Surface Rotational Velocities Research Articles

Exploring the stellar rotation of early-type stars in the LAMOST medium-resolution survey

Context. Stellar rotation significantly shapes the evolution of massive stars, yet the interplay of mass and metallicity remains elusive, limiting our capacity to construct accurate stellar evolution models and to better estimate the impact of rotation on the chemical evolution of galaxies. Aims. Our goal is to investigate how mass and metallicity influence the rotational evolution of A-type stars on the main sequence (MS). We seek to identify deviations in rotational behaviors that could serve as new constraints for existing stellar models. Methods. Using the LAMOST Median-Resolution Survey Data Release 9, we derived stellar parameters for a population of 104 752 A-type stars. Our study focused on the evolution of surface rotational velocities and their dependence on mass and metallicity in 84 683 “normal” stars. Results. Normalizing surface rotational velocities to zero age main sequence (ZAMS) values revealed a prevailing evolutionary profile from 1.7 to 4.0 M⊙. This profile features an initial rapid acceleration until t/tMS = 0.25 ± 0.1 and potentially a second acceleration peak near t/tMS = 0.55 ± 0.1 for stars heavier than 2.5 M⊙, followed by a steady decline and a “hook” feature at the end. Surpassing theoretical expectations, the initial acceleration likely stems from a concentrated distribution of angular momentum at the ZAMS, resulting in a prolonged increase in speed. A transition phase for stars with 2.0 < M/M⊙ < 2.3 emerged, a region where evolutionary tracks remain uncertain. Stellar expansion primarily drives the spin down in the latter half of the MS, accompanied by significant influence from inverse meridional circulation. The inverse circulation becomes more efficient at lower metallicities, explaining the correlation of the slope of this deceleration phase with metallicity from –0.3 dex up to 0.1 dex. The metal-poor subsample (−0.3 dex < [M/H]< − 0.1 dex) starts with lower velocities at the ZAMS, suggesting that there is a metallicity-dependent mechanism that removes angular momentum during star formation. The proportion of fast rotators decreases with an increase in metallicity, up to log(Z/Z⊙)∼ − 0.2, a trend consistent with observations of OB-type stars found in the Small and Large Magellanic Clouds.

Under Einstein’s Microscope: Measuring Properties of Individual Rotating Massive Stars from Extragalactic Microcaustic Crossings

Highly magnified stars residing in caustic crossing lensed galaxies at z ≃ 0.7–1.5 in galaxy cluster lensing fields inevitably exhibit recurrent brightening events as they traverse a microcaustic network cast down by foreground intracluster stars. The detectable ones belong to nature’s most massive and luminous class of stars, with evolved blue supergiants being the brightest ones at optical wavelengths. Considering single stars in this work, we study to what extent intrinsic stellar parameters are measurable from multifilter light curves, which can be obtained with optical/near-IR space telescopes during one or multiple caustic crossing events. We adopt a realistic model for the axisymmetric surface brightness profiles of rotating O/B stars and develop a numerical lensing code that treats finite source size effects. With a single microcaustic crossing, the ratio of the surface rotation velocity to the breakup value is measurable to a precision of ∼0.1–0.2 for feasible observation parameters with current space telescopes, with all unknown intrinsic and extrinsic parameters marginalized over and without a degeneracy with inclination. Equatorial radius and bolometric luminosity can be measured to 1/3 and 2/3 of the fractional uncertainty in the microcaustic strength, for which the value is not known at each crossing but an informative prior can be obtained from theory. Parameter inference precision may be further improved if multiple caustic crossing events for the same lensed star are jointly analyzed. Our results imply new opportunities to survey individual massive stars in star formation sites at z ≃ 0.7–1.5 or beyond.

