Early-A stars from IPHAS, and their distribution in and around the Cyg OB2 association

Stellar photometry derived from the INT/WFC Photometric Hα Survey (IPHAS) of the Northern Galactic plane can be used to identify large, reliable samples of A0–A5 stars. For every A-type star, so identified, it is also possible to derive individual reddening and distance estimates, under the assumption that most selected objects are on or near the main sequence, at a mean absolute r ' magnitude of 1.5–1.6. This study presents the method for obtaining such samples and shows that the known reddenings and distances to the open clusters NGC 7510 and NGC 7790 are successfully recovered. A sample of over 1000 A-type stars is then obtained from IPHAS data in the magnitude range  13.5 < r ' < 20  from the region of sky including the massive northern OB association Cyg OB2. An analysis of these data reveals a concentration of ∼200 A stars over an area about a degree across, offset mainly to the south of the known 1–3 Myr old OB stars in Cyg OB2: their dereddened r ' magnitudes fall in the range 11.8–12.5. These are consistent with a ∼7 Myr old stellar population at distance modulus DM = 10.8, or with an age of ∼5 Myr at DM = 11.2. The number of A-type stars found in this clustering alone is consistent with a lower limit to the cluster mass of  ∼10⁴ M_⊙  .     

Triggered star formation and evolution of T-Tauri stars in and around bright-rimmed clouds

The aim of this paper is to quantitatively testify the ' small-scale sequential star formation ' hypothesis in and around bright-rimmed clouds (BRCs). As a continuation of the recent attempt by Ogura et al., we have carried out   BVI_c   photometry of four more BRC aggregates along with deeper re-observations of two previously observed BRCs. Again, quantitative age gradients are found in almost all the BRCs studied in the present work. Archival Spitzer /Infrared Array Camera data also support this result. The global distribution of near-infrared excess stars in each H  ii region studied here clearly shows evidence that a series of radiation-driven implosion processes proceeded in the past from near the central O star(s) towards the peripheries of the H  ii region. We found that in general weak-line T-Tauri stars (WTTSs) are somewhat older than classical T-Tauri stars (CTTSs). Also the fraction of CTTSs among the T-Tauri stars (TTSs) associated with the BRCs is found to decrease with age. These facts are in accordance with the recent conclusion by Bertout, Siess & Cabrit that CTTSs evolve into WTTSs. It seems that in general the equivalent width of Hα emission in TTSs associated with the BRCs decreases with age. The mass function (MF) of the aggregates associated with the BRCs of the morphological type 'A' seems to follow that found in young open clusters, whereas 'B/C'-type BRCs show significantly steeper MF.     

Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars

We present Hα spectropolarimetry observations of a sample of 10 bright T Tauri stars, supplemented with new Herbig Ae/Be star data. A change in the linear polarization across Hα is detected in most of the T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret in terms of a compact source of line photons that is scattered off a rotating accretion disc. We find consistency between the position angle (PA) of the polarization and those of imaged disc PAs from infrared and millimetre imaging and interferometry studies, probing much larger scales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find the polarization PA to be perpendicular to the imaged disc, which is expected for single scattering. On the other hand, the polarization PA aligns with the outer disc PA for the T Tauri stars DR Tau and SU Aur and FU Ori, conforming to the case of multiple scattering. This difference can be explained if the inner discs of Herbig Ae stars are optically thin, whilst those around our T Tauri stars and FU Ori are optically thick. Furthermore, we develop a novel technique that combines known inclination angles and our recent Monte Carlo models to constrain the inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, we consider the connection of the inner disc structure with the orientation of the magnetic field in the foreground interstellar medium: for FU Ori and DR Tau, we infer an alignment of the stellar axis and the larger magnetic field direction.     

