2011;Chevance et al. 2022).During this process, the changing gravitational potential leads to the final dissolution of the associations (Krause et al. 2016).By studying the distribution of young stars with respect to that of the gas, we can obtain a systematic view of cloud evolution and gas removal.
We are now working on the ISM-6D program, in which we combine the CO map (Dame et al. 2001), dust map from Planck (Planck Collaboration et al. 2014), and Gaia astrometry measurements to study the structure and full kinematics of ISM (interstellar medium) (Gaia Collaboration et al. 2016).The 2D velocities (proper motion in the  and  directions) and locations of YSOs are from Gaia astrometry measurements.The CO data from Dame et al. (2001) provides the radial velocity as the third velocity component.In the program, we use the YSO association to trace molecular clouds.The YSO associations are groups of YSOs clustered in both the spatial and the kinematic space.In Zhou et al. (2022) we constructed a sample of 150 YSO associations.In this paper, we measure the distribution of gas in associations and study the relationship between the spatial distribution of the gas and the YSOs.We classify the YSO associations based on the similarity between the spatial distribution of the gas and that of the YSOs.We reveal a diverse range of ways through which they relate, hinting at a picture that as star formation precedes, the molecular gas is gradually removed by stellar feedback.We link YSO association types with their location in the Galaxy and in the velocity dispersion-size plane, which is a first step in constructing a sample of YSO-giant molecular cloud (YSO-GMC) complex with complete kinematic information.

YSO associations
We start with a YSO association catalog produced in our recent work (Zhou et al. 2022).We use a YSO sample from Marton et al. (2016), which contains 133980 class I/II YSOs using the Support Vector Machine method.After cross-matching the YSO sample with the Gaia DR2 data set, we applied Dendrogram method (Rosolowsky et al. 2008) to the YSO density map in , , log() space, which gives us 150 structures, including less compact branch structures and their high-density substructures.These spatially and kinematically clustered YSOs are called YSO associations.Each association contains the following parameters: location , , , ,  and , mean proper motion pm  and pm  , 2D velocity dispersion, as well as size Zhou et al. (2022).

Gas tracers
To study the gas content of the YSO associations, we use a map of 12 CO ( = 1−0) line presented in Dame et al. (2001), and a map of 870m emission produced by the Planck satellite (Planck Collaboration et al. 2014), both of which directly trace the cold molecular gas.
The 12 CO ( = 1−0) rotational line is a good tracer of the cold, molecular gas.CO data from Dame et al. (2001) is a composite CO map containing data from mostly the CfA 1.2 m telescope.The survey covers the Galactic plane at resolutions ranging from 9 ′ to 18 ′ .The map is velocity-resolved with a resolution of 1.3 km s −1 .This enables us to distinguish different clouds along the line of sight and measure the radial velocities of the clouds of interest.Although there are other newer CO surveys with higher resolutions, the complete coverage of data from Dame et al. (2001) makes it most suitable for our study.
The Planck dust map we use is taken at 857 GHz (Planck Collaboration et al. 2014).The Planck observations contain nine frequency bands, including 30, 44, 70, 100, 143, 217, 353, 545, and 857 GHz.Maps at lower frequencies are used to study the CMB fluctuations (Planck Collaboration et al. 2014).The 870m map from the Planck satellite is dominated by emissions from cold dust in molecular clouds, making it a good tracer for clouds.Compared to the CO map, the Planck dust map has a slightly higher resolution of 5 ′ .One major benefit of the map is its complete, all-sky coverage, making it particularly useful for high-latitude regions where the CO map is incomplete.

Gas counterparts of YSO associations
We aim to study how gas is distributed around the YSOs in the associations.To achieve this, we plot the member YSOs on both the CO map and the Planck map, which can trace the gas.The CO map from Dame et al. (2001) contains velocity information.At a given location, different clouds appear as different velocity components.This velocity information is useful as the YSO associations are likely to be associated with one of these velocity components, and we aim to find the appropriate component at a similar location on the sky plane.To achieve this, towards the CO data cube, first, we integrate the CO emission over a box-like area in the position-position space.The area is the footprints of the YSO associations 1 .The integrated emission is then plotted along the radial velocity axis, producing the line profile and showing gas distribution at different velocities.
The resulting CO line profiles would contain several Gaussianlike components if there are several different clouds along the line of sight, and the corresponding cloud should appear as one of these components.In our second step, we perform decompositions to the integrated line profile.The decomposition is performed using the Gaussian Mixture algorithm in python (Lindsay 1995;Pedregosa et al. 2011), where we set the maximum Gaussian component number  and input the line profile.The algorithm fits the line profile using 1 to  Gaussian components, where the optimal decomposition output is selected using the Akaike Information Criterion (AIC), which estimates the relative error between the data and the fitting model.The one with the smallest AIC value is the one we chose.
Based on the decomposition results, we produce maps of the CO emissions into different velocity components and select the corresponding cloud by comparing the spatial distribution of YSOs with the morphology of the gas or dust emission (details and examples are available in Appendix A).
The Planck dust map contains no velocity information.We plot the member YSOs in each association on the dust map directly and compare the dust distribution and morphology with the member YSOs in the - space through visual inspections.