Betelgeuse as a Merger of a Massive Star with a Companion

We investigate the merger between a 16M ⊙ star, on its way to becoming a red supergiant (RSG), and a 4M ⊙ main-sequence companion. Our study employs three-dimensional hydrodynamic simulations using the state-of-the-art adaptive mesh refinement code Octo-Tiger. The initially corotating binary undergoes interaction and mass transfer, resulting in the accumulation of mass around the companion and its subsequent loss through the second Lagrangian point (L2). The companion eventually plunges into the envelope of the primary, leading to its spin-up and subsequent merger with the helium core. We examine the internal structural properties of the post-merger star, as well as the merger environment and the outflow driven by the merger. Our findings reveal the ejection of approximately ∼0.6 M ⊙ of material in an asymmetric and somewhat bipolar outflow. We import the post-merger stellar structure into the MESA stellar evolution code to model its long-term nuclear evolution. In certain cases, the post-merger star exhibits persistent rapid equatorial surface rotation as it evolves in the H–R diagram toward the observed location of Betelgeuse. These cases demonstrate surface rotation velocities of a similar magnitude to those observed in Betelgeuse, along with a chemical composition resembling that of Betelgeuse. In other cases, efficient rotationally induced mixing leads to slower surface rotation. This pioneering study aims to model stellar mergers across critical timescales, encompassing dynamical, thermal, and nuclear evolutionary stages.

Angular momentum transport near convective-core boundaries of Gamma Doradus stars

Recent asteroseismic studies have revealed that the convective core of γ Doradus stars rotates faster than their radiative interior. We study the development of differential rotation near the convective core to test angular momentum transport processes that are typically adopted in stellar evolution models. Models that only include the advection of angular momentum by meridional circulation and shear instabilities cannot reproduce current rotational constraints, irrespective of the initial conditions. The latest formulation of internal magnetic fields based on the Tayler instability is indeed able to reproduce the internal rotation rate of post-main sequence stars; however, it appears too efficient during the main sequence and has thus been disfavoured. A less efficient version of the same transport process can simultaneously reproduce the rotation rate of the convective core, the rotation rate in radiative regions as probed by gravity-modes, and the surface rotational velocities of γ Doradus stars. Our work suggests that there are additional physical processes apart from internal magnetic fields at work in the stellar interiors of post-main sequence stars.

Modelling and dynamic behaviours of a high-contact-ratio spur-gear system considering rough surface

The dynamic characteristics of a high-contact-ratio (HCR) spur-gear system having rough surfaces generated by shot peening (SP) were studied, with specific emphasis on characterisation of the gear-surface topography as well as modelling of the gear backlash and static transmission error. Accordingly, a four-degree-of-freedom dynamic model was established. Simulation experiments were then conducted using surface roughness, rotational velocity, input torque, and shaft-bearing stiffness as the variables. The results show that the dynamic characteristics of the gear system tend toward instability with increasing surface roughness. The models developed in this study outline a method for building indirect relationships between the vibration, dynamics, and tooth-surface microscopic features. This research thus provides a theoretical basis for designing the tooth-surface topography of HCR gears in the future.

The young massive SMC cluster NGC 330 seen by MUSE

Context. The origin of the initial rotation rates of stars, and how a star’s surface rotational velocity changes during the evolution, either by internal angular momentum transport or due to interactions with a binary companion, remain open questions in stellar astrophysics. Aims. Here, we aim to derive the physical parameters and study the distribution of (projected) rotational velocities of B-type stars in the ∼35 Myr-old, massive cluster NGC 330 in the Small Magellanic Cloud. NGC 330 is in an age range where the number of post-interaction binaries is predicted to be high near the cluster turnoff (TO). Methods. We developed a simultaneous photometric and spectroscopic grid-fitting method adjusting atmosphere models on multiband Hubble Space Telescope (HST) photometry and Multi Unit Spectroscopic Explorer (MUSE) spectroscopy. This allowed us to homogeneously constrain the physical parameters of over 250 B and Be stars (i.e., B-type stars with emission lines), brighter than mF814W = 18.8 mag. Results. The rotational velocities of Be stars in NGC 330 are significantly higher than the ones of B-type stars. The rotational velocities vary as a function of the star’s position in the color-magnitude diagram, qualitatively following predictions of binary population synthesis. A comparison to younger clusters shows that stars in NGC 330 rotate more rapidly on average. Conclusions. The rotational velocities of the ∼35 Myr old population in NGC 330 quantitatively agree with predictions for a stellar population that underwent significant binary interactions: the majority of the B-type stars could be single stars or primaries in pre-interaction binaries. The rapidly spinning Be stars could be mass and angular momentum gainers in previous interactions, while those Be stars close to the TO may be spun-up single stars. The slowly rotating, apparently single stars above the TO could be merger products. The different v sin i characteristics of NGC 330 compared to younger populations can be understood in this framework.

