Chemically Peculiar Research Articles

Chemically peculiar stars on the pre-main sequence

Context. The chemically peculiar (CP) stars of the upper main sequence are defined by spectral peculiarities that indicate unusual elemental abundance patterns in the presence of diffusion in the calm, stellar atmospheres. Some of them have a stable local magnetic field of up to several kiloGauss. The pre-main-sequence evolution of these objects is still a mystery and contains many open questions. Aims. We identify CP stars on the pre-main sequence to determine possible mechanisms that lead to the occurrence of chemical peculiarities in the (very) early stages of stellar evolution. Methods. We identified likely pre-main-sequence stars by fitting the spectral energy distributions. The subsequent analysis using stellar spectra and photometric time series helped us to distinguish between CP and non-CP stars. Additionally, we compared our results to the literature to provide the best possible quality assessment. Results. Out of 45 candidates, about 70% seem to be true CP stars or CP candidates. Furthermore, 9 sources appear to be CP stars on the pre-main sequence, and all are magnetic. We finally report a possible CP2 star that is also a pre-main-sequence star and was not previously in the literature. Conclusions. The evolution of the peculiarities seems to be related to the (strong) magnetic fields in these CP2 stars.

The astrophysical parameters of chemically peculiar stars from automatic methods

Context. The chemically peculiar (CP) stars of the upper main sequence are excellent astrophysical laboratories for investigating the diffusion, mass loss, rotational mixing, and pulsation in the presence and absence of a stable local magnetic field. For this, we need a homogeneous set of parameters, such as effective temperature (Teff) and surface gravity (log g), to locate the stars in the Hertzsprung-Russell diagram so that we can then estimate the mass, radius, and age. Aims. In recent years, the results of several automatic pipelines have been published; these use various techniques and data sets, including Teff and log g values for millions of stars. Because CP stars are known to have flux anomalies, these astrophysical parameters must be tested for their reliability and usefulness. If the outcome is positive, these can be used to analyse the new and faint CP stars published recently. Methods. I compared published Teff and log g values of a set of CP stars, which are mostly based on high-resolution spectroscopy, with values from four automatic pipeline approaches. In doing so, I searched for possible correlations and offsets. Results. I present a detailed statistical analysis of a comparison between the ‘standard’ and published Teff and log g values. The accuracy depends on the presence of a magnetic field and the spectral type of the CP subgroups. However, I obtain standard deviations of between 2% and 20%. Conclusions. Considering the statistical errors, the astrophysical parameters from the literature can be used for CP stars, although caution is advised for magnetic CP stars.

TESS Exploration of Targets Investigated for the Nainital-Cape Survey Project

The Nainital-Cape Survey was initiated more than two decades ago aiming to search for and study the pulsational variability in two subclasses of chemically peculiar (CP) stars, namely the Ap and Am stars. In this paper, we present the TESS photometry of 4 targets out of the 381 sample stars observed under the survey, which were not studied before using TESS data. Our results suggest that HD34060 is a rotational variable, HD 25487 is of eclipsing nature, HD 15550 exhibits pulsational variability while HD48953 is a non-variable star. The diverse variability detected in the studied sources places important constraints for the study of the internal structure and evolution of the CP stars in the presence of surface inhomogeneity, magnetic field, rotation and pulsation.

