RRECENTECENT AASTROPHYSICALSTROPHYSICAL RRESULESULTSTS FROMFROM THETHE VLVLTITI

WE PRESENT A REVIEW OF RECENT ASTROPHYSICAL RESULTS BASED ON DATA OBTAINED WITH THE ESO VERY LARGE TELESCOPE INTERFEROMETER (VLTI). THE VERY FIRST VLTI RESULTS HAVE ALREADY BEEN REVIEWED BY RICHICHI & PARESCE (2003). REMARKABLY, THE FIRST SCIENTIFIC RESULTS FROM THE MID-INFRARED INSTRUMENT MIDI, WHICH EMERGE FROM SCIENCE DEMONSTRATION AND GUARANTEED TIME OBSERVATIONS, ALREADY COVER A VERY BROAD RANGE OF ASTROPHYSICAL TOPICS. THESE TOPICS INCLUDE ACTIVE GALACTIC NUCLEI, YOUNG STELLAR OBJECTS AND PROTOPLANETARY DISCS, THE ENVIRONMENTS OF HOT STARS, AS WELL AS EVOLVED STARS AND THEIR ENVELOPES.

MARKUS WITTKOWSKIITTKOWSKI1, FRANCESCO PARESCE1, OLIVIER CHESNEAU2, PIERREIERRE KERVELLA3, ANTHONY MEILLAND2, KLAUS MEISENHEIMER4 AND KEIICHI OHNAKA5

1EUROPEAN SOUTHERN OBSERVATORY, GARCHING, GERMANY 2OBSERVATOIRE DE LA CÔTE D’AZUR, GRASSE, FRANCE 3LESIA, OBSERVATOIRE DE PARIS, FRANCE 4MAX-PLANCK-INSTITUT FÜR ASTRONOMIE, HEIDELBERG, GERMANY 5MAX-PLANCK-INSTITUT FÜR RADIOASTRONOMIE, BONN, GERMANY

BSERVATIONS WITH THE ESO The first refereed publications based on data broad range of important astrophysical top- VLT Interferometer started obtained with the VLTI appeared in the year ics. With the large collecting areas of the with the achievement of 2003. A review of the very first VLTI results 8 m VLT Unit Telescopes, VLT Interferome- First Fringes in March 2001, was presented by Richichi & Paresce (De- try is not limited to the brightest stars, as most which employed the test cember 2003). Since then many new results interferometric facilities so far, but can be OsiderostatsO and the commissioning instru- have been published that are based on used to study relatively faint sources. Results ment VINCI. Since then, the VLTI instrumen- data obtained with VLTI instruments. The obtained so far include already, for instance, tation has been constantly expanded, and sci- ESO telescope bibliography lists for the years studies of the innermost cores of Active Ga- entific observations have continuously been 2003 and 2004 a total of 21 refereed publica- lactic Nuclei (AGN) harbouring supermas- carried out using the instrumentation that had tions directly using ESO data, which were sive black holes, the origins of stars such as been available at any given time. A report on obtained with the VLTI instruments VINCI our sun and the formation of protoplanetary the technical status of the VLT Interferome- and MIDI (see Table 1). These data were discs, fundamental parameters of stars in evo- ter was for instance presented by Glindemann taken within VINCI commissioning, VINCI lutionary stages such as our sun, the evolu- et al. (2004). A huge number of scientifical- shared risk, MIDI science demonstration, and tion and fate of stars and their mass-loss to ly interesting VINCI data were secured be- MIDI guaranteed time programmes. These the circumstellar environment, the environ- tween March 2001 and September 2004 with- publications are listed in the references be- ment of hot stars, and distance estimates in commissioning and early shared risk low. Also listed are 11 additional publications based on measurements of Cepheid pulsa- programmes, and all of these VINCI data are that appeared in 2005 or that are in press, and tions. publicly available1. Early MIDI observations that qualify to be included in the ESO tele- Remarkably, several of the publications in the framework of science demonstration scope bibliography. Here, we review these reviewed here include results that were ob- and guaranteed time programmes have been scientific results, concentrating on those that tained by combing observations using differ- conducted since 2003. The data based on were published after the review of first VLTI ent instruments and facilities. For instance, MIDI science demonstration programmes are results by Richichi & Paresce (2003). We put Chesneau et al. (2005a) combined obser- also publicly available2. The MIDI instru- a particular emphasis on the results emerg- vations obtained with MIDI, optical spec- ment has been offered to the whole astro- ing from MIDI science demonstration and tra taken with the HEROS instrument, and nomical community for general programmes guaranteed time programmes (Jaffe et al. infrared spectra from the 1.6 m Brazilian since ESO period P73 (observations starting 2004; Leinert et al. 2004; van Boekel et al. telescope. Kervella et al. (2003b), Pijpers in April 2004). First science demonstra- 2004; Chesneau et al. 2005 a, b, c; Ohnaka et et al. (2003), and di Folco (2004) combined tion and guaranteed time observations using al. 2005). Recently, Richichi et al. (2005) pre- VINCI data with asteroseismic observations. the near-infrared phase closure instrument sented an updated version of CHARM (cata- Boboltz & Wittkowski (2005) conducted AMBER were conducted in October and logue for high angular resolution measure- coordinated near-infrared (VINCI) and radio December 2004. ments), a catalogue that lists high angular (VLBA) interferometry. Woodruff et al. resolution measurements in general, includ- (2004) combined VINCI observations ing results from the VLTI. with near-infrared speckle observations. 1 www.eso.org/projects/vlti/instru/vinci/vinci_data_ The VLTI results reviewed in this arti- Wittkowski et al. (2004) combined VINCI sets.html 2 www.eso.org/projects/vlti/instru/midi/midi_data_ cle illustrate that the VLT Interferometer interferometry with available spectropho- sets.html provides excellent opportunities to address a tometry. These examples show that interfer- 36 The Messenger 119 Year VINCI MIDI Total Table 1: Refereed pub- to speculations based on indirect arguments. 