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Pavel Kroupa

Publications and source records attributed to Pavel Kroupa.

At least 55 records · Page 3Linked to original sources

A recent confirmation of the wide binary gravitational anomaly

Concerning recent published studies exploring the presence or otherwise of a gravitational anomaly at low accelerations in wide binary stars as observed by the {\it Gaia} satellite, the paper published by Cookson on the subject last year presents an interesting case. In that study, RMS values of binned relative internal velocities in 1D for wide binaries are compared to Newtonian predictions for that quantity, with the author concluding that the data presented show no indication of any inconsistency with Newtonian expectations. However, the comparison presented is critically flawed, as the Newtonian predictions used refer to wide binaries with mean total masses of 2.0 $M_{\odot}$. This is larger than the 1.56 $M_{\odot}$ value which applies to the data used in said paper. In this short note we correct the error mentioned above and show that the data and error bars as given by Cookson are in fact inconsistent with Newtonian expectations. Contrary to the assertion in that study, the data presented there actually show a clear anomaly in the low acceleration gravitational regime, { with the overall slope of the velocity-separation scaling being inconsistent with Newtonian expectations at a 3.3$σ$ level.} Based on the data presented in the paper by Cookson, wide binary systems show a clear Milgromian deviation from Newtonian dynamics.

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Tidal tails of nearby open clusters I. Mapping with Gaia DR3

Context: Tidal tails of open clusters are the result of stellar evaporation from the cluster through the Galactic potential and internal dynamics. With the recent availability of high-precision data, tidal tails are being detected for most of the nearby open clusters. Aims: We identify the tidal tail members for all open clusters within a distance of 400 pc that are older than 100 Myr and have $>$100 members. To do this, we use model-independent methods. Methods: We used the convergent-point (CP) method to identify the co-moving stars near the open clusters using Gaia DR3 data. A new method called the self-compact convergent-point (SCCP) method was proposed and applied to some of the clusters. It performed better overall in tracing the tails. We also analysed the colour-magnitude diagrams and orbital energy to diagnose possible contamination. Results: Nineteen out of 21 clusters have tidal tails. Five of them were discovered for the first time through this work. The typical span of the tidal tails is 20--200 pc, and 30--700 member stars lie in the region inside the tidal radius and the tidal tails. Four out of 19 tidal tails are tilted away from direction of the Galactic centre. This contradicts the known theory of the tidal-tail formation. The luminosity functions of the tails and clusters are consistent with each other and with the canonical stellar interstellar mass function, but systematically higher radial velocities for the trailing tail than for the leading tail were observed for the first time. Conclusions: The CP method is useful for detecting tidal tails on a scale of approx. 100 pc for clusters closer than 400 pc. A further analysis of theoretical N-body models is required to understand the incompleteness and biases in the current sample of tidal tails.

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The post-gas expulsion coalescence of embedded clusters as an origin of open clusters

The mismatch between the mass function of the Milky Way's embedded clusters (ECs) and that of open clusters (OCs) raises the question of whether each OC originates from a single EC. In this work, we explore a scenario in which OCs form as a result of post-gas expulsion coalescence of ECs within the same parental molecular cloud. We model this process using N-body simulations of ECs undergoing expansion due to gas expulsion. Our initial conditions are based on the observed spatial, kinematic, and mass distributions of ECs in three representative massive star-forming regions (MSFRs). Initially, ECs are isolated. After further expansion, interactions between ECs begin, mutually influencing their evolution. We examine this process as a function of gas expulsion timescales, spatial separations between ECs, and their relative velocities. Our results demonstrate that, within a reasonable range of these parameters, the coalescence of ECs is robust and largely insensitive to initial conditions. The mass of ECs plays a critical role in the coalescence process. More massive ECs form stable gravitational cores, which greatly facilitate coalescence and help the resulting cluster resist expansion and Galactic tidal forces. Additionally, the number of ECs also enhances coalescence. The current mass distribution of clumps in the Milky Way suggests that directly forming massive ECs is challenging. However, the coalescence of multiple low-mass ECs can account for the observed parameter space of OCs in the Milky Way.