BD +30°549: a young helium-weak silicon star in the NGC 1333 star-forming region

ABSTRACT We present results of the spectroscopic study of the chemically peculiar star BD +30°549, which is a bona fide member of the young NGC 1333 star-forming region. We found that the star possesses negligible rotation and a helium-weak spectroscopic pattern with strongly enhanced Si ii and Si iii lines. The fundamental parameters of the star, Teff = 13 100 K and log (L/L⊙) = 2.1, indicate that its age is about 2.7 Myr and that its position on the Hertzsprung–Russell diagram is at the end of the pre-main-sequence evolutionary track, close to the zero-age main sequence. Abundance analysis reveals the modest deficit of almost all elements, with the exception of Si , Fe , Ca and P, which are overabundant. We performed non-local thermodynamic equilibrium (non-LTE) calculations for Si ii/Si iii, Mg ii and Ca ii in order to check the influence of departures from LTE on line formation. Non-LTE calculations lead to much better reproduction of individual silicon line profiles, but do not completely remove the abundance discrepancy between Si ii and Si iii lines. We also investigate the effects of possible chemical stratification in BD +30°549. We suspect that the ‘Si ii/Si iii anomaly’ observed in the BD +30°549 spectrum arises under the combined action of the vertical and horizontal abundance gradients and non-LTE effects. We suppose that the evolutionary status and phenomena observed in BD +30°549 indicate that conditions favourable for the selective diffusion and formation of the surface chemical composition peculiarities (slow rotation and stabilization of the atmosphere) could already be built up at the pre-main-sequence phase.

Are Am stars and hot-Jupiter planets related?

Context. Metallic-lined A stars (Am stars) are often components of short-period binary systems, where tidal interactions would result in low rotational velocities and help to develop the chemical peculiarities observed. However, the origin of single Am stars and Am stars that belong to wide binary systems is unclear. Aims. There is very recent evidence of an Am star hosting a likely synchronized hot brown dwarf (hot BD) and other possible Am stars hosting hot Jupiter planets. Following literature suggestions, we wonder if these hot low-mass companions could play a role in the development of an Am star, that is to say, if they could help to mitigate the “single Am” problem. Methods. We carried out a detailed abundance determination via spectral synthesis of 19 early-type stars hosting hot BDs and hot Jupiter planets, in order to determine the possible presence of Am stars in this sample. The abundances were determined iteratively for 25 different species by fitting synthetic spectra using the SYNTHE program together with local thermodynamic equilibrium (LTE) ATLAS12 model atmospheres. The abundances of C I, O I, and Mg I were corrected by non-LTE effects. The complete chemical patterns of the stars were then compared to those of Am stars and other chemically peculiar stars. Results. We studied a sample of 19 early-type stars, seven of them hosting hot BDs and 12 of them hosting hot Jupiter planets. We detected four Am stars in our sample (KELT-19A, KELT-17, HATS-70, and TOI-503) and two possible Am stars (TOI-681 and HAT-P-69). In particular, we detected the new Am star HATS-70, which hosts a hot BD, and we ruled out this class for the hot Jupiter host, WASP-189. Both stars show a different composition than previously reported. For the first time, we estimated the incidence of Am stars among stars hosting hot BDs (50−75%) and among stars hosting hot Jupiters (20−42%). The incidence of Am stars hosting hot BDs has shown to be higher than the frequency of Am stars in general. This would imply that the presence of hot BDs could play a role in the development of Am stars and possibly help to mitigate the single Am problem, which is different for the case of hot Jupiter planets. Notably, these results also indicate that the search for hot BDs may benefit from the targeting of single Am stars or Am stars in wide binary systems. We encourage the analysis of additional early-type stars hosting hot companions in order to improve the significance of the initial trends found here.