Sub-stellar engulfment by a main-sequence star: Where is the lithium?

Context. Since the discovery of exoplanetary systems, questions have been raised as to the sub-stellar companions that can survive encounters with their host star, and how this interaction may affect the internal structure and evolution of the hosting star, and particularly its surface chemical composition. Aims. We study whether the engulfment of a brown dwarf (BD) by a solar-like main-sequence (MS) star can significantly alter the structure of the star and the Li content on its surface. Methods. We performed 3D smoothed particle hydrodynamics simulations of the engulfment of a BD with masses 0.01 and 0.019 M⊙, on an MS star of 1 M⊙ and solar composition, in three different scenarios: a head-on collision, a grazing collision with an impact parameter η = 0.5 R⊙, and a merger. We studied the dynamics of the interaction in detail, and the relevance of the type of interaction and the mass of the BD on the final fate of the sub-stellar object and the host star in terms of mass loss of the system, angular momentum transfer, and changes in the Li abundance on the surface of the host star. Results. In all the studied scenarios, most of the BD mass is diluted in the denser region of the MS star. Only in the merger scenario a significant fraction (∼40%) of the BD material would remain in the outer layers. We find a clear increase in the surface rotational velocity of the host star after the interaction, ranging between 25 km s−1 (grazing collision) to 50 km s−1 (merger). We also find a significant mass loss from the system (in the range 10−4 − 10−3 M⊙) due to the engulfment, which in the case of the merger may form a circumstellar disk-like structure. Assuming that neither the depth of the convective envelope of the host star nor its mass content are modified during the interaction, a small change in the surface Li abundance in the head-on and grazing collisions is found. However, in the merger we find large Li enhancements, by factors of 20 − 30, depending on the BD mass. Some of these features could be detected observationally in the host star, provided they remained for a long enough time. Conclusions. In our 3D simulations, a sizable fraction of the BD survives long enough to be mixed with the inner core of the MS star. This is at odds with previous suggestions based on 1D simulations. In some cases the final surface rotational velocity is very high, coupled with enough mass loss that may form a circumstellar disk. Merger scenarios tend to dilute considerably more BD material on the surface of the MS star, which could be detected as a Li-enhancement. The dynamic of the simulated scenarios suggests the development of asymmetries in the structure of the host star that can only be tackled with 3D codes, including the long-term evolution of the system.

Correlations in Chromospheric and Coronal Activity Indicators of Giant Stars

The main goal of this paper is to use ultraviolet photometry from the Galaxy Evolution Explorer (GALEX) satellite to study chromospheric activity among red giant and core-helium-burning (CHeB) stars. Correlations are sought between several chromospheric and coronal activity indicators for giants that were detected in soft X-rays by the ROentgen SATellite. There is an evident correlation between and , where L X is the X-ray luminosity and F Mg II is the flux from Mg ii h and k emission lines, although there is substantial scatter. Using GALEX far-ultraviolet (FUV) magnitudes, the relationship between an FUV-excess parameter (a proxy for chromospheric and transition-region emission lines) and X-ray luminosity is documented. Correlations found herein are not strong, and may be linked to binarity. There is a varied range of FUV emission among CHeB stars, indicating that giants in this phase may be exhibiting differences or cycles in outer atmospheric activity. Additionally, efforts were made to constrain a relationship between excess FUV and near-ultraviolet emission and the projected surface rotation velocity, , of giants. No clear relationship was found. Obscurities in this relationship may result from a spread in inclinations, and/or too few stars with km s−1.