2003 6 – 6 lications based on From the depth of the SiO feature one infers data obtained with VLTI 2004 12 3 15 instruments. Source: a K-Band extinction toward the hot MIDI 2005 (so far) 8 4 11 ESO Telescope Bibliog- component of AK > 12. One possible expla- 26 7 32 raphy. nation is a very porous dust torus which occa- sionally leaves holes of low extinction toward its hot centre. The compact K-band compo- ometry, and in particular VLTInterferometry, lent to AV > 50. The model is visualized in nent could then represent a direct view to the is becoming a very useful complementary Figure 1 (top). central source or the inner part of the funnel tool for general astrophysics among the many A natural interpretation of these findings viewed through such a hole. Alternatively, the tools that are available to the astronomical identifies the 320 K component with the K-band component could be located some- community. It can be expected that more syn- AGN-heated dusty torus expected in the cores what above the torus plane: in this case it is ergies between observations with the VLTI of Seyfert galaxies, while the embedded hot very unlikely that dust can be heated to and external instruments and facilities will component represents the wall of the funnel > 1200 K (unless the UV radiation from the emerge, ultimately also including ALMAand which is heated almost to the dust subli- central source is extremely collimated). More OWL. mation temperature and confines the well probably, the K-band light is then scattered studied outflows along the source axis of by dust clouds above the torus. GALACTIC NUCLEI NGC 1068. Additional support for this pic- VLTI observations of Active Galactic Nuclei ture comes from radio continuum observa- (AGN) with MIDI started soon after the in- tions with the VLBA presented by Gallimore strument was ready for scientific observa- et al. (2004): Superimposing their map of tions with two UTs: the first interferomet- the central component S1 on the MIDI model ric spectrum of the famous Seyfert II gal- (Figure 1 bottom) shows that the disc-like axy NGC 1068 was obtained during science radio source coincides in size with the hot demonstration time in June 2003. A second component. Presumably, in the radio contin- set of MIDI measurements were carried out uum we observe free-free emission of the ion- in November 2003. Also in November 2003, ized gas within the funnel which is continu- the first near infrared K-band long-baseline ously released from the walls and eventually interferometric observations of NGC 1068 feeds the central accretion disc. It is intrigu- succeeded, which employed the VINCI in- ing to identify the increased height of the strument after adaptive optics wavefront cor- radio continuum source at its outer parts with rection (MACAO). The first successful MIDI the ionized gas which has just been “eaten observation of the second brightest galactic away” from the funnel walls by the intense nucleus, that of the Circinus galaxy, became UV radiation from the central engine. possible in February 2004. Two more mea- At the same time,Wittkowski et al. (2004) surements with baseline UT2–UT3 were presented the first K-band long-baseline in- obtained during guaranteed time in June. terferometric measurement of the nucleus of The Circinus nucleus is over three times NGC 1068 (see above). A squared visibility Figure 1: Top panel: Model dust distribution in the closer and ten times less luminous than NGC amplitude of 16.3 ± 4.3 % was measured for nucleus of NGC 1068 as inferred from the MIDI observations (North is at top, East to the right). A 1068. The analysis of the data shows that the NGC 1068 at a baseline length of 46 m. This compact hot component (yellow) is embedded central dust structure is well resolved at 10 value corresponds to a FWHM of the K-band into the well resolved warm component (red). µm even with a 43 m baseline. Nevertheless, intensity distribution of 5.0 ±0.5 mas if it con- Hatching indicates the depth of the SiO absorp- tion. Bottom panel: The VLBA radio continuum due to the small distance, its physical size is sists of a single Gaussian component. Taking map at 5 GHz of the S1 component in NGC 1068 only 1 pc diameter. The observed visibilities into account K-band speckle interferom- (from Gallimore et al 2004.) superimposed on the as a function of projected baseline orientation etry observations up to a baseline of 6 m model. Note the widening of the radio component hint at a much flatter dust distribution than (Wittkowski et al. 1998), a multi-component at its western edge. that observed in NGC 1068 (see below). model is suggested for the intensity distribu- For the purpose of this overview, however, tion where a part of the flux originates from we focus on the discussion of the VLTI results scales clearly smaller than 5 mas (< 0.4 pc), of NGC 1068 since they best demonstrate the and another part of the flux from larger scales. wealth of information which already has been Figure 2 shows the measured VLTI/VINCI provided by the VLTI. A first account of the K-band visibility together with possible MIDI observations of NGC 1068 was pub- visibility models. For illustration, two dif- lished by Jaffe et al. (2004). On the basis of ferent two-component visibility curves are the interferometric spectra covering the range shown which are consistent with both, the 8 µm < λ < 13 µm taken with 78 m and 42 m VLTI/VINCI visibility measurement at a baseline, respectively, the authors argue that baseline of 46 m, and the speckle observa- two central components are discernible: a tions up to a baseline of 6 m. These models Figure 2: First K-band long-baseline interferometry well resolved, warm (T = 320 K) compo- consist of a 0.1 mas or 3 mas compact core, of NGC 1068, obtained with VINCI. The VINCI visibility