HD 60431, the CP2 star with the shortest rotational period

Context. CP2 stars show periodic photometric, spectroscopic, and magnetic variations with the rotational period. They are generally slow rotators, with rotational periods exceeding half a day, except for the late B-type star HD 60431, which has an unusually short rotational period of 0.4755 days. As slow rotation is deemed a necessary criterion for the establishment of chemical peculiarities, this characteristic renders HD 60431 a special object that might offer valuable insight into, and constraints on, the formation and evolution of CP2 stars. Aims. The aims of our study are to analyse the light variability, derive atmospheric abundances, and determine the detailed physical parameters of HD 60431 to confirm its status as the CP2 star with the shortest known rotational period, with special emphasis on the rotational period evolution. Methods. Photometric indices and high-resolution spectroscopy were employed to derive the physical parameters, evolutionary status, and atmospheric abundances of our target star. A light variability study was carried out using combined sets of photometric data from ground- and space-based facilities. A circularly polarised spectrum was employed to check the presence of a longitudinal magnetic field in the star. Results. With an age of only 10 Myr, an effective temperature of Teff = 13 000 ± 300 K, surface gravity log g = 4.10 ± 0.10, radius R = 1.97 ± 0.09 R⊙, and mass M = 3.1 ± 0.1 M⊙, HD 60431 is situated close to the zero age main sequence; it is a member of the open cluster NGC 2547 in the Vela OB2 complex. We confirm its status as a classical late B-type CP2 star showing strong overabundances of Mg (1.8 dex), Si (1.9 dex), Ca (1.6 dex), Ti (2.2 dex), and Fe (1.8 dex). No conclusive evidence for the presence of a strong magnetic field was found in the available spectroscopic data. The light curve of HD 60431 has remained constant over the last four decades. The available photometric time series data confirm the short rotational period and indicate a slight secular increase in the rotational period of Ṗ = 2.36(19) × 10−10 = 7.5(6) ms yr−1. The following quadratic ephemeris has been derived: HJDmin(E) = 2 459 212.969 35 + 0. d475 516 64 E + 5. d62 × 10−11E2. Conclusions. HD 60431 is indeed the CP2 star with the shortest known rotational period. Theory needs to explain the establishment and maintenance of chemical peculiarities in such a young and fast-rotating object. Our results furthermore raise the question of whether period variability on timescales significantly shorter than stellar evolution is inherent to all magnetic chemically peculiar stars.

Pulsation of chemically peculiar stars

AbstractThis paper reviews our current knowledge about pulsating chemically peculiar (CP) stars. CP stars are slowly rotating upper main-sequence objects, efficiently employing diffusion in their atmospheres. They can be divided into magnetic and non-magnetic objects. Magnetic activity significantly influence their pulsational characteristics. Only a handful of magnetic, classical pulsating objects are now known. The only exceptions are about 70 rapidly oscillating Ap stars, which seem to be located within a very tight astrophysical parameter space. Still, many observational and theoretical efforts are needed to understand all important physical aspects and their interrelationships. The most important steps to reach these goals are reviewed.

Chemically peculiar stars in integrated light stellar population models and local group galaxies

ABSTRACT Integrated light models that incorporate common types of chemically peculiar (CP) stars are assembled using synthetic spectra. Selected spectral features encode significant age information for populations with ages ∼50 Myr < age < ∼2 Gyr. Due to the alleviation of template mismatch, the inclusion of CP star features in model spectra improves the accuracy of recovered stellar population parameters, but we are not able to show that new or unique age information can be extracted from the weak CP features compared to continuum fitting and strong-feature strengths, at least at the present state of the art. An age-extraction routine that recovers two- and three-burst age structures is employed to analyse the spectra of local group galaxies. NGC 224 (M 31) has a stellar population too old for the types of CP stars we examine. NGC 221 (M 32) also shows no CP spectral features. It appears to contain a component at age ∼1 Gyr at 1 per cent by mass in addition to its dominant 4.7 Gyr population. Unlike Sloan Digital Sky Survey galaxy spectrum averages, NGC 205 (M 110) contains no features due to mercury–manganese (HgMn) stars. This excludes the age range associated with HgMn production, and its near-nuclear spectrum is best fitted by a 68 ± 2 Myr population superimposed on an older population with a 1.85 ± 0.1 Gyr component. Both NGC 205 and NGC 221 have an ancient component whose mass is not easy to constrain given the overwhelming light dominance of the younger populations.

