© ESO, 2012

1. Introduction

Stars between 1 and 8 M_⊙, after having exhausted the nuclear combustible at their cores, ascend the asymptotic giant branch (AGB), where they suffer heavy mass loss episodes with mass-loss rates up to several times 10^-4   M_⊙ yr ^-1 (e.g., Hoogzaad et al. 2002). The envelope expelled during this evolutionary stage will usually obscure the central star at optical wavelengths, and strong IR radiation is emitted instead (Pottasch et al. 1988). When the mass loss that characterizes the AGB ceases and the star enters the post-AGB phase (see Soker 2008, for a precise quantitative definition of this moment), the circumstellar envelope ejected in the AGB phase expands and cools while the effective temperature of the central star increases progressively (Blöcker 1995b). At the time the central star becomes hot enough to ionize the circumstellar envelope (T_eff ≥ 25 000 K), a planetary nebula (PN) arises.

The circumstellar envelope of an AGB star can be optically thick during its late AGB evolution, but its subsequent expansion during the post-AGB phase will make the star visible again at optical wavelengths. The exact time and the temperature of the star when this occurs are governed by the mass-loss rate during the AGB (and thus the optical thickness of the envelope) which is ultimately related to the stellar mass (Schönberner 1981; Vassiliadis & Wood 1993; Blöcker 1995a). The most massive stars will even traverse the whole post-AGB phase being invisible at optical wavelengths, and may reach the PN phase while still not detectable in the visible domain (e.g., IRAS 17347−3139, de Gregorio-Monsalvo et al. 2004). These optically obscured sources that may represent the most massive stars in these evolutionary phases are obviously of great interest.

Objects in the transition from the AGB to the PN phase have strong far-infrared (far-IR) excess and the use of IRAS data has generated extensive catalogues of post-AGB and young PN candidates (e.g., Pottasch et al. 1988). The identification and characterization of post-AGB sources have been so far hampered by the limited astrometric accuracy of the IRAS sources,  ~15′′−30′′. This task is particularly difficult in the case of heavily obscured sources that cannot be detected in the optical (Suárez et al. 2006). Near-IR JHK observations can be well suited for the accurate identification of IRAS post-AGB candidates. A significant effort has been carried out by Manchado et al. (1989), García-Lario et al. (1990) and García-Lario et al. (1997), who obtained JHK photometric observations of a large sample of post-AGB candidates.

The examination of more recent 2MASS data has shown that the use of single-element photometers may not be adequate for the identification of optically obscured sources (e.g., Jiménez-Esteban et al. 2006b; Szczerba et al. 2007), resulting in the misidentification of sources in crowded fields. We have further developed this technique (Ramos-Larios et al. 2009, hereafter Paper I) to search for near-IR counterparts in the 2MASS Point Source Catalog (PSC) of heavily obscured post-AGB stars, proto-PNe and PNe among a sample of 165 IRAS sources that, fulfilling the selection criteria of post-AGB candidates described by Suárez et al. (2006), show strong indications of being invisible at optical wavelengths.

The near-IR identification and characterization of  ~85 of the IRAS sources in our sample was not possible in Paper I because 2MASS has a limited sensitivity (its detection limit is J = 15.8 mag, H = 15.1 mag, and K = 14.3 mag). Therefore, the results presented in Paper I were biased towards the less obscured objects in our sample. Furthermore, Paper I purposely excluded resolved objects because the morphological information from 2MASS images, at a spatial resolution  ~2′′, was poor and their near-IR photometry was uncertain. These sources, however, may be key objects in the transition from the AGB to the PN phases.

To complete the results of Paper I and to search for circumstellar shells at a greater spatial resolution,  ~1′′, we have obtained near-IR JHK imaging observations of the IRAS sources in our sample. The improved sensitivity and spatial resolution of the new near-IR data, in conjunction with the improved coordinates provided by AKARI, have allowed us to identify near-IR counterparts for most of the sources in Paper I that could not be detected or resolved from nearby sources in 2MASS images. The photometric measurements of the new JHK images have been used to derive the spectral energy distributions (SED) in the IR domain of the sources with newly identified near-IR counterparts, and to search for near-IR variability among the sources with 2MASS PSC counterparts studied in Paper I. Finally, the morphology of the objects in our sample that are found to be extended is also presented.

The new near-IR observations and archival MSX, Spitzer GLIMPSE, and AKARI data are described in Sect. 2. The new information obtained for the objects with 2MASS PSC counterpart studied in Paper I is given in Sect. 3, while the identification of new near-IR counterparts and the morphology of extended sources are described in Sects. 4 and 5, respectively. Finally, we discuss the results in Sect. 6 and provide a summary of the main findings in Sect. 7.