Stick-Slip Vibration Suppression in Drill String Using Observer-Based LQG Controller.

Hydrocarbon exploration and production activities are guaranteed through various operations including the drilling process, which is realized by using rotary drilling systems. The process involves crushing the rock by rotating the drill bit along a drill string to create a borehole. However, during this operation, violent vibrations can occur at the level of the drill string due to its random interaction with the rocks. According to their axes of occurrence, there are three types of vibrations: axial, lateral, and torsional, where the relentless status of the torsional vibrations is terminologically known as the stick-slip phenomenon. Such a phenomenon can lead to increased fatigue of the drill string and cause its abortive fracture, in addition to reducing the efficiency of the drilling process and consequently making the exploration and production operations relatively expensive. Thus, the main objective of this paper is to eliminate the severe stick-slip vibrations that appear along the drill string of the rotary drilling system according to the LQG observer-based controller approach. The rock–bit interaction term is highly nonlinear, and the bit rotational velocity is unmeasurable; an observer was first designed to estimate the unknown inputs of the model, and then the controller was implemented in the drill string model with 10 degrees of freedom. The estimation process was essentially based on surface measurements, namely, the current and rotational velocity of the top drive. Thereafter, the performance of the proposed observer-based LQG controller was tested for different simulation scenarios in a SimScape/Matlab environment, for which the controller demonstrated good robustness in suppressing the severe stick-slip vibrations. Furthermore, the simulation and experimental results were compared to other controllers designed for the same model; the proposed observer-based LQG controller showed better performance, and it was less sensitive to structured disturbances than H∞. Thence, it is highly recommended to use the proposed approach in smart rotary drilling systems.

Friction coefficient measurements on jumping ski patterned running surfaces

The tribological interaction between patterned running surfaces and an ice track was investigated. The friction coefficient was analyzed according to the patterned running surface type, ambient temperature, and rotational track velocity. The results were analyzed based on the structural characteristics of the patterned running surface in terms of average height and skewness. Generation of the water layer was visualized using transparent replicas of the patterned running surfaces. The water layer ratio was then calculated and compared with the friction coefficient. We found that the water layer either functioned as a lubricant or offered frictional resistance depending on the water layer ratio and the type of patterned running surfaces as the lubricating friction regime changed.

Equilibrium of Planets in The Solar System-A New Approach

As per Newtonian laws of motion and gravitation, the gravitational force of attraction governs all the planets of solar system. Here we discuss equilibrium of planets in which gravitational force represents one side and the magnetic hold the other side. The planet achieves equilibrium in the solar field at that point where its centrifugal (i.e., gravitational) force balances the magnetic hold of sun. The sun develops magnetic flux on the planet according to its surface area, magnetic content and rotational velocity. As per available data, calculations in this regard are in good agreement with the functions achieving rotational and orbital motions and equilibrium of planet, thus supporting our concept. Therefore, this theory provides the missing link explaining magnetic interaction with the centrifugal force of planet to achieve equilibrium, hold and motion.

The effect of the cone rotation in the supersonic gas flow on the velocity propagation of perturbations in the boundary layer

The paper aimed at studying the peculiarities of boundary layer on a rotating body. Rotation of axisymmetric bodies is often used for their stabilization at a high velocity motion in the atmosphere. Presented are calculation of boundary layer flow on a rotating cone at zero angle of attack. Longitudinal and full enthalpy profiles in the laminar boundary layer obtained as a result of calculations were used to determine upstream disturbances propagation velocity. New integral relation was determined to find out disturbances propagation velocity for the regime of weak hypersonic viscous-inviscid interaction. Effects of surface rotation velocity along with the temperature factor influence were investigated.