Seismic modelling of the pulsating mercury–manganese star HD 29589

ABSTRACT Photometric and spectroscopic time-series of chemically peculiar mercury–manganese (HgMn) stars show variability, which in most cases is interpreted as the effect of binarity or surface spots. Until recently, pulsations have not been detected in these objects. However, the analysis of the Transiting Exoplanet Survey Satellite (TESS) photometric time-series of a large sample of HgMn stars resulted in a small sample of candidates for pulsating variables. In this paper, we present a thorough analysis of one of them, HD 29589. High-resolution spectroscopic data were used to determine the atmospheric parameters and a detailed chemical composition of the star. The obtained effective temperature, Teff = 14 400 ± 200 K, places this star among the high-temperature HgMn stars. The determined abundance pattern is typical for this class of objects. Interferometric data were used to discuss the properties of the secondary in the HD 29589 system. More than 20 independent frequencies were identified in the TESS time-series of this star, which we interpret as high-order g-modes. We found regular period spacing, which is due to the presence of consecutive prograde dipole modes. The exact matching of the six frequencies in the series and the position of the star in the Hertzsprung–Russell diagram allowed us to constrain the internal structure of HD 29589. We derived the overshooting parameter, fov = 0.028−0.03, mass M = 3.4 M⊙ and metallicity Z = 0.008. A detailed analysis of pulsating HgMn stars will allow us to understand the influence of the chemical peculiarity on the pulsation characteristics of these objects.

Spectropolarimetry of magnetic Chemically Peculiar stars in the Orion OB1 association

ABSTRACT We summarize the results of a spectropolarimetric survey of 56 chemically peculiar (CP) stars in the association of Orion OB1. We uniformly collected the observational material with the 6-m telescope BTA of the Special Astrophysical Observatory in 2013–2021. We identify 14 new magnetic CP stars with a longitudinal magnetic field exceeding approximately 500 G. The studied sample contains 31 magnetic stars or 55 per cent of the whole CP population in Orion OB1. We show that the percentage of the magnetic CP stars and the field strength drop sharply with age. The mean longitudinal magnetic field in the young subgroup OB1b (log t = 6.23) is confidently almost three times stronger than in the older subgroups OB1a (log t = 7.05) and OB1c (log t = 6.66). In the Orion Nebula, a place with the youngest stellar population (log t < 6.0), we detect the magnetic field only in 20 per cent of CP stars. Such an occurrence drastically differs from 83 per cent of magnetic CP stars in the nearby subgroup OB1c. We consider this effect an observational bias caused by a significant portion of a very young population with the signatures of Herbig Ae/Be stars. The technique we used for magnetic measurements and the quality of available data do not allow us to detect weak fields in the case of stars with a limited number of lines and emissions in spectra.

The evolutionary state of the chemically peculiar members of the open cluster NGC 2516

ABSTRACT We aim at establishing safe membership and evolutionary status of 11 chemically peculiar (CP) stars that are residing in the domain of the open cluster NGC 2516 and are frequently referred to as cluster members. We queried the Gaia EDR3 catalogue in an area with a radius of 1° and selected 37 508 stars brighter than G = 19 mag. The cluster membership was determined in parallax-proper motion-space and 719 probable and 764 possible members were found. The obtained average astrometric and photometric parameters of the cluster are in good agreement with the most recent literature data. The evolutionary status of the target stars was determined with respect to Padova isochrones. After minor adjustments including the metallicity, the reddening, and the transformation scale variation, a perfect fit of the model to the observations over the whole observed magnitude range was achieved. Only 5 of the 11 considered CP stars could be classified as highly probable cluster members. Among the Ap/Bp stars with previously detected magnetic fields HD 65987 and HD 65712 have a high membership probability and the magnetic star CPD−60 944B is a possible cluster member. Further we discuss the blue straggler nature of HD 66194 and the magnetic star HD 65987. To our knowledge, HD 65987 is currently the only known blue straggler, with a field of the order of a few hundred Gauss. The most striking result of our study is that the strongly magnetic A0p star HD 66318 with previously reported very low fractional age does not belong to the NGC 2516 cluster at a high level of confidence.