2. Observations

2.1. Near-IR observations

2.1.1. Data acquisition

Near-IR JHK images of the objects in our sample (see Table 1 in Paper I) have been obtained using the 3.58 m Telescopio Nazionale Galileo (TNG) at the Observatorio del Roque de los Muchachos (La Palma, Spain) and the 3.6 m New Technology Telescope (NTT) at La Silla Observatory (Chile). Northern objects (δ >  −5°) were observed at the TNG during three observing runs on June, August, and October 2008, while Southern objects (δ <  −5°) were observed at the NTT on July 2008. Only IRAS 08242−3828 has not been observed because this Southern source was not visible during the NTT observing run on July 2008.

The Near Infrared Camera Spectrometer (NICS), a multi-mode instrument for IR observations in the range 0.9–2.5 μm (Baffa et al. 2001), was used at the TNG. NICS employs a Rockwell 1024 × 1024 HgCdTe Hawaii array. The large field (LF) mode was used, providing a plate scale of 025 pixel^-1 and a field of view of 42  ×  42. At the NTT, the Son OF Isaac (SOFI) near-IR spectrograph and camera (Moorwood et al. 1998) was used. SOFI is equipped with a similar Rockwell 1024 × 1024 HgCdTe Hawaii array, providing a pixel scale of 0288 pixel and a field of view of 492  ×  492 in its large field (LF) mode. The observations were carried out under photometric conditions and the seeing for most of the observations was 07–08 at the TNG and 08–12 at the NTT.

Broad-band JHK images were obtained at both telescopes. We note that the characteristics of the filters used at each telescope are not exactly the same; their central wavelengths, bandwidths, and transmission peaks are listed in Table 1. The most remarkable difference among the filters used in the observations occurs in the K band. The K short (K_s) filter used at SOFI is notably different from both the K′ filter used at NICS (Wainscoat & Cowie 1992) and the standard K filter; the red edge of the K_s filter is similar to that of the K′ filter, but its blue edge is more alike that of the standard K filter. This special design allows the K_s filter to avoid both the atmospheric absorption feature at 1.9 μm and the thermal background radiation beyond 2.3 μm. In our work, we will convert both K_s and K′ magnitudes into standard K magnitudes using the expressions: provided by the SOFI User’s Manual (ESO Doc. No. LSO-MAN-ESO-40100-0004) and Wainscoat & Cowie (1992), respectively.

These near-IR observations consisted of series of several integrations repeated at dithered positions on the detector. As for the TNG NICS observations of Northern sources, 5–10 integrations of exposure time 3 to 10 s, depending on the brightness of the object, were taken at 5–7 different CCD locations for each object and filter. Therefore, total effective exposures ranging between 75 and 500 s were secured. Since the shortest exposure time of an individual frame allowed by the control software was 3 s, a grey attenuator filter was used to reduce the count rate of the brightest objects (e.g., IRAS 20406+2953) and to avoid the saturation of the signal. This filter reduced the brightness of the object by 5.1 mag, 5.5 mag, and 5.7 mag in the J, H, and K bands, respectively, but its use has the drawback of limiting the accuracy of the photometric measurements to  ~0.2 mag. Likewise, the NTT SOFI observations of Southern sources consisted of 5 integrations of exposure time from 1.2 to 8 s at 7 positions on the CCD randomly determined by the imgobsJitter template. The total effective exposure, resulting from the co-addition of the 35 short jittered exposures, ranged from 42 to 280 s.

Six of the objects that were found to present extended emission in JHK images (Table 2) were also observed using the H₂, Brγ, and K-continuum narrow-band filters whose characteristics are listed in Table 1. The observations consisted of a number of jittered images, each one being the average of five integrations. For each object, the integration time of each individual frame, the number of integrations and images, and the total exposure time are listed in Table 2. The spatial resolution of the images, as determined from the FWHM of stars in the field of view, is also listed in the last column of Table 2.

2.1.2. Data reduction, calibration and analysis

The standard reduction of near-IR data includes the dark subtraction and flat field correction of individual frames that are then median combined to build a sky image. The sky image is subsequently subtracted from the individual frames that are then shifted and finally stacked into one single image. As for the TNG NICS JHK′ data, their reduction was mostly carried out using SNAP (Speedy Near-IR data Automatic reduction Pipeline), a pipeline for the automatic reduction of near-IR data that uses software routines of IRDR¹, IRAF², Sextractor, and Drizzle³. The reduction performed by SNAP includes additional non-standard steps such as cross-talk correction, double-pass sky subtraction, and field distortion correction⁴. When SNAP was not used, the standard reduction was performed using standard routines within IRAF, but an additional cross-talk correction was carried out at the beginning of the reduction process. The reduction of the NTT SOFI JHK_s data was performed using the SOFI pipeline sofi_img_jitter which is part of the SOFI subsystem of the VLT data flow system (DFS). The sofi_img_jitter tool carries out the standard near-IR reduction, including additional steps for cross-talk effect correction and bad pixels cleaning as described in the SOFI pipeline user manual (VLT-MAN-ESO-19500-4284).

