Young Intermediate-mass Stars Research Articles

Spatially extended polycyclic aromatic hydrocarbons in circumstellar disks around T Tauri and Herbig Ae stars

Aims. Our aim is to determine the presence and location of the emission from polycyclic aromatic hydrocarbons (PAHs) towards low and intermediate mass young stars with disks using large aperture telescopes. Methods. VLT-VISIR N-band spectra and VLT-ISAAC and VLT-NACO L-band spectra of 29 sources are presented, spectrally resolving the 3.3, 8.6, 11.2, and 12.6 μm PAH features. Spatial-extent profiles of the features and the continuum emission have been derived and used to associate the PAH emission with the disks. The results are discussed in the context of recent PAH-emission disk models. Results. The 3.3, 8.6, and 11.2 μm PAH features are detected toward a small fraction of the T Tauri stars, with typical upper limits between 1 × 10^(−15) and 5 × 10^(−17) Wm^(−2). All 11.2 μm detections from a previous Spitzer survey are confirmed with (tentative) 3.3 μm detections, and both the 8.6 and the 11.2 μm features are detected in all PAH sources. For 6 detections, the spatial extent of the PAH features is confined to scales typically smaller than 0.12−0.34, consistent with the radii of 12−60 AU disks at their distances (typically 150 pc). For 3 additional sources, WL 16, HD 100546, and TY CrA, one or more of the PAH features are more extended than the hot dust continuum of the disk, whereas for Oph IRS 48, the size of the resolved PAH emission is confirmed as smaller than for the large grains. For HD 100546, the 3.3 μm emission is confined to a small radial extent of 12±3 AU, most likely associated with the outer rim of the gap in this disk. Gaps with radii out to 10−30 AU may also affect the observed PAH extent for other sources. For both Herbig Ae and T Tauri stars, the small measured extents of the 8.6 and 11.2 μm features are consistent with larger (≥100 carbon atoms) PAHs.

Chemistry in disks

Aims: To constrain the ionization fraction in protoplanetary disks, we present new high-sensitivity interferometric observations of N$_2$H$^+$ in three disks surrounding DM Tau, LkCa 15, and MWC 480. Methods: We used the IRAM PdBI array to observe the N$_2$H$^+$ J=1-0 line and applied a $\chi^2$-minimization technique to estimate corresponding column densities. These values are compared, together with HCO$^+$ column densities, to results of a steady-state disk model with a vertical temperature gradient coupled to gas-grain chemistry. Results: We report two \ndhp detections for LkCa 15 and DM Tau at the $5 \sigma$ level and an upper limit for MWC 480. The column density derived from the data for LkCa 15 is much lower than previously reported. The [N$_2$H$^+$/HCO$^+$] ratio is on the order of 0.02--0.03. So far, HCO$^+$ remains the most abundant observed molecular ion in disks. Conclusions: All the observed values generally agree with the modelled column densities of disks at an evolutionary stage of a few million years (within the uncertainty limits), but the radial distribution of the molecules is not reproduced well. The low inferred concentration of N$_2$H$^+$ in three disks around low-mass and intermediate-mass young stars implies that this ion is not a sensitive tracer of the overall disk ionization fraction.

Long-term infrared variability of the UX Ori-type star SV Cep

We investigate the long-term optical-infrared variability of SV Cep, and explain it in the context of an existing UX Ori (UXOR) model. A 25-month monitoring programme was completed with the Infrared Space Observatory in the 3.3-100 um wavelength range. Following a careful data reduction, the infrared light curves were correlated with the variations of SV Cep in the V-band. A remarkable correlation was found between the optical and the far-infrared light curves. In the mid-infrared regime the amplitude of variations is lower, with a hint for a weak anti-correlation with the optical changes. In order to interpret the observations, we modelled the spectral energy distribution of SV Cep assuming a self-shadowed disc with a puffed-up inner rim, using a 2-dimensional radiative transfer code. We found that modifying the height of the inner rim, the wavelength-dependence of the long-term optical-infrared variations is well reproduced, except the mid-infrared domain. The origin of variation of the rim height might be fluctuation in the accretion rate in the outer disc. In order to model the mid-infrared behaviour we tested to add an optically thin envelope to the system, but this model failed to explain the far-infrared variability. Infrared variability is a powerful tool to discriminate between models of the circumstellar environment. The proposed mechanism of variable rim height may not be restricted to UXOR stars, but might be a general characteristic of intermediate-mass young stars.