Mapping the β Cephei instability strip: photometric variability of stars in the central part of the Cygnus OB2 association

The central part of the Cygnus OB2 association was searched for the presence of short-period hot pulsators. Out of 16 O-type stars in the field, five turned out to be variable. None of the O-type variables showed short-period variations characteristic for the β Cephei-type stars. However, two β Cephei stars were found among the early B-type members of the association. Their pulsation periods are too long to be in agreement with the average association age. We note that the discrepancy can be explained by non-coeval star formation, which was already suggested by other authors from the investigation of the colour–magnitude diagrams of the association.   We found 29 new variables in all fields searched by us. About half of these are Cygnus OB2 members. By means of Hα photometry, we confirmed the presence of Hα emission in two stars and found one new Be star. A weak Hα emission is probably present in all observed O-type supergiants and giants. We also give the new BVRI Hα photometry for almost 300 stars. This multicolour photometry is used to determine a dereddened colour–magnitude diagram for the association and make a reddening map of the observed field. The extinction in the field, expressed as the total absorption in the Johnson V filter, A _v , ranges from 4.1 to 7.0 mag; the average A _V amounts to 5.8 mag. One O-type star is probably a background object.     

Initial data release from the INT Photometric Hα Survey of the Northern Galactic Plane (IPHAS)

The INT/WFC Photometric Hα Survey of the Northern Galactic Plane (IPHAS) is an imaging survey being carried out in  Hα, r '  and   i '  filters, with the Wide Field Camera (WFC) on the 2.5-m Isaac Newton Telescope (INT) to a depth of   r '= 20  (10σ). The survey is aimed at revealing the large scale organization of the Milky Way and can be applied to identifying a range of stellar populations within it. Mapping emission line objects enables a particular focus on objects in the young and old stages of stellar evolution ranging from early T-Tauri stars to late planetary nebulae. In this paper we present the IPHAS Initial Data Release, primarily a photometric catalogue of about 200 million unique objects, coupled with associated image data covering about 1600 deg² in three passbands. We note how access to the primary data products has been implemented through use of standard virtual observatory publishing interfaces. Simple traditional web access is provided to the main IPHAS photometric catalogue, in addition to a number of common catalogues (such as 2MASS) which are of immediate relevance. Access through the AstroGrid VO Desktop opens up the full range of analysis options, and allows full integration with the wider range of data and services available through the Virtual Observatory. The IDR represents the largest data set published primarily through VO interfaces to date, and so stands as an exemplar of the future of survey data mining. Examples of data access are given, including a cross-matching of IPHAS photometry with sources in the UKIDSS Galactic Plane Survey that validates the existing calibration of the best data.     

Formation of current helicity and emerging magnetic flux in solar active regions

We present the results of a photometric study of X-ray-active weak-lined T Tauri (WTT) stars in the η Chamaeleontis star cluster. Multi-epoch V -band photometric monitoring during 1999 and 2000 of the 10 X-ray-active WTT stars found that all were variable in one or both years, with periods ascribed to rotational modulation of starspots. Comparison between the rotational and X-ray properties of these objects indicates the saturation level,     observed in other studies of X-ray-active pre-main-sequence stars, persists in the η Cha stars from the slow- to the fast-rotator regimes. Cousins VRI photometry of the WTT stars has enabled us to investigate further the photometric properties of these stars. The stars appear sufficiently coeval to distinguish near-equal-mass binaries within the sample. A new Hertzsprung–Russell diagram for these objects suggests ages of 4–9 Myr for M-type RECX primaries using the tracks of D'Antona & Mazzitelli.     

GEMINI near-infrared spectroscopic observations of young massive stars embedded in molecular clouds