point at a baseline of 46 m indicates that nent of 3 pc diameter which is geometrically respectively, plus a more extended compo- part of the K-band flux originates from very thick (h/r ȃ 0.6), and a very compact hot nent. small scales of less than 5 mas or 0.4 pc. Two pos- (T > 800 K) component, which is marginally Taking the MIDI and VINCI results of sible two-component model visibility curves are shown which are consistent with this VINCI visibil- resolved along the source axis. The SiO NGC 1068 together, there remains a com- ity measurement and with speckle observations absorption feature increases in depth when pletely open question: How are the compact up to a baseline of 6 m. These models consist of a using higher spatial resolution, i.e. longer components at 2 and 10 µm related to each 0.1 mas and 3 mas compact core, respectively, baseline, suggesting that the central hot com- other? Without an accurate (relative) astrom- plus a more extended component. Based on Wittkowski et al. (2004b). ponent suffers strong dust extinction equiva- etry (to about 1 mas) available yet, we are left © ESO – March 2005 37 Pott et al. (this issue, page 43) describe in the star is soon transformed from ”pristine” detail their program to study the stellar pop- (small and amorphous) to ”processed” (larg- ulation close to our Galactic Centre. In par- er and crystalline) grains. Model calculations ticular, they describe the first successful show that crystalline grains should be abun- MIDI observations of the dust enshrouded dant in the inner part of the disc at the time star IRS 3 located in the central lightyear of of formation of the Earth. In fact, the mete- our Galaxy. Such studies aim at investigat- orites in our own solar system are mainly ing the star formation history and the inter- composed of this kind of silicate. Spectral action of stars with their environment in the observations of silicate grains in the mid- presence of tidal forces exerted by the grav- infrared around 10 µm should tell whether itational potential of the central supermassive they are ”pristine” or ”processed”. Earlier ob- black hole. servations of discs around young stars have Figure 3: Correlation between the mid-IR spectral In summary, it is obvious that already the shown a mixture of pristine and processed slope and the half light radius corresponding to the observed visibilities at 12.5 µm. The largest first one and a half years of VLTI observa- material to be present, but it was impossible sources are those with the reddest mid IR SED. tions of galactic nuclei have provided com- to tell where the different grains resided in From Leinert et al. (2004). pletely new views into the cores of these still the disc. mysterious objects. New facilities like the Thanks to a hundred-fold increase in angu- phase-closure instrument AMBER or the ex- lar resolution with the VLTI and the highly ternal fringe tracking system FINITO will sensitive MIDI instrument, detailed infrared soon extend the VLTI capabilities to a level spectra of the various regions of the pro- that interferometric observations of galactic toplanetary discs around the three young nuclei will become both routine and the most Herbig Ae stars, only a few million years old, important driver for our understanding of now show that the dust close to the star is AGN physics. much more processed than the dust in the outer disc regions. In one star (HD 142527) SIZES AND MINERALOGY OF CIRCUMSTELLAR the dust is processed in the entire disc (Fig- DISCS AROUND YOUNG HERBIG AE/BE STARS ure 4). In the central region of this disc, it is MIDI on the UTs was recently used to de- extremely processed, consistent with com- termine the characteristic sizes of Herbig pletely crystalline dust. In the other two stars Ae/Be star discs (Leinert et al. 2004) and their (HD 144432 and HD 163296) the dust in the detailed dust properties (Van Boekel et al. inner disc is fairly processed whereas the dust 2004). The characteristic 10 µm sizes of the in the outer disc is nearly pristine (Figure 5). discs around seven Herbig Ae/Be stars turned An important conclusion from the VLTI out to be in the range of 1–10 AU and cor- observations is, therefore, that the building related with the slope of their SED between blocks for Earth-like planets are present in 10 and 25 µm (Figure 3). This correlation circumstellar discs from the very start. This tends to give observational support even for is of great importance as it indicates that plan- such a limited sample to the hypothesis that ets of the terrestrial type like the Earth are Figure 4: The mid-IR spectrum of the inner region Herbig Ae/Be star discs can be distinguished most probably quite common in planetary of the protoplanetary disc around the young star HD 142527, as observed with MIDI (upper panel). by whether they are flaring or non-flaring. In systems, also outside the solar system. Below it are shown laboratory spectra of two crys- this context, one would expect, that as ob- The present observations also have impli- talline minerals as well as of an Interplanetary Dust served, the reddest objects should be larger cations for the study of comets. Some – per- Particle (IDP; captured in the Earth’s upper atmos- phere) with hydrated silicates and, at the bottom, since the flaring disc geometry exposes more haps all – comets in the solar system do con- a typical telescopic spectrum of dust grains in the distant material to direct illumination than the tain both amorphous and crystalline dust. interstellar space. The spectral “signatures” of non-flaring geometry. Comets were definitely formed at large dis- crystalline pyroxene and olivine, i.e. peaks at wave- The detailed spectral shapes of three stars tances from the Sun, in the outer regions