At least three different photometric standard stars were observed each night for photometric calibrations. At the TNG, we observed the standard stars AS5, AS17, AS29, AS31, AS33, and AS36 (Hunt et al. 1998). The effective exposure time for these stars varies from 75 s up to 175 s, consisting of 5–7 dithered images, each one being the average of 5 frames with integration times between 3 and 7 s. At the NTT, we observed the infrared NICMOS standard stars #9103, #9146, #9157, #9185, #9186, and #9187 (Persson et al. 1998). The effective exposure time for these stars is 50 s, consisting of 5 jittered images, each one being the average of five 2-s integrations.

Using routines of the IRAF daophot package, aperture photometry was performed to derive the JHK magnitudes listed in Tables 3–6. The tables include references to previous works reporting JHK measurements. For point sources, different aperture sizes and sky annuli were used depending on the PSF of the source. Photometric instrumental zero-points for the TNG standard stars were calculated using this method, while, for the NTT standard stars, the zero-point tool of Gasgano (a data file organizer developed and maintained by the European Southern Observatory, ESO) was used. For extended sources (e.g., IRAS 15452−5459), we used the HEASARC task fv (the interactive FITS file editor) to determine their background-subtracted fluxes and uncertainties by comparing the count numbers and deviations of source regions encompassing the extended emission with those of appropriate nearby background regions. The 1-σ error-bars of the JHK magnitudes provided in Tables 3–6 include systematic uncertainties of the calibration, aperture and sky annuli selection, and PSF estimate, as well as the photon noise. The limiting magnitudes at the NTT were found to be J = 22.7 mag, H = 21.8 mag, and K = 20.4 mag, while at the TNG these were found to be J = 22.5 mag, H = 21.4 mag, and K = 20.2 mag We note that the NTT SOFI JHK_s images of objects brighter than J > 9 mag and K > 7 mag saturated using the shortest integration time (1.2 s) that SOFI control software could afford. Consequently, we do not present new JHK magnitudes for these objects, all of them having a 2MASS PSC counterpart. Their images were nonetheless acquired and examined in order to search for extended emission.

The narrow-band images were processed using standard IRAF routines. By comparing field stars in the emission line and continuum images, the contribution of continuum emission in the H₂ and Brγ images was found to be similar, within a few percent, to the emission detected in the K-continuum images. Images were consequently scaled by this small percentage before subtracting the continuum emission.

2.2. Archival data

As in Paper I, data were extracted from the Mid-course Space Experiment (MSX) Point Source Catalogue (MSX6C) and Spitzer Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) Image Atlas and source catalogues (Price et al. 2001; Fazio et al. 2004). A description of the data products extracted from these catalogues is given in Paper I.

To complement the mid-IR data, in this paper we have made also use of recently released AKARI data. AKARI is a Japanese space mission dedicated to infrared astronomical observations (Murakami et al. 2007) that has provided an all-sky survey in the 9 and 18 μm bands of the Infrared Camera (IRC, Onaka et al. 2007), and in the 65, 90, 140, and 160 μm bands of the Far-Infrared Surveyor (FIS, Kawada et al. 2007). We extracted data of the AKARI FIS Bright Source Catalogue and IRC Point Source Catalogue using the AKARI Catalogue Archive Server (AKARI-CAS)⁵. Only data for sources that are confirmed and whose flux is reliable, as indicated by a flux quality indicator FQUAL = 3, were used.

3. Objects with 2MASS PSC counterpart

The objects in our sample that have a counterpart in the 2MASS PSC were reported in Paper I. A comparison between the new JHK magnitudes determined from our NTT and TNG observations and these derived from the 2MASS PSC (Table 2 of Paper I) is mandatory in order to check the calibration of our observations and to assess the limits for point-source detection of the 2MASS PSC data. Furthermore, this comparison can uncover long term variability or it can be used to correct the 2MASS PSC magnitudes of objects that are contaminated by unresolved sources.

Figure 1 compares the JHK magnitudes derived from the NTT and TNG observations of the objects listed in Table 3 with the JHK magnitudes of the 2MASS PSC. The plots demonstrate that the JHK magnitudes derived from the NTT and TNG observations are consistent with the 2MASS PSC JHK magnitudes, thus giving support to the calibration of our JHK measurements. For objects fainter than  ~16 mag in J,  ~15 mag in H, and  ~14 mag in K, there is a clear deviation from the 1:1 relationship, with the J, H, and K magnitudes derived from our images being generally greater than the corresponding 2MASS PSC J, H, and K magnitudes. These deviations are consistent with the 2MASS PSC 10 σ detection limit for point-sources of J = 15.8 mag, H = 15.1 mag, and K_s = 14.3 mag (Skrutskie et al. 2006).

On the other hand, a number of objects show discrepancies larger than 10-σ between one of the new JHK magnitudes and the corresponding 2MASS PSC value. To verify the likely photometric variability of these sources, compiled in Table 4, we have compared the JHK magnitudes of a number of stars in their fields of view with their 2MASS PSC JHK magnitudes and confirmed that there are no systematic variations. We thus conclude that the variations reported for the sources in Table 4 are real.