Classification criteria
The relationship between the spatial distribution of the YSOs and that of the gas can be diverse.We classify the YSO associations into 6 categories: (1) if the YSO association has a well-defined counterpart seen in the CO map or the Planck dust map, it is classified as Type 1 -direct YSO association; (2) if the YSO association is located outside of a giant molecular cloud seen from the CO map or the Planck dust map, it is classified as Type 2 -a close YSO association; (3) if the YSOs in the association appear to stay inside a bubble-like structure in the CO map or the Planck dust map, it is classified as  (2001).The CO data is integrated through the velocity range of the optimal component identified using the procedure described in Sec.2.2 for Type 1, 2, 3, and 4 and integrated over all the velocity channels for Type 5 and 6.The grid-like artifacts in the CO map for Type 6 are caused by the incomplete sampling of the legacy observations.
Type 3 -bubble YSO association; (4) for large associations, the relationship between the YSO and gas is hard to quantify, as both the gas and the YSOs have complex, patchy distributions.This kind of association is considered a Type 4 -complex association.
(5) if some diffuse gas appears in the vicinity of a YSO association, it's classified as Type 5 -diffuse association; (6) if no gas is detected around a YSO association, it's classified as Type 6 -clean association; We note that several associations are hard to classify due to sparsely sampled data from legacy observations, and they are designated as unclassified (marked as Type 7 in the tables) and discarded in the later analysis.For Type 2 close and Type 3 bubble associations, we consider them as adjacent associations due to their relation with gas.A summary of the classification criteria can be found in Table .1, and examples for each association type can be found in Fig. 1.

Diversity
The first discovery is that the YSO associations have a wide relation with nearby gas, as shown in Fig. 1.We classify these associations based on their relations with the gas.Among all our associations, 80.7 per cent structures are related with their surrounding gas, which contains the Type 1 direct associations (55.3 per cent), Type 2 close associations (4.7 per cent), Type 3 bubble associations (8 per cent) and Type 4 complex associations (12.7 per cent).In these complex associations, part of them can be associated with gas while part of them are gas-poor.They are usually the maximum structure in our YSO associations.Nevertheless, we divide their substructures into different categories whenever possible.
Associations less related to gas take up 15.4 per cent of all the associations.These include structures where only a diffuse gas component is detected (Type 5 diffuse associations: 10.7 per cent) and structures where no gas or dust counterpart exists (Type 6 clean association: 4.7 per cent).

Evolution
Among the 6 types of YSO associations, based on the size of the structures, we present two separate evolution sequences for cloudscale and supercloud-scale YSO associations in Fig. 2 and Fig. 3.The size distributions for associations in these two evolution sequences are shown in Fig. 4. The mean size for evolution sequence 1 is 35.72 pc, and for evolution sequence 2 is 81.77pc.
For the small, cloud-sized structures, the evolution sequence goes as follows (Fig. 2, evolution 1): at the beginning, all the YSOs tend to be associated with molecular gas, making them the Type 1 direct association.As the young massive stars evolve in the association, the nearby gas is partially destructed or expelled by the stellar feedback, leading to Type 2 close associations and Type 3 bubble associations.
For some supercloud-size associations, we propose the following evolution (Fig. 3, evolution 2): it starts with Type 4 complex associations, where the gas is still abundant, and these structures can evolve into Type 5 diffuse associations, and finally into type 6 clean associations.This distinction is still suggestive, and further studies are needed.