Anatomy of a Flaring Proto-Planetary Disk Around a Young Intermediate-Mass Star

Although planets are being discovered around stars more massive than the Sun, information about the proto-planetary disks where such planets have built up is sparse. We have imaged mid-infrared emission from polycyclic aromatic hydrocarbons at the surface of the disk surrounding the young intermediate-mass star HD 97048 and characterized the disk. The disk is in an early stage of evolution, as indicated by its large content of dust and its hydrostatic flared geometry, indicative of the presence of a large amount of gas that is well mixed with dust and gravitationally stable. The disk is a precursor of debris disks found around more-evolved A stars such as beta-Pictoris and provides the rare opportunity to witness the conditions prevailing before (or during) planet formation.

AO Velorum: a young quadruple system with a ZAMS eclipsing BpSi primary

Using recent spectroscopic observations, we show that the triple system AOVel with an eclipsing BpSi primary is in fact a remarkable quadruple system formed by two double-lined spectroscopic binaries with components close to the ZAMS. All available data have been reanalyzed to derive proper orbital parameters for both binary systems and to calculate absolute parameters of the eclipsing system. For the first time, direct determination of the radius and the mass have been obtained for a BpSi star. The study of the physical parameters of this unique system is especially important since it can be used as a test of evolutionary models for very young stars of intermediate mass.

Chandra Observations of a Young Embedded Magnetic B Star in the $\rho$ Ophiuchus Cloud

Abstract This paper reports on an analysis of two Chandra X-ray observations of the young magnetic B star $\rho$ Ophiuchus S1. X-ray emission from the star was detected in both observations. The average flux was almost the same in both, but during each observation the flux showed significant time variations by a factor of two on time-scales of 20–40 ks. Each spectrum could be fit by either an absorbed power-law model with a photon index of $\sim 3$ or a thin-thermal plasma model with a temperature of $\sim 2 \,\mathrm{keV}$ and an extremely low metal abundance ($\lesssim 0.1$ solar). The spectrum of the first observation has a weak-line feature at about 6.8 keV, which might correspond to highly ionized iron $\mathrm{K}\alpha$. In contrast, the spectrum of the second observation apparently shows a weak edge absorption component at $E \sim4 \,\mathrm{keV}$. The continuum emission and $\log (L_\mathrm{X} / L_{\mathrm{bol}}) \sim-6$ are similar to those of young intermediate-mass stars (Herbig Ae/Be stars), although the presence of a strong magnetic field (inferred from the detection of non-thermal radio emission) has drawn an analogy between $\rho$ Ophiuchus S1 and magnetic chemically peculiar (MCP) stars. If the X-ray emission is thermal, the small abundances that we derived might be related to the inverse first-ionization potential (FIP) effect, though there is no significant trend as a function of FIP from our model fits. If the emission is non-thermal, it might be produced by high-energy electrons in the magnetosphere.

Dense Shell around a Young Intermediate-Mass Star: NGC 2264 IRS 1

Abstract The results of $\mathrm{H}{{13\atop} \mathrm{CO}}^{+}$$J = 1 \hbox{--} 0$ line and 93-GHz continuum observations of NGC 2264 IRS 1, a luminous infrared source known as Allen$^\prime$s source, are reported. IRS 1 is a young intermediate-mass star with a surrounding YSO cluster, and is associated with a molecular outflow. High angular resolution interferometric observations were performed using the Nobeyama Millimeter Array, and mapping observations were conducted using a 45-m telescope. The continuum map reveals four sources, three of which correspond to a submillimeter source detected by Ward-Thompson et al. (2000, AAA 73.137.062). Four compact clumps are also observed in the $\mathrm{H}{{13\atop} \mathrm{CO}}^{+}$ map, two of which are associated with these millimeter / submillimeter continuum sources without bipolar outflow. These $\mathrm{H}{{13\atop} \mathrm{CO}}^{+}$ clumps as a whole form an incomplete dense shell of 0.12 pc in diameter around IRS 1, and the outer part of the shell is clearly traced on the single-dish map. The shell, or cavity, structure suggests that the outflow from IRS 1 has evacuated part of the surrounding envelope in $\sim 0.1 \,\mathrm{Myr}$. The kinematic properties can be interpreted as being two clumps of entrained or compressed material derived from steady outflow. It is concluded that the activity of IRS 1 may have triggered the formation of cluster members in the dense surrounding shell.