The photometric variations in Table 4 fail to show consistent color changes that could be interpreted as post-AGB evolution if the sources were becoming bluer. Compared to the 2MASS values, the new TNG and NTT JHK magnitudes show that some of the sources are fainter, whereas others have become brighter. These brightness variations can be attributed to pulsation (e.g. Hrivnak et al. 2010) rather than to evolution. We note that, even if the IRAS variability of these sources is not large enough to have them excluded from our sample (see selection criteria in Paper I), they could still be variable stars. Additionally, IRAS 14104−5819, IRAS 18071−1727, and IRAS 18105−1935 show OH masers (te Lintel Hekkert et al. 1991; Sevenster et al. 2001) that might make them belong to the group of OH/IR AGB stars.

3.1. Extended sources among the objects with 2MASS PSC counterpart

The new JHK images have unveiled extended emission from IRAS 09102−5101 and IRAS 13398−5951 (Fig. 2), two sources whose near-IR counterparts had already been reported in Paper I. IRAS 09102−5101 shows a round nebula  ~22 in radius that cannot be resolved in 2MASS images. Meanwhile, IRAS 13398−5951 displays low surface brightness extended emission, too faint to be detected in 2MASS images, which seems to be distributed in a discontinuous round shell,  ~10′′ in radius, centered at the source.

4. Source identification

According to the procedure described in Paper I, we have primarily used the Spitzer GLIMPSE Image Atlas and catalogue to determine the accurate position of the objects with no 2MASS PSC counterpart. The images of the 29 objects in the Spitzer GLIMPSE Image Atlas are shown in Fig. 3. In all cases but IRAS 17153−3814, the identification of the Spitzer mid-IR counterpart of the IRAS post-AGB star and PN candidates is undoubtful.

When Spitzer GLIMPSE images were not available, we took advantage of the improved positional accuracy of the sources in the AKARI IRC catalogue to refine the IRAS coordinates of our target list. After the cross-correlation of IRAS and AKARI IRC coordinates, we found AKARI counterparts within 30′′ of all IRAS sources but for IRAS 17153−3814 and IRAS 17479−3032, which are otherwise detected in Spitzer GLIMPSE images, and IRAS 17097−3624, for which a reddened near-IR counterpart is found at the location of the IRAS coordinates. For those sources that still lack an unambiguous near-IR counterpart, we examined the recently released Wide-field Infrared Survey Explorer (WISE, Wright et al. 2010) images and searched for them in the WISE All-Sky Source Catalog.

All the objects that have been unambiguously identified are listed in Table 5. The table includes the improved coordinates, as derived from our near-IR observations (Cols. 2 and 3), a comment on the spatial properties of the source, describing whether it is extended or a point-source (Col. 4), a note on their detection on DSS red or 2MASS images (Cols. 5 and 6), the JHK magnitudes derived from the new observations (Cols. 7–9), and the reference to previous JHK measurements as listed in Table 3.

The two objects that have multiple possible counterparts within the AKARI IRC and IRAS error boxes, IRAS 11544−6408 and IRAS 17153−3814, are listed in Table 6. This table lists the near-IR coordinates of the possible counterparts and their positional offset with respect to the AKARI IRC coordinates for IRAS 11544−6408 and to the IRAS coordinates for IRAS 17153−3814 (Cols. 2–6), information on the extended nature of the sources (Col. 7), and our JHK magnitudes and references of previous measurements (Cols. 8–11). From the near-IR images at an angular resolution of 10, it is unclear whether a number of unresolved stars is found at the location of IRAS 11544−6408-B, or whether it is extended.

Sources that do not have an evident near-IR counterpart within the AKARI IRC and IRAS error boxes are listed in Table 7. For these objects we provide their AKARI IRC coordinates, except for IRAS 18113−2503 and IRAS 18529 + 0210. For IRAS 18113−2503 we provide the source coordinates derived from the water maser structure detected with EVLA by Gómez et al. (2011). The absolute error of this position is 12, whereas for IRAS 18529 + 0210, detected in all Spitzer IRAC bands, the Spitzer coordinates listed in Table 7 are known with an accuracy better than 2′′. Despite the accurate positioning of these two sources, they have not been detected in our near-IR images, with detection limits J > 22.5 mag, H > 21.4 mag, and K > 20.2 mag, suggesting that they are highly obscured sources.

Composite pictures of the broad-band near-IR images of the sources in Table 5 are presented in Fig. 4. The location of the near-IR counterparts are overlaid on the pictures. The sources with multiple possible counterparts or with unidentified counterparts listed in Tables 6 and 7 are presented in Fig. 5. For sources with multiple possible counterparts, these different counterparts are labeled. When no counterpart was found, the IRAS ellipse error and AKARI IRC position error are overlaid.