of length 9.2 and 11.3 µm, respectively, are clearly visible in the spectrum of the inner stellar disc, de- of this sample (HD 142527, HD 144432, and the solar system where it has always been monstrating the presence of these species in that HD 163296) were then studied as a function very cold. It is therefore not clear how region of the disc. From Van Boekel et al. (2004). of spatial scale in order to better understand processed dust grains may end up in comets. the properties of circumstellar dust in these In one theory, processed dust is transported the original material from which the solar sys- very young stars. It has been known for some outward from the young Sun by turbulence in tem was formed. time that most of the dust in discs around new- the rather dense circumsolar disc. Other the- born stars is made up of silicates. In the natal ories claim that the processed dust in comets HOT STARS AND THEIR ENVIRONMENT cloud this dust is amorphous, i.e. the atoms was produced locally in the cold regions over Be stars are hot stars that exhibit Balmer lines and molecules that make up a dust grain are a much longer time, perhaps by shock waves in emission and free-free infrared excess, in- put together in a chaotic way, and the grains or lightning bolts in the disc, or by frequent terpreted as due to a compact equatorial gas- are fluffy and very small, typically about collisions between bigger fragments. eous disc around these objects. Be stars are 10–4 mm in size. However, near the young star The MIDI observations now imply that the relatively frequent among the B-type objects. where the temperature and density are high- first theory is the most likely explanation for α Ara (HD 158427, B3Ve), which is one of est, the dust particles in the circumstellar disc the presence of processed dust in comets. This the closest Be stars with an estimated distance tend to stick together so that the grains be- also implies that the long-period comets that of 74 pc (Hipparcos parallax) and an infrared come larger. Moreover, the dust is heated by sometimes visit us from the outer reaches of excess among the highest of its class, has been stellar radiation and this causes the molecules our solar system are truly pristine bodies, dat- chosen for the first MIDI observations of in the grains to re-arrange themselves in geo- ing back to an era when the Earth and the other a Be star (Chesneau et al. 2005a). Surpris- metric (crystalline) patterns. planets had not yet been formed. Studies of ingly, α Ara could not be resolved with Accordingly, the expectation is that the such comets, especially when performed in- the 102 m projected baseline of the UT1 and dust in the disc regions that are closest to situ, will therefore provide direct access to UT3 telescopes, putting an upper limit to 38 The Messenger 119 rapidly rotating Be star Achernar. They found the VLTI instruments at near (AMBER) and an oblateness of Achernar which exceeds the- mid-infrared (MIDI) wavelengths are expect- oretical predictions and, hence, puts new per- ed to lead to new important insights into this spectives on our understanding of the Be phe- stage of stellar evolution, and in particular nomenon. α Ara presents a rotational veloc- into the atmospheric structure, the mass loss ity v sin i which is even larger than that of process, the formation of circumstellar enve- Achernar, and is, hence, likely to be very dis- lopes, and the evolution of AGB stars toward torted, as well. The oblateness of the surface planetary nebulae. Figure 7 shows a schemat- of α Ara can be studied with AMBER, and ic diagram of a typical Mira variable and its this will likely provide additional implica- surrounding environment. The various re- tions on the wind model. gions shown in this diagram can be probed by The famous Luminous Blue Variable combining observations from various inter- (LBV) η Car was recently studied by ferometric instruments and facilities in a mul- Chesneau et al. (2005b) by combining MIDI tiwavelength study. In particular, mid-infra- observations and adaptive optics measure- red interferometric observations are adequate ments using NACO at the VLT. They reached for probing the outer atmosphere, i.e. the re- a spatial resolution of 60–80 mas with decon- gion between the top of the photosphere and volved NACO images in the L-band. MIDI in the dust envelope where mass outflows are interferometric mode provided a mean reso- expected to be initiated, and the expanding lution of about 20 mas. Using this combina- dust shell. tion of MIDI and NACO observations, the Ohnaka et al. (2005) presented the results Figure 5: Schematic view of a circumstellar disc authors were able to study the clumpiness of of the first mid-infrared interferometric and the observed MIDI-spectra of the inner the nebula as well as to isolate the flux of the observations of the Mira variable RR Sco and outer regions of the discs around three young Herbig Ae stars, HD 163296, HD 144432 and central star. The latter enabled them to con- with MIDI, coordinated with K-band obser- HD 142527 (black lines). In all of them, there are strain the spectral energy distribution (SED) vations using VINCI. The MIDI instrument clear spectral differences between the inner of the central source by itself for the wave- has made it possible to measure the wave- and outer regions, indicating a difference in miner- alogy. The general broadening of the spectral length range from 2.2–13.5 µm. length dependence of the visibility between “mountain” in the inner discs is a sign of larger 8 and 13 µm with a spectral resolution of grains and the spectral peak at wavelength STARS ON THE ~ 30 (Figure 8, panel a). In this spectral re- 11.3 µm indicates the presence of crystalline sili- ASYMPTOTIC GIANT BRANCH (AGB) gion, a huge number of molecular lines due cates (cf Figure 4). Also shown are best-fit model spectra (red lines), based on mixtures of the Interferometry is known to be a valuable tool to H2O and SiO are located, which allows one mentioned mineral species. From Van Boekel for studying fundamental stellar parameters, to directly study the parameters of the mo- et al. (2004). the atmospheric structure, and the circumstel- lecular gas in the outer atmosphere. Figure 8 lar environment of AGB and post-AGB stars. (panel b) shows the obtained wavelength the disc size in the Ν-band of the order of The spectro-interferometric capabilities of dependence of the angular diameter of