Zeeman Doppler mapping revisited: Force-free fields, regularisation functions, and abundance maps

Aims. For the observational modelling of horizontal abundance distributions and of magnetic geometries in chemically peculiar (CP) stars, Zeeman Doppler mapping (ZDM) has become the method of choice. Comparisons between abundance maps obtained for CP stars and predictions from numerical simulations of atomic diffusion have always proved unsatisfactory. This study is intended to explore the reasons for the discrepancies. Methods. We cast a cold eye (evoking the epitaph on Nobel laureate W.B. Yeats’ gravestone: Cast a cold Eye / On Life, on Death. / Horseman, pass by) on essential assumptions underlying ZDM, in particular, the formulae governing the magnetic field geometry, but also the regularisation functionals. Results. Recognising that the observed strong magnetic fields in most well-mapped stars require the field geometry to be force free, we show that the formulae used so far to describe the magnetic geometry do not meet this condition. It follows that the published magnetic maps and the abundance maps of these stars are all spurious. Conclusions. To obtain observational constraints for the modelling of atomic diffusion, the use in ZDM of the correct formulae for force-free or potential magnetic fields is paramount. Extensive simulations are required to quantify the effects of chemical stratifications and of regularisation functions on the recovered magnetic and abundance maps.

A rare phosphorus-rich star in an eclipsing binary from TESS

Context. Few exoplanets around hot stars with radiative envelopes have been discovered, although new observations from the TESS mission are improving this. Stars with radiative envelopes have little mixing at their surface, and thus their surface abundances provide a sensitive test case for a variety of processes, including potentially star–planet interactions. Atomic diffusion is particularly important in these envelopes, producing chemically peculiar objects such as Am and HgMn stars. Aims. An exoplanet candidate around the B6 star HD 235349 was identified by TESS. Here we determine the nature of this transiting object and identify possible chemical peculiarities in the star. Methods. HD 235349 was observed using the long-slit spectrograph at Tartu Observatory, as well as photometrically by the TESS mission. The spectra were modeled to determine stellar parameters and chemical abundances. The photometric light curve was then analyzed in the context of the stellar parameters to determine properties of the transiting object. Results. We find the transiting object is a low-mass stellar companion, not a planet. However, the primary of this eclipsing binary is a rare type of chemically peculiar star. A strong overabundance of P is found with overabundances of Ne and Nd and mild overabundances of Ti and Mn, while He is mildly underabundant. There is also clear evidence for vertical stratification of P in the atmosphere of the star. The lack of Hg and the weak Mn overabundance suggests that this is not a typical HgMn star. It may be in the class of helium-weak phosphorus-gallium (He-weak PGa) stars or an intermediate between these two classes. Conclusions. We show that HD 235349 is a rare type of chemically peculiar star (He-weak PGa) in an eclipsing binary system with a low-mass stellar companion. This appears to be the first He-weak PGa star discovered in an eclipsing binary.

Échelle spectroscopy of the chemically peculiar star θ1 Ori F

ABSTRACT We analyse Échelle spectra of θ1 Ori F obtained by us on six nights unevenly distributed along 6 yr; we identify several hundred spectral lines and measure, for the first time, the star’s heliocentric radial velocity. We also collect and discuss previously published photometry of θ1 Ori F. We find that θ1 Ori F is a chemically peculiar (CP) star with overabundant silicon and phosphorus, and possibly other elements as well. From the singly ionized Fe, Cr and Ti lines we estimate its spectral type to be between B7 and B8. The radial velocity of θ1 Ori F is possibly marginally variable, with an average of 24 ± 4.2 km s−1 (standard deviation), in good agreement with the mean radial velocity of the Orion Nebula Cluster members, and about 5 km s−1 smaller than the average of the other Trapezium components. We cast doubt on the coeval nature of this star relative to the other Trapezium components, and present arguments that almost certainly exclude its membership to the Orion Trapezium. θ1 Ori F turns out to be enigmatic in several respects, and is probably an important link for understanding the evolutionary stage at which the CP phenomenon sets on.