4.1. IRAS, AKARI IRC and near-IR coordinates

The coordinates of the IRAS and AKARI sources are compared to these of their near-IR counterparts in Fig. 6. The rms of the difference between the IRAS coordinates is  ~17′′ in right ascension and  ~5′′ in declination, i.e., within the size of the median IRAS 1-σ error ellipse of our sample, 24′′  ×  6′′. There are only 6 sources for which their near-IR and IRAS coordinates show a difference notoriously larger than the error ellipse of their IRAS coordinates. In these cases, an inspection of the original IRAS images revealed the presence of multiple nearby sources that, not being resolved by IRAS, resulted in an erroneous determination of the IRAS coordinates.

The comparison between the near-IR and AKARI IRC coordinates (Fig. 6-center) offers a sharper view. The rms of the difference between these coordinates is  ≃ 10 both in right ascension and declination. The cumulative frequency of the differences in coordinates (Fig. 6-right) shows that  ~90% of the sources have near-IR coordinates within 2′′ of their AKARI IRC coordinates, whereas the same plot for the IRAS coordinates showed a greater dispersion.

4.2. Comparison with previous JHK measurements

There are 73 sources in our sample with previous JHK photometric measurements as listed in Tables 3, 5, 6, and 7. The comparison between the values of the JHK magnitudes in these tables and those in previous works is revealing: 28 out of the 73 sources in common have very similar JHK magnitudes, but the remaining 45 sources are considerably brighter in these previous works. Some of these works are based on observations carried out with a photometer (e.g., García-Lario et al. 1997) which may lead to the selection of an erroneous near-IR counterpart in crowded stellar regions using IRAS coordinates. Indeed, the inspection of the near-IR images of these objects in Fig. 4 confirms the presence of nearby bright sources that have certainly induced the misidentification of their counterparts in these works (e.g., IRAS 09370−4826 and IRAS 13500−6106). The use of 2MASS images has certainly improved the situation, but the limited sensitivity of 2MASS and the uncertainties in IRAS coordinates pose limits to this technique too, as was shown in Paper I.

5. Extended sources

We list in Table 8 the 18 sources found to be resolved or to present extended emission in near-IR JHK and/or mid-IR Spitzer IRAC images. Among these sources, 12 display extended emission in the near-IR, 6 show spatial profiles in mid-IR Spitzer IRAC images that are definitely inconsistent with those of point sources in their respective FoVs, and three others show indications of being marginally resolved at certain IRAC bands. For completeness, we also include IRAS 18229−1127, IRAS 18454 + 0001, and IRAS 18576 + 0341 that were already presented in Paper I as resolved in Spitzer IRAC images.

Near-IR narrow-band H₂, Brγ, and continuum images have been obtained for six of the sources listed in Table 8 as described in Sect. 2.2. The narrow-band images of these sources are presented in Fig. 7. According to the morphology seen in these images, and in the broad-band JHK (Fig. 4) and mid-IR Spitzer IRAC (Fig.  3), the extended sources can be classified into three different categories: young stellar object (YSOs) candidates, round post-AGB source and PN candidates, and bipolar post-AGB source and PN candidates. The objects in these categories are described in more detail below.

5.1. Young stellar object candidates

Among the sources with extended emission in our sample, IRAS 06499+0145 and IRAS 12405−6219 present irregular morphologies that do not fit the round/elliptical, bipolar, or multi-polar description typical of most post-AGB shells and young PNe (Sahai et al. 2011).

The broad-band JHK images of IRAS 06499 + 0145 (Fig. 4) show a resolved core embedded within diffuse emission, as well as a relatively bright blob and an arc-like filament are found  ~45′′ southeast from the central source. The nature of this source is disputed as its IRAS fluxes place it in the region of the IRAS color–color diagram common to post-AGB stars (García-Lario et al. 1997) and ultra-compact H ii regions (Bronfman et al. 1996). The SEST detection of CS(2–1) molecular line emission (Bronfman et al. 1996) suggests a YSO nature, as this line is excited in the dense gas (10⁴−10⁵ cm^-3) typically found in the interior of molecular clouds, but its detection in AGB stars is rare (e.g., Olofsson et al. 1998). The abundance of CS decreases even further during the post-AGB and PN phases (Bachiller et al. 1997). Moreover, the LSR velocity of the CS emission, 45.1 km s^-1 (Bronfman et al. 1996), is close to that of the molecular cloud [MAB97] 211.50+1.00 (47.6 km s^-1, May et al. 1997) whose projection on the sky encloses the IRAS source. These evidences indicate that IRAS 06499 + 0145 is a young stellar object associated with the molecular cloud [MAB97] 211.50+1.00.

As for IRAS 12405−6219, its broad-band K_s image (Fig. 4) reveals a bright central source embedded within diffuse emission that, on the screen of our computer, extends up to  ~35′′ along the direction at position angle  ~300° (measured north to east). Relatively bright knots of diffuse emission are also detected in the K_s band up to  ~11′′ towards the south. Since these are the only patches of diffuse emission seen in the images around this source, it is likely that the diffuse emission is certainly associated to IRAS 12405−6219. Also the stars around IRAS 12405−6219 show a suspicious excess in the K_s image that may be indicative of a large absorption along the line of sight. The narrow-band near-IR images of IRAS 12405−6219 (Fig. 7) show a bright core with a patch of neutral material towards the northwest and south, similar to the K_s image. In addition, bright H₂ emission is detected up to  ~26′′ towards the south of the central source. In the mid-IR (Fig. 3), the source appears embedded within bright 8 μm diffuse emission and shows a bright patch of 4.5 μm emission towards the south of the bright core which is spatially coincident with the bright patch of H₂ emission.