φmax = 4 mas ± 1.5, i.e. 14 R1 at 74 pc and Figure 6: Schematic assuming R1 = 4.8Rी, based on the spectral type. view of the proposed model for the Be star On the other hand, a high density of the α Ara and its circum- disc is mandatory in order to reproduce the stellar environment. strong Balmer emission lines and the infrared From Chesneau et al. (2005). excess. Optical spectra from the HEROS in- strument, and infrared ones from the 1.6 m Brazilian telescope have been used together with the MIDI visibilities to constrain the sys- tem parameters. Using the circumstellar pa- rameters from the SIMECA code, a good agreement was found with spectroscopic and interferometric data only if the disc is some- how truncated at about 25 R1. The authors found variations of the hydrogen recombina- tion line profiles and of the radial velocities of α Ara, and they argue that this may be an Figure 7: Schematic indication for a possible low-mass compan- view of a Mira variable star. ion that could cause the truncation of the disc. Using the stellar parameters from the mod- el, i.e. a mass of 10 Mी, a 70 day period would give a radius of about 32 stellar radii, a value in agreement with the estimate based on the

MIDI data for a disc truncated at 25 R1, i.e. somewhat smaller than the companion orbit. Figure 6 shows a schematic view of the pro- posed model for the Be star α Ara and its cir- cumstellar environment, including the possi- ble unseen companion. Previously, Domiciano de Souza et al. (2003) reported on VINCI observations of the Wittkowski M. et al., Recent Astrophysical Results from the VLTI © ESO – March 2005 39 RR Sco. The angular diameter of 15–24 mas Figure 8: MIDI observations of the Mira variable between 8 and 13 µm is significantly larger RR Sco. Comparison between the visibility observed with MIDI (a), uniform-disc diameter (b), than the K-band diameter of 10.2 mas meas- and spectrum (c) and those predicted by the best- ured using VINCI, only three weeks after the fit model for RR Sco consisting of a warm MIDI observations. This wavelength depend- H2O + SiO envelope and an optically thin dust shell. a: The diamonds and triangles represent the visibil- ence of the observed angular diameter of RR ities observed with projected baseline lengths of Sco can be interpreted in terms of the pres- 99.9 m and 73.7 m, respectively, while the corre- ence of a warm molecular envelope and an sponding predicted visibilities are represented with expanding dust shell. Ohnaka et al. (2005) the solid and dashed lines, respectively. b: The filled diamonds represent the observed uni- derived physical parameters of these compo- form-disc diameters, while the solid line represents nents using the N-band visibilities and the the model prediction. The dotted line represents spectrum obtained with MIDI together with the continuum angular diameter estimated from the VINCI observation. the K-band visibility measured with VINCI c: The filled circles represent the calibrated spec- as observational constraints. Their model cal- trum of RR Sco obtained from the MIDI obser- culations show that optically thick emission vations. The solid line represents the spectrum pre- dicted from the best-fit model. From Ohnaka et al. from the H2O + SiO envelope extending to ~ (2005). 2.3 stellar radii with a temperature of 1400 K makes the apparent mid-infrared diameter much larger than the near-infrared angular diameter. The increase of the angular diame- ter longward of 10 µm can be explained by the presence of an optically thin dust shell consisting of corundum and silicate with an inner radius at 7–8 stellar radii. Additional insights into the conditions of Figure 9: First-ever coordinated ob- the molecular gas in the outer atmospheres servations between ESO’s VLTI and NRAO’s VLBA: SiO maser emis- and circumstellar environment of AGB stars sions toward the Mira variable S Ori can be achieved by measuring and mapping measured with the VLBA, together the maser radiation that some of these mole- with the near-infrared diameter meas- ured quasi simultaneously with the cules emit. Boboltz & Wittkowski (2005) VLTI (red stellar disc). From Boboltz conducted concurrent observations of the & Wittkowski (2005). Mira variable S Ori as part of a program in- tended to exploit the power of long baseline interferometry at infrared and radio wave- lengths to study the photospheres and nearby circumstellar envelopes of evolved stars. Fig- ure 9 shows the results of the first-ever coordinated observations between NRAO’s VLBA (Very Long Baseline Array) and ESO’s VLTI. The VLBA was used to observe the 43 GHz SiO maser emission (represent- ed by the circles color-coded in bins of radi- al velocity) concurrent with near-infrared K-band VINCI observations of the stellar photosphere (represented by the red disc in the centre of the distribution). The SiO masers were found to lie at a distance of about 1.7 stellar radii or 1.5 AU. With concurrent observations such as these, parameters of the circumstellar gas, as traced by the SiO ma- sers, can be related to the star