Rotational variation in the chemically peculiar B0 star θ Car as seen by TESS

ABSTRACT θ Car is a bright well studied B0 type star in a close binary (Porb = 2.2 d) with an unseen companion. The system has enjoyed extensive spectroscopic characterization at multiple wavelengths, revealing narrow X-ray lines, a soft X-ray tail, and chemical peculiarities. The star was recently observed in high-cadence photometric time-series by the NASA TESS satellite. In this paper, we investigate the photometric variability and demonstrate that it is likely associated with surface brightness inhomogeneities rather than orbital modulation. We investigate the potential distribution of surface features required to produce the observed variability, and report a new rotational period of Prot = 2.2034 d to be commensurate with the orbital period.

Magnetic Fields of CP Stars in the Orion OB1 Association. IV. Stars of Subgroup 1b

The paper presents magnetic field measurements for 15 chemically peculiar (CP) stars of subgroup~1b in the OrionOB1 association. We have found that the proportion of stars with strong magnetic fields among these 15 CP stars is almost twice as large as in subgroup 1a. Along with this, the age of subgroup 1b is estimated as 2 Myr, and the age of subgroup~1a is in the order of 10 Myr. The average root-mean-square magnetic field Be for stars in subgroup 1b is 2.3 times higher than that for stars in subgroup 1a. The conclusions obtained fall within the concept of the fossil origin of large-scale magnetic fields in B and A stars, but the rate of field weakening with age appears anomalously high. We present our results as an important observational test for calibrating the theory of stellar magnetic field formation and evolution.

Rubidium in Barium stars

ABSTRACT Barium (Ba) stars are chemically peculiar stars that display in their atmospheres signatures of the slow neutron-capture (s-process) mechanism that operates within asymptotic giant branch (AGB) stars, an important contributor to the cosmic abundance. The observed chemical peculiarity in these objects is not due to self-enrichment, but to mass transfer between the components of a binary system. The atmospheres of Ba stars are therefore excellent astrophysical laboratories, providing strong constraints for the nucleosynthesis of the s-process in AGB stars. In particular, rubidium (Rb) is a key element for the s-process diagnostic because it is sensitive to the neutron density and hence its abundance points to the main neutron source of the s-process in AGB stars. We present Rb abundances for a large sample of 180 Ba stars from high-resolution spectra (R = 48 000), and we compare the observed [Rb/Zr] ratios with theoretical predictions from s-process models in AGB stars. The target Ba stars in this study display [Rb/Zr] < 0, showing that Rb was not efficiently produced by the activation of the branching points at 85Kr and 86Rb. Model predictions from the Monash and FRUITY datasets of low-mass (≲4 M⊙) AGB stars are able to cover the Rb abundances observed in the program Ba stars. These observations indicate that the 13C(α,n)16O reaction is the main neutron source of the s-process in the low-mass AGB companions of the observed Ba stars. We have not found in the present study candidate companions for former IR/OH massive AGB stars.

V772 Cas: an ellipsoidal HgMn star in an eclipsing binary

ABSTRACT The late B-type star V772 Cas (HD 10260) was previously suspected to be a rare example of a magnetic chemically peculiar star in an eclipsing binary system. Photometric observations of this star obtained by the TESS satellite show clear eclipses with a period of 5.0137 d accompanied by a significant out-of-eclipse variation with the same period. High-resolution spectroscopy reveals V772 Cas to be an SB1 system, with the primary component rotating about a factor two slower than the orbital period and showing chemical peculiarities typical of non-magnetic HgMn chemically peculiar stars. This is only the third eclipsing HgMn star known and, owing to its brightness, is one of the very few eclipsing binaries with chemically peculiar components accessible to detailed follow-up studies. Taking advantage of the photometric and spectroscopic observations available for V772 Cas, we performed modelling of this system with the phoebe code. This analysis provided fundamental parameters of the components and demonstrated that the out-of-eclipse brightness variation is explained by the ellipsoidal shape of the evolved, asynchronously rotating primary. This is the first HgMn star for which such variability has been definitively identified.