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The redshift dependence of the inferred $H_0$ in a local void solution to the Hubble tension

Galaxy number counts suggest that we are located within the Gpc-scale KBC void. The Hubble tension might arise due to gravitationally driven outflow from this void, as explored in detail by Haslbauer et al. We explore how the impact of the void on redshift decays at large distances. We define $H_0(z)$ as the present expansion rate $H_0$ that would be inferred from observations in a narrow redshift range centred on $z$. We find $H_0(z)$ in three different ways, all of which give similar results. We then compare these results with the observations of Jia et al., who were careful to minimise the impact of correlations between $H_0$ measurements from data in different redshift bins. We find reasonable agreement with their results for the Gaussian and Exponential void underdensity profiles, although the agreement is less good in the Maxwell-Boltzmann case. The latter profile causes severe disagreement with the observed bulk flow curve at $z < 0.1$ (Mazurenko et al.), so the tension with higher redshift data further highlights that the deepest part of the KBC void is probably near its centre. The observations show a decline of $H_0(z)$ towards the background $Planck$ value in qualitative agreement with the considered models, even if we use a larger void. The good overall agreement with the recent results of Jia et al. suggests that the local supervoid evident from the galaxy luminosity density out to a Gpc might also solve the Hubble tension while retaining a low background $H_0$ consistent with $Planck$ data, assuming enhanced structure formation on $>100$ Mpc scales.

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The initial mass function of stars

The initial mass function (IMF) is one of the most important functions in astrophysics because it is key to reconstructing the cosmological matter cycle, understanding the formation of super-massive black holes, and deciphering the light from high-redshift observations. The IMF's dependency on the physical conditions of the gas and its connection to the galaxy-wide IMF connects the molecular clumps to the cosmological scale. The extraction of the IMF from observational data requires a thorough understanding of stellar evolution, the time-dependent stellar multiplicity, the stellar-dynamical evolution of dense stellar populations, and the structures, star formation histories, and chemical enrichment histories of galaxies. The IMF in galaxies, referred to as the galaxy-wide IMF (gwIMF), and the IMF in individual star-forming regions (the stellar IMF) need not be the same, although the former must be related to the latter. Observational surveys inform on whether star-forming regions provide evidence for the stellar IMF being a probability density distribution function. They may also indicate star formation to optimally follow an IMF shaped by the physical conditions of the star-forming gas. Both theoretical and observational evidence suggest a relationship between the initial mass function of brown dwarfs and that of stars. Late-type stars may arise from feedback-regulated fragmentation of molecular cloud filaments, which build up embedded clusters. In contrast, early-type stars form under more violent accretion and feedback-regulated conditions near the centers of these clusters. The integration over all star-forming molecular cloud clumps and their stellar IMFs in a galaxy via the IGIMF theory yields its gwIMF which sensitively depends on the physical properties of the molecular cloud clumps and the range of their masses that depends on the SFR of the galaxy.

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The currently observed clumps cannot be the "direct" precursors of the currently observed open clusters

We categorized clumps, embedded clusters, and open clusters and conducted a comparative analysis of their physical properties. Overall, the radii of open clusters are significantly larger than those of embedded clusters and clumps. The radii of embedded clusters are larger than those of clumps, which may be due to the expansion of embedded clusters. The open clusters have significantly higher masses than embedded clusters, by about one order of magnitude. Given the current mass distribution of clumps in the Milky Way, the evolutionary sequence from a single clump evolving into an embedded cluster and subsequently into an open cluster cannot account for the observed open clusters with old ages and high masses, which is also supported by N-body simulations of individual embedded clusters. To explain the mass and radius distributions of the observed open clusters, initial embedded clusters with masses higher than 3000 M$_{\odot}$ are necessary. However, the upper limit of the embedded cluster sample is less than 1000 M$_{\odot}$, and only a few ATLASGAL clumps have a mass higher than 3000 M$_{\odot}$. Thus, the currently observed clumps cannot be the "direct" precursors of the currently observed open clusters. If the Milky Way has a burst-like and time-dependent star formation history, the currently observed open clusters with old ages and high masses may come from massive clumps in the past. There is also a very real possibility that these open clusters originate from post-gas expulsion coalescence of multiple embedded clusters. We compared the separation of open clusters and the typical size of molecular clouds, and find that most molecular clouds may only form one open cluster, which supports the scenario of post-gas expulsion coalescence. Further study is necessary to distinguish between the different scenarios.