K -band spectra of young stellar candidates in four Southern hemisphere clusters have been obtained with the Gemini Near-Infrared Spectrograph in Gemini South. The clusters are associated with IRAS sources that have colours characteristic of ultracompact H  ii regions. Spectral types were obtained by comparison of the observed spectra with those of a near-infrared (NIR) library; the results include the spectral classification of nine massive stars and seven objects confirmed as background late-type stars. Two of the studied sources have K -band spectra compatible with those characteristic of very hot stars, as inferred from the presence of C  iv , N  iii and N  v emission lines at 2.078, 2.116 and 2.100 μm, respectively. One of them, I16177_IRS1, has a K -band spectrum similar to that of Cyg OB2 7, an O3If* supergiant star. The nebular K -band spectrum of the associated Ultra-Compact (UC) H  ii region shows the s-process [Kr  iii ] and [Se  iv ] high excitation emission lines, previously identified only in planetary nebula. One young stellar object was found in each cluster, associated with either the main IRAS source or a nearby resolved Midecourse Space eXperiment ( MSX ) component, confirming the results obtained from previous NIR photometric surveys. The distances to the stars were derived from their spectral types and previously determined JHK magnitudes; they agree well with the values obtained from the kinematic method, except in the case of IRAS  15408−5356  , for which the spectroscopic distance is about a factor of 2 smaller than the kinematic value.     

Towards the main sequence: detailed analysis of weak line and post-T Tauri stars

A detailed study was performed for a sample of low-mass pre-main-sequence (PMS) stars, previously identified as weak-line T Tauri stars, which are compared to members of the Tucanae and Horologium Associations. Aiming to verify if there is any pattern of abundances when comparing the young stars at different phases, we selected objects in the range from 1 to 100 Myr, which covers most of PMS evolution. High-resolution optical spectra were acquired at European Southern Observatory and Observatório do Pico dos Dias . The stellar fundamental parameters effective temperature and gravity were calculated by excitation and ionization equilibria of iron absorption lines. Chemical abundances were obtained via equivalent width calculations and spectral synthesis for 44 per cent of the sample, which shows metallicities within 0.5 dex solar. A classification was developed based on equivalent width of Li  i 6708 Å and Hα lines and spectral types of the studied stars. This classification allowed a separation of the sample into categories that correspond to different evolutive stages in the PMS. The position of these stars in the Hertzsprung–Russell diagram was also inspected in order to estimate their ages and masses. Among the studied objects, it was verified that our sample actually contains seven weak-line T Tauri stars, three are Classical T Tauri, 12 are Fe/Ge PMS stars and 21 are post-T Tauri or young main-sequence stars. An estimation of circumstellar luminosity was obtained using a disc model to reproduce the observed spectral energy distribution. Most of the stars show low levels of circumstellar emission, corresponding to less than 30 per cent of the total emission.     

The properties of cataclysmic variables in photometric Hα surveys

We report on the properties of 71 known cataclysmic variables (CVs) in photometric Hα emission-line surveys. Our study is motivated by the fact that the Isaac Newton Telescope (INT) Photometric Hα Survey of the northern galactic plane (IPHAS) will soon provide r ',  i ' and narrow-band Hα measurements down to   r '≃ 20  for all northern objects between  − 5° < b < +5°  . IPHAS thus provides a unique resource, both for studying the emission-line properties of known CVs and for constructing a new CV sample selected solely on the basis of Hα excess. Our goal here is to carry out the first task and prepare the way for the second. In order to achieve this, we analyse data on 19 CVs already contained in the IPHAS data base and supplement this with identical observations of 52 CVs outside the galactic plane.
Our key results are as follows: (i) the recovery rate of known CVs as Hα emitters in a survey like IPHAS is ≃70 per cent; (ii) of the ≃30 per cent of CVs which were not recovered ≃75 per cent were clearly detected but did not exhibit a significant Hα excess at the time of our observations; (iii) the recovery rate depends only weakly on CV type; (iv) the recovery rate depends only weakly on orbital period; (v) short-period dwarf novae tend to have the strongest Hα lines. These results imply that photometric emission-line searches provide an efficient way of constructing CV samples that are not biased against detection of intrinsically faint, short-period systems.     