itself at a par- ticular phase in its pulsation cycle. Such tions for the elusiveness of a common angu- strument, they measured K-band visibilities measurements can be used to constrain mod- lar diameter value which includes temporal of the M4 giant ψ Phe in the first and second els of stellar pulsation, envelope chemistry, variations as well as a dependence of the stel- lobe of the visibility function. These obser- maser generation, and stellar evolution. lar diameter on wavelength. vations were found to be consistent with pre- Fundamental parameters, most important- Through the direct measurement of the dictions by PHOENIX and ATLAS model ly radii and effective temperatures, of the centre-to-limb intensity variation (CLV) atmospheres, the parameters for which were photospheres of the central cool giant stars across stellar discs and their close environ- constrained by comparison to available spec- themselves have been frequently obtained ments, interferometry also probes the verti- trophotometry and theoretical stellar evolu- with interferometric and other high angular cal temperature profile, the chemical com- tionary tracks. resolution techniques, thanks to the favorable position, and horizontal inhomogeneities. For cool pulsating Mira stars, the CLVs are brightness and size of these stars. Richichi Limb-darkening studies have already been expected to be more complex than for non- and Roccatagliata (2005) recently conducted accomplished for a relatively small number pulsating M giants due to the effects of molec- new accurate measurements of the angular of star using different interferometric facili- ular layers close to the continuum-forming diameter of the cool giant Aldebaran using ties. Wittkowski et al. (2004) presented the layers. Broad-band CLVs may appear as near-infrared interferometric and lunar oc- first limb-darkening observation that was ob- Gaussian-shaped or multi-component func- cultation techniques. They discuss explana- tained with the VLTI. Using the VINCI in- tions. Woodruff et al. (2004) and Fedele et al. 40 The Messenger 119 (2005) presented K-band VINCI observations Figure 10: VINCI observations of the of the prototype Mira stars o Cet and R Leo, K-band intensity profile of the Mira variable R Leo and comparison to respectively. These measurements at post- dynamic model atmosphere predic- maximum stellar phases indicate K-band tions (Ireland et al. 2004 and refer- CLVs which are clearly different from a uni- ences therein; Scholz & Wood, pri- vate communication). The top panel form disc profile already in the first lobe of shows the CLV prediction by model the visibility function. The measured visibil- P22 (green lines: monochromatic ity values were found to be consistent with CLVs; red line: filter-averaged CLV), predictions by recent self-excited dynamic and the bottom panel shows the cor- responding visibility curve (red line) Mira model atmospheres (Ireland et al. 2004 compared to the measured R Leo and references therein; Scholz & Wood 2004, visibility values. Also indicated in the private communication) that include molec- top panel are the mean positions of the SiO maser shells observed by ular shells close to continuum-forming lay- Cotton et al. (2004). Based on Fedele ers. Figure 10 shows as an example the com- et al. (2005). parison of the VINCI observation of the Mira stars R Leo (Fedele et al. 2005) with the pre- dictions by the dynamic model atmospheres mentioned above.

POST-AGB STARS Further steps towards our better understand- ing of the stellar mass loss process are inter- ferometric measurements of post-AGB stars. One of the basic unknowns in the study of CEPHEID PULSATIONS AND VERY LOW MASS STARS AND STELLAR late-type stars is the mechanism by which DISTANCE ESTIMATES EVOLUTION NEAR THE MAIN SEQUENCE spherically symmetric AGB stars evolve to For almost a century, Cepheid variable stars The VLTI was used to precisely measure fun- form axisymmetric planetary nebulae (PNe). have occupied a central role in distance deter- damental parameters of very low mass dwarf Antoniucci et al. (2005) presented the first de- minations. This is thanks to the existence stars and of stars close to the main sequence tection of the envelope which surrounds the of the Period-Luminosity (P – L) relation (MS). During evolution after the main se- post-AGB binary source HR 4049 by K-band M = a log P + b which relates the logarithm quence, up to the subgiant and red giant VINCI observations. They report on a phys- of the variability period P of a Cepheid to its stages, the stellar diameter increases enor- ical size of the envelope in the near-infrared absolute mean magnitude M. This relation is mously, up to several hundred times that of K-band of about 15 AU (Gaussian FWHM). the basis of the extragalactic distance scale, the present Sun. Even while the star is still These measurements provide information on but its calibration is still uncertain at a close the the original MS, it inflates slowly. the geometry of the emitting region and cover ∆M = ± 0.10 mag level. This uncertainty can This means that the size of a given MS star is a range of position angles of about 60 deg. be overcome by an independent estimate of linked to its age. When combined with a the- They show that there is only a slight varia- the distance to a sufficient number of nearby oretical stellar evolution model, a direct tion of the size with position angle covered Cepheids. Trigonometric parallaxes are gen- measurement of the size of a star therefore within this range. These observations are, erally too uncertain to provide strong enough provides an estimate of its age. thus, consistent with a spherical envelope at constraints, and an alternative is provided by Until recently, the mass-radius