Locations in the velocity-size (𝜎 𝑣 − 𝑟) diagram
Turbulence is one of the most important controlling factors of molecular cloud evolution.A convenient way to study turbulence is to plot the velocity dispersions of different structures against their sizes.This plot was made by Larson (1981) and has been called the "Larson relation" ever since.In Zhou et al. (2022), we derived the velocity dispersion-size relation towards all our YSO associations, where we found a slope of 0.67 which is steeper than the 0.38 found in early studies (e.g.Larson 1981).
In Fig. 5, we plot the different types of associations on the velocity dispersion (  ) -size () plane.The size and velocity dispersion information toward each association is present in Tab. 2. The sizes have been estimated in our last work (Zhou et al. 2022).It refers to the FWHM (full width of half maximum) of the spatial distribution for the YSO association.  is the standard deviation of the 2D velocities for a certain YSO association using proper motion in  and  directions.
In Fig. 5, different types of YSO associations occupy different parts of the diagram.At smaller scale (r ≲ 50 pc), the majority of the YSO associations are either directly associated with the gas (Type 1 direct association), or partially associated with the gas (Type 2 close & 3 bubble association), and they follow the relation of   ∼  0.68 as found by Zhou et al. (2022), which is steeper than the green line found in Larson (1981).On the large side, the velocity dispersions of the associations reach the range of 10 − 30 km s −1 .This size range is dominated by Type 4 complex, Type 5 diffuse, and Type 6 clean associations.Among these included in the evolution II sequence, there tends to be a velocity dispersion increase from Type 4 complex, Type 5 diffuse to Type 6 clean associations.

Locations in the Galaxy and on the sky
To study the distributions of different types of YSO associations in our Galaxy, we use a three-dimensional dust map from Lallement et al. (2019).In Fig. 6, we plot the dust distribution in the Galactic - plane, upon which the locations of the different types of YSO associations are overlaid.We notice that there is a group of Type 4 complex, Type 5 diffuse, and Type 6 clean associations distributed on the right of Fig. 6, and most Type 1 direct, Type 2 close, and Type 3 Bubble associations are located on the two filaments near the Sun.We mark these filaments in Fig. 9 and make a discussion in Sec.4.3 In Fig. 7, we plot the locations of different types of YSO associations on the Planck dust map to show their location on the sky plane.At higher Galactic latitudes, our YSO associations match with some well-known molecular clouds like Perseus, Taurus, and Orion molecular clouds.We also recovered some small clouds at high latitudes.The majority of our YSO associations stay close to the Galactic mid-plane.

Gas mass and virial parameter
In Sec.2.2, we find the CO counterpart for most YSO associations, especially for gas-rich associations: Type 1 direct and Type 4 complex associations.This gas-rich property provides us with an opportunity to study the gas mass in the associations.As the widely used tracer for H 2 molecular gas, CO emission can be converted into the column density of Hydrogen gas using the factor (CO).We adopt  (CO) = 2 × 10 20 cm −2 (K km s −1 ) −1 (Bolatto et al. 2013).The gas mass is calculated using:  =  CO ×  (CO) × m H 2 × 1.35A, where  CO is the CO emission,  H 2 is the molecular mass of Hydrogen and A is the area defined in by [ mean, ± 3std(  ),  mean, ± 3std(  )], where  mean, and  mean, are the mean galactic latitude and longitude for association , std(  ) and std(  ) are the standard deviations of galactic latitudes and longitudes for association .The multiple factor 1.35 is due to the metallicity of the molecular gas.To calculate the gas mass without background contamination, we just use the emission of the chosen gas component (more details about CO components in Appendix.A).We integrate the CO emission of the gas counterpart between   −std(  ) and   +std(  ), where   is the centroid velocity for association  and std(  ) is the velocity dispersion of the associated component for association .
After having derived gas mass for those YSO associations, we study the importance of gravity in the molecular gas by estimating the virial parameter  vir = 5 2 /G (Bertoldi & McKee 1992), where  is the 1D velocity dispersion ( =  2D / √ 2, using our  2D derived from YSO proper motion in  and  directions),  is the  size of the YSO association,  is the cloud mass derived from CO emission and  is the gravitational constant.The virial parameter describes the ratio between kinetic energy and gravitational energy of a molecular cloud.A cloud with  ≤ 1 is gravity-dominated and is likely to collapse.We plot the virial parameter versus the gas mass in Fig. 8, where nearly all presented types of YSO associations have virial parameters larger than 1, showing subcritical properties.This is consistent with previous studies that molecular clouds tend to be gravitationally unbound (Heyer et al. 2009