5.2. Round post-AGB source and PN candidates

There are three extended sources with round morphology: IRAS 09102−5101 and IRAS 13398−5951, presented in Fig. 2 and described in Sect. 3.2, and IRAS 17009−4154. The latter displays a round morphology in the near-IR JHK_s images (Fig. 4) with an angular size  ~52 in J,  ~64 in H, and  ~80 in K_s, and resolved diffuse emission in the mid-IR Spitzer IRAC images (Fig. 3) at 3.6, 4.5, and 5.8 μm with a deconvolved PSF  ~ 44 FWHM. Previous mid-IR N and near-IR K images presented by Van de Steene et al. (2000) showed a roundish faint nebulosity, 7′′ in diameter, surrounding the star. Their near-IR K spectrum displays Brγ emission from ionized material, thus suggesting that the source has already reached the PN stage.

5.3. Bipolar post-AGB source and PN candidates

Seven sources are found to display bipolar morphology: IRAS 05573+3156, IRAS 15103−5754, IRAS 15452−5459, IRAS 15534−5422, IRAS 16333−4807, IRAS 17150−3224 (the Cotton Candy Nebula), and IRAS 19071 + 0857. These objects exhibit a variety of bipolar morphologies, with IRAS 15103−5754 and IRAS 15452−5459 (Fig. 4) displaying a dark equatorial region that marks a waist in their bipolar lobes, while IRAS 16333−4807, IRAS 17150−3224, and IRAS 19071 + 0857 (Fig. 4) display bright central regions from which fainter bipolar lobes protrude. The JHK′ images of IRAS 05573 + 3156 (Fig. 4) show a bright core that is resolved along the direction at PA  ~  45° and two bipolar lobes. The lobe along the southwest direction has an angular extent  ~90′′, and bright emission at its tip, whereas the northeast lobe seems to lack emission on its east wall. Among the objects with bipolar morphology, only the bipolar lobes of IRAS 19071 + 0857 and IRAS 17150−3224 show some hints of point-symmetric brightness distribution.

Narrow-band H₂, Brγ, and K continuum images (Fig. 7) reveal prominent Brγ emission in IRAS 15534−5422 and IRAS 19071 + 0857. The ionization of their nebular material confirms they have reached the PN stage. On the other hand, the extended emission with bipolar morphology from IRAS 15103−5754, IRAS 15452−5459, and IRAS 16333−4807 is mainly continuum emission that can most likely be attributed to reflected emission from proto-PNe. The detection of radio continuum emission from IRAS 15103−5754 and IRAS 16333−4807 (Van de Steene & Pottasch 1993), however, suggests they are likely young PNe.

Two of the objects described above, IRAS 15452−5459 and IRAS 17150−3224, had been previously studied in detail. IRAS 15452−5459 has been described by Sahai et al. (2007) as an hourglass-shaped proto-PN using HST images in the F814W ACS filter and the F110W, F160W, and F205W NICMOS filters. Our JHK and H₂, Brγ and K continuum images confirm this morphology, including the narrow dark lane at the equatorial region separating the two bipolar lobes (Figs. 4 and 7). The Spitzer IRAC images of this source are remarkable (Fig. 3): the central region is barely resolved, but the source itself is found to be embedded within a region of diffuse emission with a 8 μm bright wavy filament crossing the source center. It is unclear whether this filament is related to IRAS 15452−5459, but a physical connection is suggested by its noticeable point-symmetry with respect to the source.

The proto-PN IRAS 17150−3224 has been amply studied (e.g., Kwok et al. 1998; Su et al. 2003; Hrivnak et al. 2006). The morphology revealed by our NTT JHK images is consistent with that shown in greater detail by previous broad-band optical and narrow- and broad-band near-IR HST images.

5.4. Morphology of resolved objects with water maser emission

In the case of O-rich sources, water maser emission has been claimed to trace the first signs of axisymmetric mass loss in the late evolutionary stages of the AGB and early post-AGB phase (Imai et al. 2002; Boboltz & Marvel 2005; Imai et al. 2007; Gómez et al. 2008; Suárez et al. 2009). Indeed, all post-AGB stars with water fountain characteristics (i.e., showing water maser components separated  ≥ 100 km s^-1) and water-maser-emitting PNe whose optical or IR morphology have been resolved, are bipolar or highly axisymmetric: IRAS 16342−3814 (Sahai et al. 1999; Verhoelst et al. 2009), IRAS 17347−3139 (de Gregorio-Monsalvo et al. 2004), IRAS 18061−2505 (Suárez et al. 2006; Gómez et al. 2008), IRAS 19134 + 2131 (Imai et al. 2007), K 3-35 (Miranda et al. 2001), W43 A (Imai et al. 2002), and IRAS 15445−5449 (Pérez-Sánchez et al. 2011; Suárez et al., in prep.).