Stellar contents and star formation in the young star cluster Be 59

We present   UBV  I _c   CCD photometry of the young open cluster Be 59 with the aim to study the star formation scenario in the cluster. The radial extent of the cluster is found to be ∼10 arcmin (2.9 pc). The interstellar extinction in the cluster region varies between   E ( B − V ) ≃ 1.4  to 1.8 mag. The ratio of total-to-selective extinction in the cluster region is estimated as  3.7 ± 0.3  . The distance of the cluster is found to be  1.00 ± 0.05 kpc  . Using near-infrared (NIR) colours and slitless spectroscopy, we have identified young stellar objects (YSOs) in the open cluster Be 59 region. The ages of these YSOs range between <1 and ∼2 Myr, whereas the mean age of the massive stars in the cluster region is found to be ∼2 Myr. There is evidence for second-generation star formation outside the boundary of the cluster, which may be triggered by massive stars in the cluster. The slope of the initial mass function, Γ, in the mass range  2.5 < M /M_⊙≤ 28  is found to be  −1.01 ± 0.11  which is shallower than the Salpeter value (−1.35), whereas in the mass range  1.5 < M /M_⊙≤ 2.5  the slope is almost flat. The slope of the K -band luminosity function is estimated as  0.27 ± 0.02  , which is smaller than the average value (∼0.4) reported for young embedded clusters. Approximately 32 per cent of Hα emission stars of Be 59 exhibit NIR excess indicating that inner discs of the T Tauri star (TTS) population have not dissipated. The Midcourse Space Experiment (MSX) and IRAS-HIRES images around the cluster region are also used to study the emission from unidentified infrared bands and to estimate the spatial distribution of optical depth of warm and cold interstellar dust.     

Surface magnetic fields on two accreting T Tauri stars: CV Cha and CR Cha

We have produced brightness and magnetic field maps of the surfaces of CV Cha and CR Cha: two actively accreting G- and K-type T Tauri stars in the Chamaeleon I star-forming cloud with ages of 3–5 Myr. Our magnetic field maps show evidence for strong, complex multipolar fields similar to those obtained for young rapidly rotating main-sequence stars. Brightness maps indicate the presence of dark polar caps and low-latitude spots – these brightness maps are very similar to those obtained for other pre-main-sequence and rapidly rotating main-sequence stars.
Only two other classical T Tauri stars have been studied using similar techniques so far: V2129 Oph and BP Tau. CV Cha and CR Cha show magnetic field patterns that are significantly more complex than those recovered for BP Tau, a fully convective T Tauri star.
We discuss possible reasons for this difference and suggest that the complexity of the stellar magnetic field is related to the convection zone; with more complex fields being found in T Tauri stars with radiative cores (V2129 Oph, CV Cha and CR Cha). However, it is clearly necessary to conduct magnetic field studies of T Tauri star systems, exploring a wide range of stellar parameters in order to establish how they affect magnetic field generation, and thus how these magnetic fields are likely to affect the evolution of T Tauri star systems as they approach the main sequence.     

High spatial resolution Galactic 3D extinction mapping with IPHAS

We present an algorithm (MEAD, for 'Mapping Extinction Against Distance') which will determine intrinsic  ( r '− i ')  colour, extinction, and distance for early-A to K4 stars extracted from the IPHAS   r '/ i '/Hα  photometric data base. These data can be binned up to map extinction in three dimensions across the northern Galactic plane. The large size of the IPHAS data base (∼200 million unique objects), the accuracy of the digital photometry it contains and its faint limiting magnitude  ( r '∼ 20)  allow extinction to be mapped with fine angular (∼10 arcmin) and distance (∼ 0.1 kpc) resolution to distances of up to 10 kpc, outside the solar circle. High reddening within the solar circle on occasion brings this range down to ∼2 kpc. The resolution achieved, both in angle and depth, greatly exceeds that of previous empirical 3D extinction maps, enabling the structure of the Galactic Plane to be studied in increased detail. MEAD accounts for the effect of the survey magnitude limits, photometric errors, unresolved interstellar medium (ISM) substructure and binarity. The impact of metallicity variations, within the range typical of the Galactic disc is small. The accuracy and reliability of MEAD are tested through the use of simulated photometry created with Monte Carlo sampling techniques. The success of this algorithm is demonstrated on a selection of fields and the results are compared to the literature.     