Study of the low-order Δν–ρ¯ relation for moderately rotating δ Scuti stars and its impact on their characterization

ABSTRACT The large separation in the low-radial order regime is considered as a highly valuable observable to derive mean densities of δ Scuti stars, due to its independence with rotation. Up to now, theoretical studies of this Δν–${\bar{\rho}}$ relation have been limited to 1D non-rotating models and 2D pseudo-evolutionary models. The present work aims at completing this scenario by investigating quantitatively the impact of rotation in this relation on a large grid of 1D asteroseismic models representative of δ Scuti stars. These include rotation effects on both the stellar evolution and the interaction with pulsation. This allowed us to compute the stellar deformation, get the polar and equatorial radii, and correct the stellar mean densities. We found that the new Δν–${\bar{\rho}}$ relation for rotating models is compatible with previous works. We explained the dispersion of the points around the linear fits as caused mainly by the distribution of the stellar mass, and partially by the evolutionary stage. The new fit is found to be close to the previous theoretical studies for lower masses ($1.3\!-\!1.81\, \mathrm{M}_{\odot }$). However, the opposite holds for the observations: For the higher masses ($1.81\!-\!3\, \mathrm{M}_{\odot }$), the fit is more compatible with the empirical relation. To avoid such discrepancies, we provided new limits to the fit that encompass any possible dependence on mass. We applied these results to characterize the two well-known δ Scuti stars observed by CoRoT, HD 174936 and HD 174966, and compared the physical parameters with those of previous works. The inclusion of rotation in the modelling causes a tendency towards greater masses, radii, and luminosities, and lower density values. Comparison between Δν and Gaia’s luminosities also allowed us to constrain the inclination angles and rotational velocities of both stars. The present results pave the way to systematically constrain the angle of inclination (and thereby the actual surface rotation velocity) of δ Scuti stars.

Asteroseismic Analyses of Slowly Pulsating B Star KIC 8324482: Ultraweak Element Mixing beyond the Central Convective Core

Abstract Asteroseismology is a powerful tool for probing the inner structure and determining the evolutionary status and the fundamental parameters of stars. The oscillation spectra of slowly pulsating B (SPB) stars show almost uniform period spans, whose pattern is sensitive to the structure of the μ-gradient shell beyond the central convective core and hence can be used to efficiently describe their shapes and constrain the convective core. The SPB star KIC 8324482 was observed by Kepler for over 1470 days with a long-cadence pattern. Nearly equidistant period spacing patterns are found for the 14 connective dipolar modes that are extracted from the oscillation spectrum by Zhang et al. In the present work, we analyze them in depth with the χ 2-matching method and determine their χ 2-minimization models (CMMs). Based on those calculated CMMs, we find that KIC 8324482 is a young ( Myr), metal-poor ( dex), and intermediate-mass ( ) star with a convective core of in mass and in radius and with a surface rotation velocity . The central hydrogen abundance is of . Asteroseismic analyses indicate that the “propagation time” of g-mode in KIC 8324482 is of μHz. To well match with the observed period spacing pattern, an extra diffusion mixing ( ) should be considered, but the normal core overshooting f ov must be fixed as zero in the best-fitting model. Such ultraweak mixing beyond the convective core corresponds to a fast rotation that is about 20–30 times the asteroseismic suggested rotation Ωrot ≃ 0.2 μHz, if it is thought of as shear mixing induced completely by differential rotation.