relation for this distance from the stellar source, while an the Baade-Wesselink (BW) method. Its basic very low mass stars was poorly constrained. asymmetric envelope cannot be completely principle is to compare the linear and angu- The VINCI observations of four nearby ruled out due to the limitation in azimuth lar size variation of a pulsating star, in order M dwarfs using the 8 m telescopes UT1 and range, spatial frequency, and wavelength to derive its distance through a simple divi- UT3 allowed the measurement of accurate range. Further investigations using the near- sion. Interferometry allows one to measure angular sizes, and improved our knowledge infrared instrument AMBER can reveal the directly the angular diameter variation dur- of these faint stars (Ségransan et al. 2003). In geometry of this near-infrared component in ing the pulsation cycle, while the linear size particular, our nearest neighbour Proxima more detail, and MIDI observations can add variation can be obtained by high resolution (M5.5V) was measured for the first time, with information on cooler dust at larger distances spectroscopy (through the integration of the a very small radius of 0.145 Rी, only slight- from the stellar surface. radial velocity curve). As described and re- ly bigger than Jupiter (0.103 Rी). Chesneau et al. (2005c) very recently pre- viewed in detail by Kervella et al. (ESO Mes- The angular diameters of a number of sented MIDI observations of the circumstel- senger, 2004), the VINCI observations of nearby dwarf stars were measured with high lar environment of the massive OH/IR star 7 southern Cepheids (Kervella et al. 2004c) precision using VINCI, with the goal to estab- OH26.5+0.6. This source is assumed to be at allowed the calibration of the zero point of lish their age based on numerical models of the tip of the AGB phase, and to have entered the P – L relation using interferometric BW their evolution. The addition of the diameter the superwind stage only about 200 years ago. distance measurements. The resulting zero as a constraint reduces dramatically the un- The emission of the dusty envelope, appeared point value is identical to the one obtained certainty of the evolutionary state of the star. to be resolved by a single UT. The MIDI ac- using a large number of Hipparcos paral- For instance, the two components of our near- quisition image taken at 8.7 µm exhibited laxes by Lanoix et al. (1999). This encourag- est neighbor, the binary star α Centauri, were clearly an asymmetry of the envelope of this ing result strengthens confidence that no resolved for the first time using VINCI star. In interferometric mode, no fringes were large bias is present on the P – L calibration (Kervella et al. 2003b). The 0.2% accuracy detected. The authors argue that this failure (Kervella et al. 2004b). Additionally, the of the angular diameter measurement of to detect fringes, caused by an over-resolved VINCI measurements provided new calibra- α Cen A is among the highest precisions ever size of the source at this wavelengh range, tions of the Cepheid Period-Radius and Sur- achieved by interferometry. The numerical gives additional constraints on the opacities face brightness-Color relations (Kervella et modeling of α Cen A and B and constraints of the inner regions of the dust shell or close al. 2004a), two important relations to con- from observed asteroseismic frequencies vicinity of the star. strain theoretical models of Cepheids. available in the literature reproduces the di- Wittkowski M. et al., Recent Astrophysical Results from the VLTI © ESO – March 2005 41 Figure 11: Evolutionary tracks in the Hertzsprung- Jaffe, W., Meisenheimer, K., Röttgering, H. et al. Russel diagram of three models of Procyon A. The 2004, Nature, 429, 47 (MIDI) dashed rectangle delimits the classical uncertainty Kervella, P., Bersier, D., Mourard, D. et al. 2004a, domain for luminosity and effective temperature, A&A, 428, 587 (VINCI) while the hatched area delimits the much reduced Kervella, P., Bersier, D., Mourard, D., Nardetto, N. uncertainty when considering the interferometric & Coudé du Foresto, V. 2004b, A&A, 423, 327 radius. Model a is the most probable and indicates (VINCI) an age of 2.3 Gyr. From Kervella et al. (2004f). Kervella, P., Nardetto, N., Bersier, D., Mourard, D. & Coudé du Foresto, V. 2004c, A&A, 416, 941 (VINCI) Kervella, P., Ségransan, D. & Coudé du Foresto, V. 2004d, A&A, 425, 1161 (VINCI) Kervella, P., Thévenin, F. & Di Folco, E. 2004e, ameters obtained by VLTI within their very 2001, using six different siderostat and five A&A, 426, 297 (VINCI) Kervella, P., Thévenin, F., Morel, P. et al. 2004f, small error bars. different UT baselines. Based on angular di- A&A, 413, 251 (VINCI) The VINCI measurements of the bright ameters of these stars that have previously Kervella, P., Fouqué, P., Storm, J. et al. 2004g, ApJ, stars Sirius A (Kervella et al. 2003a) and been available in the literature, the authors 604, L113 (VINCI) Procyon A(Kervella et al. 2004f) allowed the also discuss the evolution of the measured Le Bouquin, J. B., Rousselet-Perraut, K., Kern, P. et al. 2004, A&A, 424, 719 (VINCI) estimate of a very young age of 200–250 Myr VLTI transfer function for the period 2001 to Leinert, Ch., van Boekel, R., Waters, L. B. F. M. et for the dwarf Sirius, and an older age of mid 2004. Work in progress aims at calculat- al. 2004, A&A, 423, 537 (MIDI) 2.3 Gyr for the subgiant Procyon. Figure 11 ing a global VLTI-based solution that will Wittkowski, M., Aufdenberg, J. P. & Kervella, P. shows as an example three evolutionary track determine uniquely new diameters of a sub- 2004a, A&A, 413, 711 (VINCI) models of Procyon. The interferometric con- set of about 50 chosen calibration stars by Wittkowski, M., Kervella, P., Arsenault, R. et al. 2004b, A&A, 418, L39 (VINCI) straint reduces considerably the uncertainty minimizing the scatter of the residuals for Woodruff, H. C., Eberhardt, M., Driebe, T. et al. domain in the HR diagram, especially when each VLTI night under consideration. 