Among the seven objects of our sample that are resolved (Table 8), three display water maser emission⁶: IRAS 15103−5734 IRAS 15452−5459, and IRAS 16333−4807 (Deacon et al. 2007; Suárez et al. 2009). All these objects display IR bipolar morphologies, thus strengthening the hypothesis that water maser emission in post-AGB sources and PNe is associated with axisymmetric mass loss.

6. Obscured sources

We have compiled in Table 10 the Spitzer GLIMPSE 3.6, 4.5, 5.8, and 8.0 μm and MSX 8.28, 12.13, 14.65, and 21.3 μm fluxes extracted from the corresponding point source catalogs for the IRAS post-AGB star and PN candidates without 2MASS PSC counterpart in Paper I (Table 5 of Paper I provides this information for the IRAS sources with 2MASS PSC counterparts). Similarly, we list in Table 11 the AKARI IRC 9 and 18 μm and FIS 65, 90, 140, and 160 μm fluxes for these sources. Using these values, in conjunction with the IRAS fluxes from Table 1 of Paper I, and the NTT and TNG JHK magnitudes listed in Tables 3, 5, and 7, we have constructed the IR spectral energy distributions (SEDs) shown in Figs. 4 and 5.

The inspection of the individual IR SEDs of the IRAS post-AGB source and PN candidates in these figures discloses noticeable differences. Due to the selection criteria of this sample, most of the IR SEDs peak in the wavelength range 10–30 μm. Above 20 μm, the SED shows a steep decline for most sources, although there are a few of them that show a plateau extending at long wavelengths (e.g., IRAS 15534−5422). Below 20 μm, the SEDs of the sources in this sample show a great variety of shapes, showing a peak (e.g., IRAS 17548−2753), a flat shape (e.g., IRAS 17506−2955), a sharp decline (e.g., IRAS 18514+0019), or flux increase towards the shortest wavelengths in the JHK range (e.g., IRAS 18385+1350).

The most obscured sources can be selected among those whose SEDs show the steepest declines bluewards of 20 μm. The J and K magnitudes and the AKARI IRC 9 μm flux, which are available for most sources, can be used to select the most obscured objects. Figure 8 shows the distribution of the sources in the infrared color–color J−K vs. K−IRC_9   μm diagram⁷ Sources whose flux in the J band is 1000 (100) times fainter than at 9 μm are located above the dotted (solid) line in this diagram and are listed in Table 9. These sources represent the most obscured post-AGB and PN candidates in our sample. In particular, sources with F_9   μm/F_J > 1000 would be intrinsically 100 times fainter in the J band that at 9 μm and would still be located above the solid line in this diagram even if they suffered from a Galactic interstellar extinction of A_V = 10 mag These are basically the sources undetected in the J band in Tables 3 and 5 and sources in Table 7 with no detectable near-IR source within their coordinate error boxes (i.e., IRAS 09370−4826, IRAS 18113−2503 and IRAS 18529 + 0210).

6.1. Galactic distribution of obscured sources

The Galactic distribution of different types of PNe and post-AGB stars is commonly used to assess the properties (mainly average initial mass) of their progenitors (e.g., Corradi & Schwarz 1995; Manchado et al. 2000; Stanghellini et al. 1993). Differences in Galactic height or latitude reported for elliptical/round and bipolar PNe (Corradi & Schwarz 1995; Stanghellini et al. 2002), and for SOLE (star-obvious low-level-elongated) and DUPLEX (dust-prominent longitudinally extended) post-AGB objects (Ueta et al. 2000; Siódmiak et al. 2008) have been interpreted as a result of bipolar PNe and DUPLEX post-AGB objects descending from high progenitor mass AGB stars.

It is thus interesting to study the location on Galactic coordinates of the sources with different degrees of obscuration (Fig. 9-top), and their Galactic latitude distributions (Fig. 9-bottom). Following Corradi & Schwarz (1995), we have fitted these distributions using an exponential fit N = N_oe^{−b/ ⟨ b ⟩} , where  ⟨ b ⟩  is the mean and standard deviation of this distribution. The widths of the best fits to these distribution, shown in Fig. 9-bottom, are 20 for obscured (F_9   μm/F_J > 100) sources and 30 for the less obscured sources (F_9   μm/F_J < 100). Similar fits have been computed for the samples of SOLE and DUPLEX sources (Ueta et al. 2000; Siódmiak et al. 2008), bipolar and elliptical PNe (Corradi & Schwarz 1995; Stanghellini et al. 2002), and optically bright post-AGB stars (Suárez et al. 2006). As shown in Fig. 9-bottom, these fits suggest that both our sample of obscured and less obscured sources and the sample of bipolar PNe and DUPLEX sources have almost indistinguishable Galactic latitude distributions with  ⟨ b ⟩  equal to 20, 30, 28 and 33, respectively. Optically bright post-AGB stars and SOLE sources have slightly larger values of  ⟨ b ⟩ , 66 and 77, respectively, whereas the distribution of elliptical PNe has the largest width, 93.