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The masses of open star clusters and their tidal tails and the stellar initial mass function

Unresolved binaries have a strong influence on the observed parameters of stellar clusters (SCs). We quantify this influence and compute the resulting mass underestimates and stellar mass function (MF). N-body simulations of realistic SCs were used to investigate the evolution of the binary population in a SC and its tidal tails. Together with an empirically gauged stellar mass-luminosity relation, the results were then used to determine how the presence of binaries changes the photometric mass and MF of the SC and its tails as deduced from observations. Tail 1 (T1), which is the tidal tail caused by gas expulsion, contains a larger fraction of binaries than both the SC and tail 2 (T2), which forms after gas expulsion. Additionally, T1 has a larger velocity dispersion. Using the luminosity of an unresolved binary, an observer would underestimate its mass. This bias sensitively depends on the companion masses due to the structure of the stellar mass-luminosity relation. Combining the effect of all binaries in the simulation, the total photometric mass of the SC is underestimated by 15%. Dark objects (black holes/neutron stars) increase the difference between the real and observed mass of the SC further. For both the SC and the tails, the observed power-law index of the MF between a stellar mass of 0.3 and 0.7 $M_\odot$ is smaller by up to 0.2 than the real one, the real initial mass function (IMF) being steeper by this amount. This difference is larger for stars with a larger velocity dispersion or binary fraction. Since the stars formed in SCs are the progenitors of the Galactic field stars, this work suggests that the binary fractions of different populations of stars in the Galactic disc will differ as a function of the velocity dispersion. The direction of this correlation is currently unclear and a complete population synthesis will be needed to investigate this effect.

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Failed supernova explosions increase the duration of star formation in globular clusters

Context. The duration of star formation (SF) in globular clusters (GCs) is an essential aspect for understanding their formation. Contrary to previous presumptions that all stars above 8 M explode as core-collapse supernovae (CCSNe), recent evidence suggests a more complex scenario. Aims. We analyse iron spread observations from 55 GCs to estimate the number of CCSNe explosions before SF termination, thereby determining the SF duration. This work for the first time takes the possibility of failed CCSNe into account, when estimating the SF duration. Methods. Two scenarios are considered: one where all stars explode as CCSNe and another where only stars below 20 M lead to CCSNe, as most CCSN models predict that no failed CCSNe happen below 20 M . Results. This establishes a lower ($\approx$ 3.5 Myr) and an upper ($\approx$ 10.5 Myr) limit for the duration of SF. Extending the findings of our previous paper, this study indicates a significant difference in SF duration based on CCSN outcomes, with failed CCSNe extending SF by up to a factor of three. Additionally, a new code is introduced to compute the SF duration for a given CCSN model. Conclusions. The extended SF has important implications on GC formation, including enhanced pollution from stellar winds and increased binary star encounters. These results underscore the need for a refined understanding of CCSNe in estimating SF durations and the formation of multiple stellar populations in GCs.

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Has the Palomar 14 globular cluster been captured by the Milky Way?

We examine a new scenario to model the outer halo globular cluster (GC) Pal 14 over its lifetime by performing a comprehensive set of direct N-body calculations. We assume Pal 14 was born in a now detached/disrupted dwarf galaxy with a strong tidal field. Pal 14 evolved there until the slope of the stellar mass function (MF) became close to the measured value which is observed to be significantly shallower than in most GCs. After about 2-3 Gyr, Pal 14 was then captured by the Milky Way (MW). Although the physical size of such a cluster is indistinguishable from a cluster that has lived its entire life in the MW, other parameters like its mass and the MF-slope, strongly depend on the time the cluster is taken from the dwarf galaxy. After being captured by the MW on a new orbit, the cluster expands and eventually reaches the appropriate mass and size of Pal 14 after 11.5 Gyr while reproducing the observed MF. These simulations thus suggest that Pal 14 may have formed in a dwarf galaxy with a post-gas-expulsion initial half-mass radius and mass of $r_h=7$ pc and $8<M/10^4<10 $ M$_{\odot}$, respectively, with a high degree of primordial mass segregation.