2004, A&A, 421, 703 (VINCI) coupled with asteroseismic observations. Also, over the past two years, 16 new an- van Boekel, R., Min, M., Leinert, C. et al. 2004, The age estimates based on the modeling of gular diameter measurements of nearby MS Nature, 432, 479 (MIDI) the interferometric data are confirmed by the dwarfs and subgiants were obtained with the PUBLICATIONS IN 2005 (SOFAR) BASED ON cooling ages of the two white dwarfs that orbit VINCI instrument, with the goal to calibrate VLTI DATA4 Sirius A and Procyon A. the specific surface brightness-colour (SB) Antoniucci, S., Paresce, F. & Wittkowski, M. 2005, Similar modeling studies were also con- relations of these stars with high accuracy A&A, 429, L1 (VINCI) ducted for the debris disc stars α PsA, β Pic, (Kervella et al. 2004e). The smallest disper- Boboltz, D. A. & Wittkowski M. 2005, ApJ, 618, ε Eri and τ Cet (Di Folco et al. 2004). These sions are obtained for the visible-infrared 953 (VINCI) stars are MS or pre-main sequence stars with based relations, in particular those based on Chesneau, O., Meilland, P., Stee, P. et al. 2005a, A&A, in press (astro-ph/0501162) (MIDI) spectral type A to K presenting a far infrared the B and K magnitudes. These relations Chesneau, O., Min, M., Herbst, T. et al. 2005b, excess, which has been commonly identified allow the prediction of very accurate (within A&A, in press (astro-ph/0501159) (MIDI) with circumstellar dust in optically thin discs. 1%) angular diameters of candidate calibra- Chesneau, O., Verhoelst, T., Lopez, B. et al. 2005c, These discs are in turn often associated tion stars for long baseline interferometry. A&A, in press (astro-ph/0501187) (MIDI) Davis, J., Richichi, A., Ballester, P. et al. 2005, AN, with planetary systems presumably already The MIDI consortium has established a 326, 25 (VINCI) formed. The determination of the stellar ages list of calibration stars for the MIDI instru- Fedele, D., Wittkowski, M., Paresce, F., et al. 2005, is an essential information to study the evo- ment based on spectro-photometric observa- A&A, 431, 1019 (VINCI) lution of the discs. tions of candidate stars and fitting of the data Ohnaka, K., Bergeat, J., Driebe, T. et al. 2005, to atmosphere models (B. Stecklum, ESO A&A, 429, 1057: (MIDI,VINCI) Richichi, A. & Percheron I. 2005, A&A, in press DIAMETER ESTIMATES OF calibrator workshop 2003; van Boekel et al., (astro-ph/0501532) (VINCI) INTERFEROMETRIC CALIBRATION STARS submitted). Richichi, A. & Roccatagliata, V. 2005, A&A, Interferometric measurements require the 433, 305 (VINCI) monitoring of the interferometric transfer ACKNOWLEDGEMENTS Thévenin, F., Kervella, P., Pichon, B. et al. 2005, A&A, in press (astro-ph/0501420) (VINCI) function, which in turn is derived from obser- The figures that appeared earlier in other journals were reproduced by kind permission of their vations of interferometric calibration stars. authors and the journals in which they originally OTHER REFERENCES Usually those stars are selected as calibration appeared. We are grateful for valuable discussions Cotton, W. D., Mennesson, B., Diamond, P. J. et al. stars that do not show any peculiar character- on the topics of this review with several colleagues. 2004, A&A, 414, 275 istics, and that have a well-known angular Gallimore, J. F., Baum, S. A. & O’Dea, C. P. 2004, 3 diameter and are as little resolved as possi- PUBLICATIONS IN 2003 BASED ON VLTI DATA ApJ, 613, 794 Domiciano de Souza, A., Kervella, P., Jankov, S.et Glindemann, A., Albertsen, M., Andolfato, L. et al. ble. The need for well-known interferomet- al. 2003, A&A, 407, L47 (VINCI) 2004, Proc. SPIE 5491, 447 ric calibration stars has stimulated work aim- Kervella, P., Thévenin, F., Morel, P., Bordé, P. & Ireland, M. J., Scholz, M. & Wood, P. R. 2004 ing at measuring angular diameters of such Di Folco, E. 2003a, A&A, 408, 681 (VINCI) MNRAS, 352, 318 candidates as well as establishing surface Kervella, P., Thévenin, F., Ségransan, D., et al. Kervella, P., Bersier, D., Nardetto, N. et al. 2004, The Messenger, 117, 53 brightness-colour relations that allow the 2003b, A&A, 404, 1087 (VINCI) Pijpers, F. P., Teixeira, T. C., Garcia, P. J. et al. 2003, Lanoix, P., Paturel, G. & Garnier, R. 1999, MNRAS, estimate of angular diameters based on A&A, 406, L15 (VINCI) 308, 969 known brightness and colour. Ségransan, D., Kervella, P., Forveille, T., & Richichi, A. & Paresce, F. 2003, The Messenger, Richichi & Percheron (2005) recently de- Queloz, D. 2003, A&A, 397, L5 (VINCI) 114, 26 scribed a massive ongoing effort, that was van Boekel, R., Kervella, P., Schöller, M. et al. Richichi, A., Percheron, I. & Khristoforova, M. 2003, A&A, 410, L37 (VINCI) 2005, A&A, 431, 773 started in 2001, to accumulate observations Wittkowski, M., Balega, Y., Beckert, T. et al. 1998, on calibration stars with the aim of develop- A&A, 329, L45 PUBLICATIONS IN 2004 BASED ON VLTI DATA3 ing a VLTI-based system of high accuracy Di Folco, E., Thévenin, F., Kervella, P. et al. 2004, calibration stars. Observations of 191 calibra- A&A, 426, 601 (VINCI) tion star candidates have been conducted with 4 Publications that have appeared or are in press and that qualify to be included in the ESO tele- the VLTI and the near-infrared K-band com- 3 From the ESO telescope bibliography scope bibliography (as of 15 February 2005). Note missioning instrument VINCI since March (archive.eso.org/wdb/wdb/eso/publications/form) that this list may be incomplete. 42 The Messenger 119