Doppler images and chromospheric variability of TWA 17

We present Doppler imaging and a Balmer line analysis of the weak-line T Tauri star TWA 17. Spectra were taken in 2006 with the University College London Echelle Spectrograph on the Anglo-Australian Telescope. Using least-squares deconvolution to improve the effective signal-to-noise ratio, we produced a Doppler map of the surface spot distribution. This shows similar features to maps of other rapidly rotating T Tauri stars, i.e. a polar spot with more spots extending out of it down to the equator.
In addition to the photospheric variability, the chromospheric variability was studied using the Balmer emission. The mean Hα profile has a narrow component consistent with rotational broadening and a broad component extending out to 220 km s⁻¹. The variability in Hα suggests that the chromosphere has at least one slingshot prominence  3 R _*  above the surface.     

Hipparcos observations of pre-main-sequence stars

We present first results of Hipparcos observations of nearby low-mass pre-main-sequence (PMS) stars (T Tauri and Herbig Ae/Be stars). The data obtained by Hipparcos allow us to derive weighted mean parallaxes for three major nearby star-forming regions (SFRs), Lupus, Chamaeleon I and Taurus–Auriga. Furthermore, results on the isolated objects AB Dor and TW Hya are presented. Finally, we discuss the evolutionary status of Herbig Ae/Be (HAEBE) stars on the basis of Hipparcos results.     

A moving group of young stars in Carina–Vela

Accurate two-colour photometry and proper motions of 7096 young X-ray stars in the ROSAT All-Sky Survey Bright Star Catalogue, version 1RXS, are extracted from the Tycho-2 Catalogue. The sample is dominated by red main-sequence and possibly pre-main-sequence stars. On a global proper motion convergence map, two features are very prominent: the nearby section of the Gould Belt and the Hyades convergent point. The appearance of the Gould Belt feature with its peak at ( l =2443, b =−126) is quite similar to that of Hipparcos OB stars. When only stars with proper motions drawing close to that point are selected, strong concentrations of stars in the direction of the Sco–Cen complex are found. Another concentration, not corresponding to any known OB association, is detected between the position of the Lower Centaurus Crux and Vela OB2 associations. It is a new young moving group located in Carina and Vela, and a near extension of the Sco–Cen complex. Contrary to the classical Gould Belt OB associations, the Carina–Vela moving group has a considerable geometric depth, the closest members being as near as 30 pc from the Sun. IC 2391, one of the youngest and closest open clusters on the sky, is a part of the Carina–Vela moving group. The Carina–Vela moving group does not link the Sco–Cen complex with the Vela OB associations, because the latter is much more distant than the outer limit of the sample. It is more likely that the young late-type population of the Scorpio–Centaurus–Carina moving group stretches towards the Sun and possibly beyond it.     

The stellar composition of the star formation region CMa R1 – II. Spectroscopic and photometric observations of nine young stars

We present new high- and low-resolution spectroscopic and photometric data of nine members of the young association CMa R1. All the stars have circumstellar dust at some distance, as could be expected from their association with reflection nebulosity. Four stars (HD 52721, HD 53367, LkH α  220 and LkH α  218) show H α emission and we argue that they are Herbig Be stars with discs. Our photometric and spectroscopic observations of these stars reveal new characteristics of their variability. We present first interpretations of the variability of HD 52721, HD 53367 and the two LkH α stars in terms of a partially eclipsing binary, a magnetic activity cycle and circumstellar dust variations, respectively. The remaining five stars show no clear indications of H α emission in their spectra, although their spectral types and ages are comparable with those of HD 52721 and HD 53367. This indicates that the presence of a disc around a star in CMa R1 may depend on the environment of the star. In particular we find that all H α emission stars are located at or outside the arc-shaped border of the H  ii region, which suggests that the stars inside the arc have lost their discs through evaporation by UV photons from nearby O stars, or from the nearby (<25 pc) supernova, about 1 Myr ago.     