Properties of Density and Velocity Gaps Induced by a Planet in a Protoplanetary Disk

Abstract Gravitational interactions between a protoplanetary disk and its embedded planet are one of the formation mechanisms of gaps and rings found in recent ALMA observations. To quantify the gap properties measured in not only surface density but also rotational velocity profiles, we run two-dimensional hydrodynamic simulations of protoplanetary disks by varying three parameters: the mass ratio q of a planet to a central star, the ratio of the disk scale height h p to the orbital radius r p of the planet, and the viscosity parameter α. We find that the gap depth δ Σ in the gas surface density depends on a single dimensionless parameter as , consistent with the previous results of Kanagawa et al. The gap depth δ V in the rotational velocity is given by . The gap width, in both surface density and rotational velocity, has a minimum of about 4.7h p when the planet mass M p is around the disk thermal mass M th, while it increases in a power-law fashion as M p /M th increases or decreases from unity. This minimum in the gap width arises because spirals from sub-thermal planets have to propagate before they shock the disk gas and open a gap. We compare our relations for the gap depth and width with the previous results, and discuss their applicability to observations.

Rotation rate of the solar core as a key constraint to magnetic angular momentum transport in stellar interiors

Context. The internal rotation of the Sun constitutes a fundamental constraint when modelling angular momentum transport in stellar interiors. In addition to the more external regions of the solar radiative zone probed by pressure modes, measurements of rotational splittings of gravity modes would offer an invaluable constraint on the rotation of the solar core. Aims. We study the constraints that a measurement of the core rotation rate of the Sun could bring on magnetic angular momentum transport in stellar radiative zones. Methods. Solar models accounting for angular momentum transport by hydrodynamic and magnetic instabilities were computed for different initial velocities and disc lifetimes on the pre-main sequence to reproduce the surface rotation velocities observed for solar-type stars in open clusters. The internal rotation of these solar models was then compared to helioseismic measurements. Results. We first show that models computed with angular momentum transport by magnetic instabilities and a recent prescription for the braking of the stellar surface by magnetized winds can reproduce the observations of surface velocities of stars in open clusters. These solar models predict both a flat rotation profile in the external part of the solar radiative zone probed by pressure modes and an increase in the rotation rate in the solar core, where the stabilizing effect of chemical gradients plays a key role. A rapid rotation of the core of the Sun, as suggested by reported detections of gravity modes, is thus found to be compatible with angular momentum transport by magnetic instabilities. Moreover, we show that the efficiency of magnetic angular momentum transport in regions of strong chemical gradients can be calibrated by the solar core rotation rate independently from the unknown rotational history of the Sun. In particular, we find that a recent revised prescription for the transport of angular momentum by the Tayler instability can be easily distinguished from the original Tayler–Spruit dynamo, with a faster rotating solar core supporting the original prescription. Conclusions. By calibrating the efficiency of magnetic angular momentum transport in regions of strong chemical gradients, a determination of the solar core rotation rate through gravity modes is of prime relevance not only for the Sun, but for stars in general, since radial differential rotation precisely develops in these regions during the more advanced stages of evolution.

Mechanical Feedback from Black Hole Accretion as an Energy Source of Core-collapse Supernova Explosions

Abstract According to the traditional scenario for core-collapse supernovae, the core of the collapsing star forms a neutron star (NS) and its gravitational energy release sends out a shock wave into the stellar envelope. However, in a significant number of numerical simulations, the shock stalls and the star cannot be exploded successfully, especially for a massive, compact star. We consider an alternative scenario in which, with mass fallback, the collapsing star forms a black hole in the center, surrounded by a dense, hot accretion disk, which blows out an intense outflow (wind). The kinetic energy of the wind may result in a successful stellar explosion. With an improved version of the formalism in Kohri et al., who studied NS accretion of minor fallback, we study this disk wind-driven explosion by calculating the accretion history for a suite of pre-SN stellar models with different initial surface rotational velocities, masses and metallicities, and by comparing the disk wind energy with the binding energy of the infalling stellar envelope. We show that the most promising models to be exploded successfully by this new channel are those relatively compact pre-SN stars with relatively low metallicities and not too low specific angular momenta. The total energies of the explosions are ∼1051–52 erg, and a more massive progenitor may produce a more energetic explosion.