Non-emission-line young stars of intermediate mass

We present optical spectra of four intermediate-mass candidate young stellar objects that have often been classified as Herbig Ae/Be stars. Typical Herbig Ae/Be emission features are not present in the spectra of these stars. Three of them, HD 36917, HD 36982 and HD 37062, are members of the young Orion nebula cluster (ONC). This association constrains their ages to be ≲1 Myr. The lack of appreciable near-infrared excess in them suggests the absence of hot dust close to the central star. However, they do possess significant amounts of cold and extended dust as revealed by the large excess emission observed at far-infrared wavelengths. The fractional infrared luminosities (Lir/L★) and the dust masses computed from IRAS fluxes are systematically lower than those found for Herbig Ae/Be stars but higher than those for Vega-like stars. These stars may thus represent the youngest examples of the Vega phenomenon known so far. In contrast, the other star in our sample, HD 58647, is more likely to be a classical Be star, as is evident from the low Lir/L★, the scarcity of circumstellar dust, the low polarization, the presence of Hα emission and near-infrared excess, and the far-infrared spectral energy distribution consistent with free–free emission similar to other well-known classical Be stars.

Constraints on Disk Sizes around Young Intermediate-Mass Stars: Nulling Interferometric Observations of Herbig A[CLC]e[/CLC] Objects

Physical models of circumstellar disks surrounding pre-main sequence stars are currently constrained primarily by the spectral energy distribution (SED) of the system. Here we present first results from a survey of nearby Herbig Ae stars using nulling interferometry. We measure the spatial extent of the mid-IR emission for the stars HD 150193, HD 163296, and HD 179218. The results indicate the size of mid-infrared emission around Herbig Ae stars may be much smaller than current models predict. The observations constrain ~90% of the 10 micron flux to be within an approximately 20 AU diameter region, providing significant spatial limits for disk models. These results demonstrate the scientific potential for nulling interferometry to study circumstellar disks with better spatial discernment than is possible with standard mid-infrared imaging.

Stellar Winds in Herbig Ae/Be Stars

The winds observed in the pre-main-sequence phase of intermediate-mass young stars are discussed in light of a model originally developed by Mestel for describing stellar winds ejected by rotating stars. In addition to the acceleration mechanisms of thermal expansion and stellar radiation pressure, which typically play an important role in driving the winds of low- and high-mass stars, respectively, centrifugal acceleration by a corotating magnetic field and the presence of hydromagnetic waves are taken into account. The relevance of these mechanisms for the acceleration of stellar winds from the surface of young stars is discussed in the context of growing evidence for the presence of magnetic fields and surface convection in pre-main-sequence evolutionary phases. In the particular case of Herbig Ae/Be stars, the deposition of a fraction of the convection energy in Alfven waves seems to be required in order to explain the winds observed in these pre-main-sequence objects. Under some simplifying assumptions, the velocity and density fields around the central stellar object are derived and the emerging line profiles are computed. A test case is discussed in which the observed line profiles of the prototypical object AB Aur are used to constrain the model parameters, showing that the magnetic rotator model offers a physically consistent description of this system.

Observational Overview of Young Intermediate-Mass Objects: Herbig Ae/Be Stars

Recent discoveries of planets orbiting several nearby solar-mass stars have focussed renewed attention on the frequency and evolution of planetary systems and their evolutionary precursors around both solar and intermediate (2 ≤ M/M ⊙ ≤ 8) mass stars. As a result of a wealth of new observations at all wavelengths of the circumstellar material around the nearest of the young intermediate-mass stars, the so-called Herbig Ae/Be (HAeBe) stars, we are beginning to see how these systems are similar to the solar mass objects, and how they differ. A review of the recent literature is presented, including the evolutionary status of the stars, binary frequency and the star forming environment, the morphology of the circumstellar material, including the first direct detections of disks in Keplerian rotation around these objects, and mass loss and infall phenomena. Prospects for advances in this research area as a result of advances in instrumentation are reviewed.