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The evolution of the mass--radius relation of expanding very young star clusters

The initial mass--radius relation of embedded star clusters is an essentially important boundary condition for understanding the evolution of embedded clusters in which stars form to their release into the galactic field via an open star cluster phase. The initial mass--radius relation of embedded clusters deduced by Marks \& Kroupa is significantly different from the relation suggested by Pfalzner et al. Here, direct N-body simulations are used to model the early expansion of embedded clusters after the expulsion of their residual gas. The observational data of radii of up to a few~Myr old clusters collated from various sources are found to be well fit by the N-body models, implying these observed very young clusters to most likely be in an expanding state. We show that the mass-radius relation of Pfalzner et al. reflects the expansion of embedded clusters following the initial mass-radius relation of Marks \& Kroupa. We also suggest that even the embedded clusters in ATLASGAL clumps with HII regions are probably already in expansion. All here collected clusters from different observations show a mass-size relation with a similar slope, which may indicate that all clusters were born with a profile resembling that of the Plummer phase-space distribution function.

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The Magellanic Clouds are very rare in the IllustrisTNG simulations

The Large and Small Magellanic Cloud (LMC and SMC) form the closest interacting galactic system to the Milky Way, therewith providing a laboratory to test cosmological models in the local Universe. We quantify the likelihood for the Magellanic Clouds (MCs) to be observed within the $Λ$CDM model using hydrodynamical simulations of the IllustrisTNG project. The orbits of the MCs are constrained by proper motion measurements taken by the $Hubble~Space~Telescope$ and $Gaia$. The MCs have a mutual separation of $d_{\mathrm{MCs}}~=~24.5\,\mathrm{kpc}$ and a relative velocity of $v_{\mathrm{MCs}}~=~90.8\,\mathrm{km\,s^{-1}}$, implying a phase-space density of $f_{\mathrm{MCs,obs}}~\equiv~(d_{\mathrm{MCs}} \cdot v_{\mathrm{MCs}})^{-3}~=~9.10\times10^{-11}\,\mathrm{km^{-3}\,s^{3}\,kpc^{-3}}$. We select analogues to the MCs based on their stellar masses and distances in MW-like halos. None of the selected LMC analogues have a higher total mass and lower Galactocentric distance than the LMC, resulting in $>3.75σ$ tension. We also find that the $f_{\mathrm{MCs}}$ distribution in the highest resolution TNG50 simulation is in $3.95σ$ tension with observations. Thus, a hierarchical clustering of two massive satellites like the MCs in a narrow phase-space volume is unlikely in $Λ$CDM, presumably because of short merger timescales due to dynamical friction between the overlapping dark matter halos. We show that group infall led by an LMC analogue cannot populate the Galactic disc of satellites (DoS), implying that the DoS and the MCs formed in physically unrelated ways in $Λ$CDM. Since the $20^\circ$ alignment of the LMC and DoS orbital poles has a likelihood of $P=0.030$ ($2.17σ$), adding this $χ^2$ to that of $f_{\mathrm{MCs}}$ gives a combined likelihood of $P = 3.90\times10^{-5}$ ($4.11σ$).

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On the dynamical evolution of Cepheid multiplicity in star clusters and its implications for B-star multiplicity at birth

Classical Cepheid variable stars provide a unique probe to binary evolution in intermediate-mass stars over the course of several tens to hundreds of Myr. We studied the binary and multiple properties of Cepheids, assuming that all mid-B stars form in binaries inside star clusters. The binaries were subjected both to stellar evolution and dynamical encounters with other stars in the cluster. The dynamical cluster environment results in a higher binary fraction among the Cepheids that remain in star clusters ($\approx 60$%) than among the Cepheids which have escaped to the field ($\approx 35$%). In clusters, the binary, triple, and multiple fraction decreases with increasing cluster mass. More massive clusters have binaries of shorter orbital periods than lower mass clusters and field Cepheids. Mergers are very common with $\approx 30$% of mid-B stars not evolving to Cepheids because of the interaction with their companion. Approximately $40$ % of Cepheids have merged with their companion, and the merger event impacts stellar evolution; the age of Cepheids expected from their mass can differ from the age of their host cluster. Our models predict that one in five Cepheids is the result of a merger between stars with mass below the lower mass limit for Cepheids; in clusters, these objects occur substantially later than expected from their mass. Approximately $3$ to $5$ % of all Cepheids have a compact companion ($\approx 0.15$ % of all Cepheids are accompanied by a black hole). The binary fraction derived from our simulations (42%) underestimates the observed binary Cepheid fraction by approximately a factor of 2. This suggests that the true multiplicity fraction of B-stars at birth could be substantially larger than unity and, thus, that mid-B stars may typically form in triple and higher order systems.