Timescale of gas displacement by stellar feedback
Although YSOs are very young objects born from molecular clouds, and one would naturally expect them to be spatially associated with gas, we observe some associations that are associated but not fully associated with gas.These include the Type 2 close and Type 3 bubble associations.In those cases, the member YSOs are located at the edge of clouds or inside bubbles.We propose that the displacement and decreasing gas fraction are likely to be the results for Type 1 direct associations after gas removal by the massive stellar feedback.The fraction of feedback-affected YSO associations (Type 2 close & Type 3 bubble associations) can be used to infer the feedback timescale.We set a size range of 5−50 pc for the associations used in the calculation.
Towards the association numbers with sizes ranging between 5 -50 pc, there are 62 Type 1 associations, 5 Type 2 associations, and 8 Type 3 associations in the sample, respectively.Considering the uncertainty of the YSO age, we adopt an upper limit lifetime of 10 Myr for YSOs.We use the ratio between different types among gasrelated associations to estimate the time that takes for stellar feedback to displace gas: ≈ 10 Myr × 0.17 ≈ 1.7 Myr .
(1) This upper limit of the feedback time is comparable to a short disruption time of about 1.5 Myr estimated by Kruĳssen et al. (2019) towards a face-on, star-forming disc galaxy NGC300.Compared to the typical age of several million years of YSO (Dunham et al. 2015) and a 30-Myr lifetime of molecular clouds (Bash et al. 1977;Elmegreen 1991;Larson 1994;Kawamura et al. 2009), the feedback time, which is around 17% of the upper limit of the YSO time, is quite short.A timeline is summarized in Fig. 2. Considering the location of our sample, this estimation only represents the feedback time in the solar neighborhood.

Gas-free YSO associations
Our analyses also reveal several Type 6 clean associations that contain little or no gas.Their location can be seen in Fig. 6 and Fig. 9.This lack of gas is examined and confirmed again using the 3D dust map from Chen et al. (2019), which allows us to compare the YSO association with dust distribution in 3D.
Type 6 clean associations represent a class of YSO associations where the gas of the ambient cloud is expelled.From a map of the locations of these associations in our Galaxy (Fig. 9), we find that many gas-poor associations reside in large, superbubble-like cavities, consistent with their gas-poor nature These gas-free structures occupy a higher part of the velocity dispersion size when compared to the Type 4 complex and Type 5 diffuse associations at similar sizes.This higher velocity dispersion of the gas-free indicates energy injection during the evolution, which can be caused by gas removal and stellar feedback.
Simulations studying the effect of residual-gas expulsion in Baumgardt & Kroupa (2007) showed radially anisotropic velocity dispersions increase for star clusters with gas removed within the initial crossing time.In Pang et al. (2020), they have found the increasing trend of the 2D velocity dispersion in some young star clusters under the gas expulsion.This has been mentioned when Goodwin & Bastian (2006); Krause et al. (2016) studied the gas expulsion in the young star clusters.Also, the increased velocity dispersion is found in simulations when there is supernovae and stellar feedback added (e.g.Bending et al. 2022).

Variations across Galactic-scale filaments
Gas in the Milky Way is organized in kpc-sized filaments.Early studies like Li et al. (2013), Goodman et al. (2014) and Ragan et al. (2014) have revealed these giant coherent molecular structures in observations.Alves et al. (2020) and Li & Chen (2022) have studied a 2.7-kpc dense filament structure called Radcliffe Wave in solar neighborhood (pink square in Fig. 9), which contains most of the nearby star-forming regions.
In Fig. 9, we plot the distribution of dust in the Galactic disk plane, where we mark three Galactic-scale filament structures, including the Radcliffe Wave, a filament studied in Chen et al. ( 2020) (Lower Sagittarius-Carina Spur) and a smaller filament between them, called the "Split" in Lallement et al. (2019).Combined with Fig. 6, we found that the gas-richness and association types of different Galactic-scale filaments vary significantly.For example, the Radcliffe Wave contains mostly gas-rich associations, yet the Lower Sagittarius-Carina Spur filament (Chen et al. 2020;Kuhn et al. 2021), which is a kpc-size gas filament located at the Sagittarius arm, containing a significantly higher fraction diffuse, or clean YSO associations, indicating a lot of stellar feedback.More studies about star formation and filament structures and properties are needed to help us better understand the difference between these filaments.