The stellar composition of the star formation region CMa R1 – I. Results from new photometric and spectroscopic classifications

A new photometric and spectroscopic survey of the star formation region (SFR) CMa R1 is described. In a sample of 165 stars brighter than 13th mag, 88 stars were found to be probable members of the SFR. They are defined as early-type stars with E ( B − V )0.16 mag, which corresponds to a distance of about 1 kpc. 74 of the probable members are B stars. 19 stars are possibly associated with an IRAS point source. We derive a most probable distance of 1050±150 pc to the association. It appears that about 80 candidate members are pre-main-sequence stars with ages lower than 6 million years, while the main sequence extends over 6.0–7.6 mag, which is consistent with star formation starting about 8 million years ago and continuing until at least half a million years ago. Two bright B stars in the association (GU CMa and FZ CMa) seem to be much older and probably do not originate from the same star formation episode. The star formation efficiency appears to increase roughly monotonically with time up to half a million years ago. From our data, we conclude that only a minor fraction of the stars has been created through the scenario suggested by Herbst & Assousa, in which the members of CMa R1 form by compression of ambient material by a supernova shock wave. An extensive search for candidate members with H α emission did not reveal new Herbig Ae/Be candidates, so that the number of stars in this class seems to be limited to four: Z CMa, LkH α 218, LkH α 220 and possibly HD 53367.     

Magnetic field evolution of accreting neutron stars – IV. Effect of the curvature of space–time

We report near-infrared molecular hydrogen and Brackett γ observations towards the massive star formation site G323.74−0.26. The region contains an H  ii region, ∼30 arcsec across, and two Class II methanol maser sites, which are separate from the H  ii region. We show that the spectral type of the star powering the H  ii region is B0. We also show that at least one of the maser sites is powered by an infrared source that appears to be at least as luminous as the star responsible for the H  ii region. However, neither of the two stars associated with the methanol maser sites shows any signs of radio continuum emission above 0.2 mJy. For at least one of these maser sites, this shows a real deficiency in the radio continuum flux, which we suggest is an indication that the star is in an early stage of development, before its H  ii region becomes visible, or it is a multiple intermediate mass star system. A shocked molecular hydrogen outflow is seen extending from one of the maser sites towards the west and possibly in a fan shape, suggesting that the stars associated with the maser sites are indeed at a very early stage of evolution.     

Massive young stellar objects in the Large Magellanic Cloud: water masers and ESO-VLT 3–4 μm spectroscopy

We investigate the conditions of star formation in the Large Magellanic Cloud (LMC). We have conducted a survey for water maser emission arising from massive young stellar objects in the 30 Doradus region (N 157) and several other H  ii regions in the LMC (N 105A, N 113 and N 160A). We have identified a new maser source in 30 Dor at the systemic velocity of the LMC. We have obtained 3–4 μm spectra, with the European Southern Observatory (ESO)-Very Large Telescope (VLT), of two candidate young stellar objects. N 105A IRS1 shows H recombination line emission, and its Spectral Energy Distribution (SED) and mid-infrared colours are consistent with a massive young star ionizing the molecular cloud. N 157B IRS1 is identified as an embedded young object, based on its SED and a tentative detection of water ice. The data on these four H  ii regions are combined with mid-infrared archival images from the Spitzer Space Telescope to study the location and nature of the embedded massive young stellar objects and signatures of stellar feedback. Our analysis of 30 Dor, N 113 and N 160A confirms the picture that the feedback from the massive O- and B-type stars, which creates the H  ii regions, also triggers further star formation on the interfaces of the ionized gas and the surrounding molecular cloud. Although in the dense cloud N 105A star formation seems to occur without evidence of massive star feedback, the general conditions in the LMC seem favourable for sequential star formation as a result of feedback. In an Appendix , we present water maser observations of the galactic red giants R Doradus and W Hydrae.     

Near-infrared imaging observations of the southern massive star-forming region G333.6−0.2

We present near-infrared broad-band JHK ' images of the southern massive star-forming region G333.6−0.2. The slope of the K -luminosity function towards the region  (0.24 ± 0.01)  is considered to be equivalent to that expected for main-sequence stars in the solar neighbourhood. Point sources with their ( H − K ) colour greater than 1 are more likely to be located in extended emission and it is suggested that these objects are physically associated with the H  ii region.