A search for Herbig-Haro objects near Herbig Ae/Be stars

I have conducted a [S II] imaging search for Herbig-Haro objects in the vicinity of 32 Herbig Ae/Be stars. This survey should provide the basis for the study of optical outflows associated with intermediate-mass young stars. New Herbig-Haro objects were found near HK Ori, BD + 46° 3731, and probably BD + 41° 3471. Possible jets were found associated with a faint star near HK Ori, another faint star near Elias 1, and AS 310. Only the HK Ori Herbig-Haro object has been confirmed spectroscopically

Infrared studies of circumstellar matter around Herbig Ae/Be and related stars

The study presents high spatial resolution data at 50 and 100 microns for seven young intermediate-mass stars with flat or rising spectral energy distributions. Five stars, namely, LkH-alpha 198, R Mon, CD -42 deg 11721, R CrA, and V645 Cyg, are resolved at 100 microns. The emission comes from spatially extended envelopes, with sizes varying between 0.025 and 0.4 pc. The wavelengths are optically thin at mid-IR and FIR wavelengths but become optically thick in the visual where the extinction amounts to 5-10 mag. In two cases, R CrA and V 645 Cyg, dust density distributions with steep density gradients are obtained, alpha is approximately equal to 2, while for LkH-alpha, R Mon, and CD -42 deg 11721, alpha is approximately equal to 0.5. Model fitting to the spectral energy distributions, combined with the constraints on the size of the emitting regions, indicates that inside the circumstellar envelopes a large fraction of the stellar radiation is already degraded to longer wavelengths.

Circumstellar disks deduced from sub-arcsecond polarization observations of two young stars

HERBIG Ae/Be objects are young stars of intermediate mass (3–5 solar masses) surrounded by reflection nebulae composed of dust and gas remaining from the star-forming cloud1–3. Scattering by dust particles strongly polarizes light from the circumstellar nebula. Using imaging polarimetry in excellent conditions of atmospheric seeing (0.4 arcsec) from the La Palma observatory, we have resolved an optically thick disk around the Herbig Ae/Be object V376 Cassiopeiae. Its effective radius increases from 0.8 arc-sec in the near infrared (∼1 μm wavelength) to 1.2 arcsec at optical wavelengths (∼550 nm) because of the higher opacity of the disk material at shorter wavelengths. The corresponding linear disk radius is 500–750 AU, about an order of magnitude larger than our Solar System. The polarization behaviour within a 0.5-arcsec diameter circle around the companion star V633 Cas suggests the presence of a more compact (<0.2 arcsec) unresolved disk, possibly a protoplanetary disk, around this star. The presence of circumstellar disks around these two pre-main-sequence stars in the same star-forming cloud suggests that such disks may be common features of early stellar evolution.

Herbig Ae/Be stars - Intermediate-mass stars surrounded by massive circumstellar accretion disks

The proposition that Herbig Ae/Be stars are young intermediate mass stars surrounded by optically thick accretion disks is explored. From a study of 47 such objects, a subset of 30 stars is identified whose spectral energy distributions can be interpreted convincingly in terms of pre-main sequence stars surrounded by massive optically thick circumstellar accretion disks. Constraints on the physical properties of the disks, such as size, mass, accretion rate, lifetime, and radial structure are derived from the photometric data.

Theoretical and observational aspects of young stars of intermediate mass

An account is given of the observational and theoretical properties that characterize young stars of intermediate-mass (2 ≤ M∗ / M⊙ ≤ 10), known as Herbig Ae/Be stars. The mass range of these objects is an interesting one, since it involves the occurrence of complex phenomena associated with the transition from fully convective configurations, typical of T Tauri stars, to radiatively stable objects, as in the case of massive stars. An overview of relevant observations testifying the variety of surface phenomena associated with the Herbig Ae/Be stars is presented. Recent developments in the theory of the formation on intermediate-mass protostars will also be discussed.

Theoretical and observational aspects of young stars of intermediate mass

An account is given of the observational and theoretical properties that characterize young stars of intermediate-mass (2 ≤ M∗ / M⊙ ≤ 10), known as Herbig Ae/Be stars. The mass range of these objects is an interesting one, since it involves the occurrence of complex phenomena associated with the transition from fully convective configurations, typical of T Tauri stars, to radiatively stable objects, as in the case of massive stars. An overview of relevant observations testifying the variety of surface phenomena associated with the Herbig Ae/Be stars is presented. Recent developments in the theory of the formation on intermediate-mass protostars will also be discussed.