CONCLUSIONS
Combining the YSO associations, CO data and Planck dust map, we study the surrounding gas for a sample of 150 YSO associations and classify these associations based on their relation with the gas.Our findings include: (i) Diversity.The YSO association sample sources have diverse relations with the surrounding gas.Based on their relations with gas, they are classified into six types: (1) Type 1 direct associations with clear gas counterpart; (2) Type 2 close associations partially associated with gas; (3) Type 3 bubble associations partially associated with bubble-like gas structures; (4) Type 4 complex associations with complex gas counterparts.Type 4 tends to be the parent structures for some smaller structures; (5) Type 5 diffuse associations with very diffuse gas counterparts; (6) Type 6 clean associations with no gas counterpart.
(ii) Evolution of cloud-sized ( ≈ 30 pc) objects.The different gas-richness results from evolution.From Type 1 direct associations Solid red circle, translucent red circle, and hollow red circle refer to Type 1 direct associations, Type 2 close associations, and Type 3 bubble associations respectively.Solid blue triangles, translucent blue triangles, and hollow triangles represent the Type 4 complex associations, Type 5 diffuse associations, and Type 6 clean associations respectively.The grey line shows the velocity dispersionsize relation with fitting error added:  ,2D = 10 −0.13±0.08  0.67±0.05(Zhou et al. 2022).The green line is the Larson's relation from Larson (1981):   ∝  0.38 .

Local Bubble
Sun  to Type 2 close and Type 3 bubble associations, the gas displacement is likely caused by stellar feedback of the massive stars.
(iii) Evolution of  ≳ 100 pc objects.We found an evolution sequence from Type 4 complex, Type 5 diffuse to Type 6 clean associations.This sequence also shows the gas decrease in the associations due to the stellar feedback.
(iv) Different types occupy different Locations in the velocitysize plane.The gas-rich Type 1 direct, Type 2 close, and Type 3 bubble associations follow a velocity-size relation which is consistent with our previous results.
For associations in evolution 2, going from the Type 4 complex, Type 5 diffuse to Type 6 clean YSO associations, there is a continuous increase in the velocity dispersion, indicating the energy injection during stellar feedback.
(v) Short feedback time and rapid gas removal.From the number of feedback-related types of YSO associations within 5-50 pc, we derive an upper limit timescale of around 1.7 Myr ( feedback < 0.17  YSO ) as the time it takes for stellar feedback to remove gas from a cloud.
(vi) Gas-free YSO associations.We also discovered a population of gas-free YSO associations.They are located either inside kpc-sized superbubble-like cavities or on a Galactic-scale diffuse filament.These gas-free YSO associations are at a very late stage of evolution where gas has been removed.These structures might later evolve into other forms of the concentrations of young stars, such as the Gaia stellar strings (Kounkel et al. 2020).
(vii) Different association types on different filaments.We found the YSO association types differ across different filament structures, indicating different stages of the filaments.More detailed studies are needed.
(viii) Gas-rich structures are gravitationally unbound.By computing the virial parameter for Type 1 direct and Type 4 complex associations where most gas has not yet been removed, we find that nearly all these YSO associations have  vir > 1, indicating they are gravitationally unbound, consistent with previous studies.
Our sample sets a solid foundation to explore the relation between interstellar medium evolution, star formation, and Galaxy structure, and this potential will be exploited in our future papers.
Table 2 containing the location and classification type information will be made available online upon publication.

Figure 1 .
Figure 1.Examples of different types of YSO associations.Red dots in all the subplots represent the member YSOs in the corresponding YSO associations.The left panels are the member YSOs plotted against the dust map form Planck Collaboration et al. (2014), and the right panels show member YSOs plotted against integrated CO data fromDame et al. (2001).The CO data is integrated through the velocity range of the optimal component identified using the procedure described in Sec.2.2 for Type 1, 2, 3, and 4 and integrated over all the velocity channels for Type 5 and 6.The grid-like artifacts in the CO map for Type 6 are caused by the incomplete sampling of the legacy observations.
t YSO N Close + N Bubble N Close + N Bubble + N Direct Within cloud scale: 5-50 pc

Figure 2 .Figure 3 .
Figure 2. YSO association classification and evolution 1 for cloud-scale associations.Three inset images show the examples of Type 1 direct, Type 2 close, and Type 3 bubble associations.Member YSOs are plotted as red dots on the Planck dust map.The red part in each pie chart shows the fraction of this type in the whole sample.The arrow shows the proposed evolution sequence of the YSO associations.
; Dobbs et al. 2011; Miville-Deschênes et al. 2017).All the virial parameters and mass values can be found in Tab. 2. In Fig. 8, we compare the virial parameter of our gas-rich sample with that of 8107 molecular clouds from Miville-Deschênes et al. (2017).Miville-Deschênes et al. (2017) found a modest correlation between the virial parameter and the cloud mass in their sample:  virial ∝  −0.53±0.3 .Although this correlation can not be seen in our Type 1 and Type 4 associations, our sample covers a wide range in the virial paramater-mass diagram.