To check whether the populations from which these samples are drawn are different or not, we have applied Kolmogorov-Smirnov statistical tests to each pair of samples. These tests did not show any significant statistical differences between the Galactic latitude distributions of the samples of obscured and less obscured sources, bipolar PNe and DUPLEX sources, whereas these distributions are distinct at a 95% confidence level from those of optically bright post-AGB stars, elliptical PNe and SOLE sources (the latter only marginally owing probably to the small size of this sample). The obscured and less obscured sources in our sample and the stellar progenitors of bipolar PNe and DUPLEX sources may arise from the same population of sources, which seems to be different from those of the stellar progenitors of optically bright post-AGB stars, SOLE sources and elliptical PNe.

7. Summary

This is the second paper in a series aimed at investigating the spectral properties and determining the nature of a sample of 165 presumably obscured IRAS post-AGB star and PN candidates. In Paper I (Ramos-Larios et al. 2009), 2MASS, MSX and Spitzer GLIMPSE archival data were used to search for near-IR counterparts in the 2MASS PSC, but the limited sensitivity of 2MASS observations did not allow the identification of  ~80 sources in the sample.

To overcome this problem, near-IR JHK images have been obtained for 164 objects in the sample of IRAS post-AGB star and PN candidates. The new near-IR JHK images, in conjunction with MSX, Spitzer GLIMPSE and recently available AKARI archival data, have allowed the unambiguous identification of near-IR counterparts for 154 out of the 165 sources in the sample. Accurate finding charts are provided in this paper for the sources lacking a PSC counterpart, thus complementing those presented in Paper I for the sources with PSC counterparts.

We have selected the sources with the greatest obscuration degree between the mid-IR 9 μm band and the near-IR J band. The Galactic latitude distribution of these sources, close to the Galactic Plane, is consistent with that of bipolar PNe and DUPLEX sources, and thus it does not disproof the possible link between obscured post-AGB stars and PNe, high progenitor mass AGB stars and bipolar PNe.

Sources that have a near-IR 2MASS PSC counterpart have also been investigated for long-term variability. The JHK magnitudes of a dozen sources have been found to have varied between the time of the 2MASS observations and that of the NTT and TNG observations. The color changes implied by these variations are not consistent with post-AGB evolution, but they can be rather interpreted as the result of pulsation.

Extended or resolved emission has been found in 18 objects and near-IR Brγ, H₂, and K continuum images have been obtained for 6 of them. Among these sources, IRAS 06499 + 0145 and IRAS 12405−6219 are proposed to be young stellar object candidates, and 10 sources are proposed to be post-AGB source and PN candidates with either round (IRAS 09102−5101, IRAS 13398−5951, and IRAS 17009−4154) or bipolar (IRAS 05573 +  3156, IRAS 15103−5754, IRAS 15452−5459, IRAS 15534−5422, IRAS 16333−4807, IRAS 19071 + 0857, and IRAS 17150− 3224) morphologies. The detection of near-IR Brγ line emission or radio continuum suggests that IRAS 15103−5754, IRAS 15534−5422, IRAS 16333−4807, IRAS 17009−4154, and IRAS 19071 + 0857 have already reached the PN stage.

We find that all sources in our sample that are resolved and have water maser emission are bipolar. This strengthens the hypothesis that water maser emission is associated with axisymmetric mass loss in evolved stars.

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Acknowledgments

This research has made use of: the SIMBAD database, operated at CDS, Strasbourg, France, data products from the 2MASS, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation, and data based on observations obtained with AKARI, a JAXA project with the participation of ESA, and on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by an award issued by JPL/Caltech. G.R.L. acknowledges support of a postdoctoral fellowship from CONACyT (Mexico) grant 75861. Important support has been provided by several grants of the Spanish Ministerio de Ciencia e Innovación (MICINN): AYA2008-01934 (GRL, LFM, MAG, and OS), AYA2008-06189-C03-01 (JFG and OS), AYA2011-29754-C03-02 (MAG), and AYA 2011-30228-C03-01 (JFG, LFM, and OS). J.F.G., M.A.G., and L.F.M. acknowledge partial support of grant TIC-126 from Consejería de Innovación, Ciencia y Empresa of Junta de Andalucía. L.F.M. is supported partially by grants INCITE09E1R312096ES and INCITE09312191PR of the Galician INCITE research program of the Dirección Xeral de Investigación, Desenvolvemento y Innovación of the Spanish Xunta de Galicia. Finally, we thank Martin Cohen for helpful discussion on the accuracy of Spitzer GLIMPSE data.

References

All Tables

All Figures