New BRITE-Constellation observations of the roAp star α Cir

Context. Chemically peculiar (CP) stars with a measurable magnetic field comprise the group of mCP stars. The pulsating members define the subgroup of rapidly oscillating Ap (roAp) stars, of which α Cir is the brightest member. Hence, α Cir allows the application of challenging techniques, such as interferometry, very high temporal and spectral resolution photometry, and spectroscopy in a wide wavelength range, that have the potential to provide unique information about the structure and evolution of a star. Aims. Based on new photometry from BRITE-Constellation, obtained with blue and red filters, and on photometry from WIRE, SMEI, and TESS we attempt to determine the surface spot structure of α Cir and investigate pulsation frequencies. Methods. We used photometric surface imaging and frequency analyses and Bayesian techniques in order to quantitatively compare the probability of different models. Results. BRITE-Constellation photometry obtained from 2014 to 2016 is put in the context of space photometry obtained by WIRE, SMEI, and TESS. This provides improvements in the determination of the rotation period and surface features (three spots detected and a fourth one indicated). The main pulsation frequencies indicate two consecutive radial modes and one intermediate dipolar mode. Advantages and problems of the applied Bayesian technique are discussed.

Photometry and high-resolution spectroscopy of comet 21P/Giacobini-Zinner during its 2018 apparition

We report on photometry and high-resolution spectroscopy of the chemically peculiar Jupiter-family comet (hereafter JFC) 21P/Giacobini-Zinner. Comet 21P is a well-known member of the carbon-chain-depleted family, but also displays a depletion of amines. We continuously monitored the comet over more than seven months with the two TRAPPIST telescopes (TN and TS), covering a wide heliocentric distance range from 1.60 au inbound to 2.10 au outbound with a perihelion at 1.01 au on September 10, 2018. We computed and followed the evolution of the dust- (represented by Afρ) and gas-production rates of the daughter species OH, NH, CN, C3, and C2 and their relative abundances to OH and to CN over the cometary orbit. We compared them to those measured in the previous apparitions. The activity of the comet and its water production rate reached a maximum of (3.72 ± 0.07) × 1028 mol s−1 on August 17, 2018 (rh = 1.07 au), 24 days before perihelion. The peak value of A(0)fρ was reached on the same date (1646 ± 13) cm in the red filter. Using a sublimation model for the nucleus, we constrained the active surface of the nucleus using the slow-rotator model. The abundance ratios of the various species are remarkably constant over a wide range of heliocentric distances before and after perihelion, showing a high level of homogeneity of the ices in the surface of the nucleus. The behaviour and level of the activity of the comet is also remarkably similar over the last five orbits. In the coma dust colour, 21P shows reflective gradients similar to JFCs. We obtained a high-resolution spectrum of 21P with UVES at ESO Very Large Telescope one week after perihelion. Using the CN B-X (0, 0) violet band, we measured 12C/13C and 14N/15N isotopic ratios of 100 ± 10 and 145 ± 10, respectively, both in very good agreement with the ratios commonly found in comets. We measured an ortho-para abundance ratio of NH3 of 1.16 ± 0.02, corresponding to a nuclear spin temperature of Tspin = 27 ± 1 K, which is similar to other comets. While the abundance ratios in the gaseous coma reveal a peculiar composition, the isotopic and ortho-to-para ratios appear entirely normal. We performed a dynamical simulation of 21P and found that it is likely a young member of the JFC population. We favour a pristine composition scenario to explain the chemical peculiarities of this comet.