Figure 4 .
Figure 4. Size distribution of YSO associations for evolution sequence 1 and 2. The red histogram shows the size distribution for the associations used in evolution 1: Type 1 direct associations, Type 2 close associations, and Type 3 bubble associations.The blue histogram shows the size distribution for the associations in evolution 2: Type 4 complex associations, Type 5 diffuse associations, and Type 6 clean associations.

Figure 5 .
Figure 5. Locations of YSO associations of different types on   −  diagram.Solid red circle, translucent red circle, and hollow red circle refer to Type 1 direct associations, Type 2 close associations, and Type 3 bubble associations respectively.Solid blue triangles, translucent blue triangles, and hollow triangles represent the Type 4 complex associations, Type 5 diffuse associations, and Type 6 clean associations respectively.The grey line shows the velocity dispersionsize relation with fitting error added:  ,2D = 10 −0.13±0.08  0.67±0.05(Zhou et al. 2022).The green line is the Larson's relation fromLarson (1981):   ∝  0.38 .

Figure 6 .Figure 7 .
Figure 6.Locations of YSO associations of different types in the Galactic  plane.The background is the dust map from Lallement et al. (2019).Red circles and blue triangles represent associations involved in evolution sequences 1 and 2, and different styles refer to different association types.The Galactic Center is located at (0,0) and the Sun is located at (−8.34, 0) (black star).The green dashed lines mark several cavities in the solar neighborhood.

Figure 8 .
Figure 8. Viral parameter vs. gas mass for Type 1 direct and Type 4 complex YSO associations.Blue dots are the clouds from Miville-Deschênes et al. (2017).Red dots refer to the Type 1 direct associations and Orange dots refer to Type 4 complex associations.

Figure 9 .
Figure 9. Locations of clean associations on  - plane.The background is a dust map from Lallement et al. (2019).The Sun is located at (−8.34 kpc, 0) marked by the black star.Blue triangles refer to the clean associations (Type 6).The pink, green, and red squares mark the Radcliffe Wave filament, Split, and Lower Sagittarius-Carina Spur, respectively.The black dashed lines mark several cavities in the solar neighborhood.

Figure A1 .
Figure A1.Components selection in the CO data towards a Type 1 direct association.The upper left panel shows the PDF (Probability density function) distribution for the best-fitting CO line profile integrated over the association region.The grey, red, orange, and green lines refer to the PDF of the total distribution and three components, respectively.The red dots in the rest three subplots show the location of member YSOs.The background is CO maps obtained by integrating over velocity range in the title.The cyan contours mark the region with CO emission larger than 2.21 K.

Figure B1 .
Figure B1.Example for a Type 1 direct association.The red dots in both subplots are member YSOs.The background in the left panel is the Planck 857 GHz map and the background in the right panel is the CO map integrated from 10.4 to 13 km s −1 .In both panels, the cyan contours mark the area with emission larger than 3.Almost all the YSOs are located inside the gas region.

Figure B2 .
Figure B2.Example foofr a Type 2 close association.All the markers are the same as Fig. B1.In the right panel, the CO map is integrated from 7.8 to 10.4 km s −1 .The member YSOs are partially associated with the gas distribution.

Figure B3 .
Figure B3.Example of a Type 3 bubble association.All the markers are the same as Fig. B1.In the right panel, the CO map is integrated from 7.8 to 11.7 km s −1 .The member YSOs partially match the bubble-like gas distribution.

Figure B4 .
Figure B4.Example of a Type 4 complex association.All the markers are the same as Fig. B1.In the right panel, CO map is integrated from −5.2 to 3.9 km s −1 .Part of the member YSOs are associated with gas and part are only associated with very diffuse gas structure.

Figure B5 .
Figure B5.Example of a Type 5 diffuse association.All the markers are the same as Fig. B1.In the right panel, the CO map is integrated over the whole velocity range.The YSO distribution matches a very diffuse gas distribution.

Figure B6 .
Figure B6.Example for a Type 6 clean association.All the markers are the same as Fig. B1.In the right panel, the CO map is integrated over the whole velocity range.The YSO distribution seldom matches the gas distribution.

Table 1 .
Classification criteria: criteria and information of different types of YSO associations

Table 2 .
Lists of information of the 150 YSO associations.The table contains location, association types based on relation with nearby gas, 2D velocity dispersion, size, mass, virial parameter and YSO number.The full table can be downloaded online through ...