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The star formation histories, star formation efficiencies and ionizing sources of ATLASGAL clumps with HII regions

1226 ATLASGAL clumps with HII regions were matched with radio sources in the CORNISH-North/South surveys, and 392 of them have corresponding radio sources. We determined the stellar luminosity according to the Lyman continuum flux. When the bolometric luminosity of HII-clumps is less than $\approx$ 10$^{3.7}$ L$_{\odot}$, corresponding to a clump mass $\approx$ 10$^{2.55}$ M$_{\odot}$, the stellar luminosities derived from the Lyman continuum flux overestimate the actual stellar luminosities, because the accretion onto the protostars contributes significantly to the radio emission. After subtracting the accretion luminosity, we obtained reasonable estimates of the stellar luminosity. Using the 0.5 Myr isochrone, we calculated the stellar masses according to the stellar luminosities, and found that they roughly follow the $m_{\rm max}-M_{\rm ecl}$ relation of embedded clusters, consistent with the ionizing sources representing the most massive stars in the embedded clusters of HII-clumps. We also studied the contribution of the possible flaring activity to the observed stellar luminosity and found that they can be neglected. We further studied the change of SFE with the clump mass. According to the derived mass of the most massive star in each HII-clump, using the theoretical $m_{\rm max}-M_{\rm ecl}$ relation, we calculated the mass of the corresponding embedded cluster and then the SFE of the clump. The SFE decreases with increasing clump mass, with a median value of $\approx$0.3. We also independently derived the SFE for each HII-clump based on the model developed in our previous work. The SFEs of HII-clumps derived from the observation and the model are in good agreement. Concerning the star formation histories of the ATLASGAL clumps, low-mass clumps may reach the peak of star formation earlier than high-mass clumps, consistent with the shorter free-fall time of low-mass clumps.

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Physical properties of embedded clusters in ATLASGAL clumps with HII regions

Using the optimal sampling model, we synthesized the embedded clusters of ATLASGAL clumps with HII regions (HII-clumps). The 0.1 Myr isochrone was used to estimate the bolometric luminosity of each star in an embedded cluster, we also added the accretion luminosity of each star in the embeded cluster. The total bolometric luminosity of synthetic embedded clusters can well fit the observed bolometric luminosity of HII-clumps. More realistically, we considered the age spread in the young star and protostar populations in embedded clusters of HII-clumps by modeling both constant and time-varying star formation histories (SFHs). According to the age distribution of the stellar population, we distributed the appropriate isochrones to each star, and sorted out the fraction of stellar objects that are still protostars (Class 0 and Class I phases) to properly add their accretion luminosities. Compared to a constant SFH, burst-like and time-dependent SFHs can better fit the observational data. We found that as long as 20\% of the stars within the embedded cluster are still accreting, the contribution of accretion luminosity will be significant to the total bolometric luminosity of low-mass HII-clumps with mass log$_{10}$(M$_{\rm cl}$/M$_{\odot}$) $<$ 2.8. Variations in the accretion rate, the SFE and the initial mass function (IMF) and more physical processes like the external heating from HII regions and the flaring from pre-main sequence (PMS) stars and protostars need to be investigated to further explain the excess luminosity of low-mass HII-clumps.