Hot subdwarfs formed from the merger of two He white dwarfs

We perform stellar evolution calculations of the remnant of the merger of two He white dwarfs (WDs). Our initial conditions are taken from hydrodynamic simulations of double WD mergers and the viscous disc phase that follows. We evolve these objects from shortly after the merger into their core He-burning phase, when they appear as hot subdwarf stars. We use our models to quantify the amount of H that survives the merger, finding that it is difficult for $\gtrsim 10^{-4}\;{\rm M}_\odot$ of H to survive, with even less being concentrated in the surface layers of the object. We also study the rotational evolution of these merger remnants. We find that mass loss over the $\sim 10^4\;\rm yr$ following the merger can significantly reduce the angular momentum of these objects. As hot subdwarfs, our models have moderate surface rotation velocities of $30-100\; {\rm km\,s^{-1}}$. The properties of our models are not representative of many apparently-isolated hot subdwarfs, suggesting that those objects may form via other channels or that our modelling is incomplete. However, a sub-population of hot subdwarfs are moderate-to-rapid rotators and/or have He-rich atmospheres. Our models help to connect the observed properties of these objects to their progenitor systems.

The Magnetic Effect on Dynamical Tide in Rapidly Rotating Astronomical Objects

Abstract By numerically solving the equations of rotating magnetohydrodynamics, we study the magnetic effect on dynamical tide. We find that a magnetic field has a significant impact not only on the flow structure, i.e., the internal shear layers in a rotating flow can be destroyed in the presence of a moderate or stronger magnetic field (in the sense that the Alfvén velocity is at least of the order of 0.1 of the surface rotational velocity), but also on the dispersion relation of waves excited by tidal force such that the range of tidal resonance is broadened by a magnetic field. A major result is that the total tidal dissipation scales as a square of the field strength, which can be used to estimate the strength of the internal magnetic field in the astronomical object of a binary system. Moreover, with a moderate or stronger field, the ratio of magnetic dissipation to viscous dissipation is almost inversely proportional to the magnetic Prandtl number (i.e., the ratio of viscosity to magnetic diffusivity); thus, in the astrophysical situation at a small magnetic Prandtl number magnetic dissipation dominates over viscous dissipation with a moderate or stronger field.

A study of the effect of rotational mixing on massive stars evolution: surface abundances of Galactic O7-8 giant stars

Context. Massive star evolution remains only partly constrained. In particular, the exact role of rotation has been questioned by puzzling properties of OB stars in the Magellanic Clouds. Aims. Our goal is to study the relation between surface chemical composition and rotational velocity, and to test predictions of evolutionary models including rotation. Methods. We have performed a spectroscopic analysis of a sample of fifteen Galactic O7-8 giant stars. This sample is homogeneous in terms of mass, metallicity and evolutionary state. It is made of stars with a wide range of projected rotational velocities. Results. We show that the sample stars are located on the second half of the main sequence, in a relatively narrow mass range (25–40 M⊙). Almost all stars with projected rotational velocities above 100 km s-1 have N/C ratios about ten times the initial value. Below 100 km s-1 a wide range of N/C values is observed. The relation between N/C and surface gravity is well reproduced by various sets of models. Some evolutionary models including rotation are also able to consistently explain slowly rotating, highly enriched stars. This is due to differential rotation which efficiently transports nucleosynthesis products and allows the surface to rotate slower than the core. In addition, angular momentum removal by winds amplifies surface braking on the main sequence. Comparison of the surface composition of O7-8 giant stars with a sample of B stars with initial masses about four times smaller reveal that chemical enrichment scales with initial mass, as expected from theory. Conclusions. Although evolutionary models that include rotation face difficulties in explaining the chemical properties of O- and B-type stars at low metallicity, some of them can consistently account for the properties of main-sequence Galactic O stars in the mass range 25–40 M⊙.