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Self-similar cluster structures in massive star-forming regions: Isolated evolution from clumps to embedded clusters

We used the dendrogram algorithm to decompose the surface density distributions of stars into hierarchical structures. These structures were tied to the multiscale structures of star clusters. A similar power-law for the mass-size relation of star clusters measured at different scales suggests a self-similar structure of star clusters. We used the minimum spanning tree method to measure the separations between clusters and gas clumps in each massive star-forming region. The separations between clusters, between clumps, and between clusters and clumps were comparable, which indicates that the evolution from clump to embedded cluster proceeds in isolation and locally, and does not affect the surrounding objects significantly. By comparing the mass functions of the ATLASGAL clumps and the identified embedded clusters, we confirm that a constant star formation efficiency of $\approx$ 0.33 can be a typical value for the ATLASGAL clumps.

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Open star clusters and their asymmetrical tidal tails

Stars that evaporate from their star cluster by the energy equipartition process end up either in a leading or a trailing tidal tail. In Newtonian gravitation and for open star clusters in the Solar vicinity, the tidal threshold, or prah, for escape is symmetrical, such that the leading and trailing tails are equally populated. The data by six independent teams that applied the convergent point method to map out the tidal tails of four open clusters (the Hyades, the Praesepe, Coma Berenices and COIN-Gaia13) using Gaia DR2 and DR3 are here applied to test for the expected symmetry. All tidal tails contain more stars in the leading tail. The combined confidence amounts to an 8 sigma falsification of the prah symmetry. The same test using Milgromian dynamics leads to consistency with the data. More effort needs to be exerted on this matter, but the data indicate with high confidence that the tidal prah of an open star cluster is asymmetrical with the corresponding confidence that Newtonian gravitation is falsified. Open star clusters depopulate more rapidly in Milgromian than in Newtonian dynamics and the COIN-Gaia13 cluster is here found to be nearly completely dissolved. In view of these results, the wide-binary star test and the Keplerian Galactic rotation curve finding are briefly discussed.

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The Variation of the Galaxy-Wide IMF for Low-Mass Stars: Modeling and Observational Insights

The Stellar Initial Mass Function (IMF) characterizes the mass distribution of newly formed stars in various cosmic environments, serving as a fundamental assumption in astrophysical research. Recent findings challenge the prevalent notion of a universal and static IMF, proposing instead that the IMF's shape is contingent upon the star formation environment. In this study, we analyze the galaxy-wide variation of the IMF for low-mass stars in both dwarf and massive galaxies with diverse observational methods. Despite systematic discrepancies between different approaches, an IMF model with a metallicity-dependent slope for the low-mass stars aligns with the majority of observations, indicating a high degree of uniformity in the star formation processes across the universe. We also emphasize the need for a more comprehensive understanding of the variation of the low-mass IMF, considering measurement biases and factors beyond metallicity.

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The effect of the environment-dependent stellar initial mass function on the photometric properties of star-forming galaxies

(Abridged) Observational estimates of galaxy properties rely on the inherent galaxy-wide initial mass function (gwIMF), which systematically varies with the global SFR and metallicity, as proposed by the integrated-galactic IMF (IGIMF) theory and supported by empirical evidence. We incorporate PARSEC and COLIBRI stellar isochrones into the GalIMF code, a galaxy chemical evolution (GCE) model featuring real-time updates of environment-dependent gwIMFs. This newly developed photometric GalIMF (photGalIMF) code allows the calculation of photometric properties for galaxies with diverse stellar populations. Subsequently, we analyze observed luminosities and metallicities of local star-forming galaxies to deduce their stellar masses assuming that they have constant SFRs over 13.6 Gyr. We also compute SFR$-$H$α$ luminosity relations for varying stellar metallicities using a separate stellar population synthesis code based on PEGASE. Comparing the IGIMF theory to the canonical universal IMF, our analysis reveals that estimates of the stellar masses and SFRs for local star-forming galaxies differ by factors of $\approx 2$ and 10, respectively. The computed gas-depletion timescale increases with gas mass, implying lower star formation efficiencies in more massive galaxies, possibly due to stronger feedback regulation, aligning with theoretical expectations. Additionally, the characteristic stellar mass buildup timescale increases with stellar mass, indicating that massive disk galaxies initiate star formation earlier than their low-mass counterparts. The photGalIMF code enables self-consistent computations of galactic photometry, self-consistently with GCE modelling within the context of an environment-dependent gwIMF. Utilizing Ks-band and H$α$ luminosities of galaxies, the outcomes include galaxy mass, SFR, and fitting functions for the SFR correction factor.

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